Acta Materialia

Editors for Acta Materialia

2010-04-01

On intrinsic brittleness and ductility of intergranular fracture along symmetrical tilt grain boundaries in copper

Y. Cheng, Z.-H. Jin, Y.W. Zhang, H. Gao — 2009-12-07

Abstract: The intrinsic brittleness and ductility of intergranular fracture along a number of symmetrical [110] tilt grain boundaries (GBs) in Cu are investigated via combined atomistic and continuum studies of dislocation nucleation from an atomically sharp crack tip. In all cases investigated, the classical model of Rice predicts a directional anisotropy in that, along a given GB, brittle cleavage is favored for crack propagation in one direction while dislocation emission from the crack tip is preferred in the opposite direction. This prediction is validated by atomistic simulations of crack propagation along coherent GBs, including and . However, for incoherent GBs such as and , such directional anisotropy in intrinsic ductility is not observed; instead, we show that dislocation emission is favored in both crack propagation directions. The reason for this discrepancy is shown to be dislocation emission at a distance ahead of the crack tip along an incoherent GB, which violates the assumption in Rice’s model that dislocation emission occurs directly at the crack tip.

Damage evolution and domain-level anisotropy in metal/ceramic composites exhibiting lamellar microstructures

Siddhartha Roy, Benjamin Butz, Alexander Wanner — 2010-01-11

Abstract: Aluminium/alumina composites based on ceramic preforms prepared via freeze-casting are examined. Domains composed of alternating but also interpenetrating ceramic and metallic lamellae are observed. Single-domain samples were extracted from composites processed under different conditions. In situ scanning electron microscopy analyses were carried out to investigate the damage evolution under compressive load. The composite is strong and brittle when loaded along directions parallel to the freezing direction. When compressed in other directions, the behavior is controlled by the soft metal. The plastic anisotropy is less pronounced than theoretical predictions for laminates, which is explained by the presence of bridges between the ceramic lamellae. Coating the preform with Cu or Cu2O prior to melt infiltration reduces the compressive strengths of the composites. Transmission electron microscopy analysis shows that even in the case of a Cu coating a Cu2O layer is formed during processing, weakening the interface and preventing the dissolution of Cu in the aluminium alloy.

Grain size–wear rate relationship for titanium in liquid nitrogen environment

Aditya Jain, B. Basu, B.V. Manoj Kumar, Harshavardhan, J. Sarkar — 2009-12-25

Abstract: This paper presents the results of sliding wear experiments conducted on high-purity titanium (Ti) against bearing-steel in liquid nitrogen (LN2; boiling point: 77K) environment. Ti samples of three different grain sizes (9, 17 and 37μm) were used to study the effect of hardness, derived from grain refinement as well as cryogenic test temperature, on the wear properties of Ti. In our experiments, a constant load of 10N and sliding speeds of 0.67, 1.11 and 4.19ms−1 were used. The coefficient of friction (COF) for this tribo-couple varied between ∼0.25 and ∼0.50. While a steady state was always achieved, a peak in the COF was always noted in case of coarse-grain (37μm) Ti tested at a sliding speed of 4.19ms−1. Under the investigated sliding conditions, the wear rate was found to be of the order of 10−3–10−4mm3N−1m−1. The lowest wear rate was recorded in the fine-grain (9μm) Ti at the highest sliding speed of 4.19m s−1. The critical analysis of the worn surface topography reveals that the reduced wear rate was due to the formation of adherent and strain-hardened tribolayer. In order to show various dominant wear mechanisms of Ti, a qualitative map was developed in sliding speed–grain size space. Substructure evaluation revealed the formation of a dense array of deformation twins because of the plastic deformation, which often resulted in the subdivision of grains.

Studies on recrystallization of single-phase copper alloys by resistance measurements

2009-12-28

Abstract: Copper-based solid solutions with different contents of solute elements (Zn, Al, Ga, Sn, Ge) were deformed at room temperature and at liquid nitrogen temperature. The recrystallization behaviour of these alloys has been investigated by means of dynamic and isothermal measurements of the resistivity . Variations of from are interpreted in terms of changes in defect densities by recovery and recrystallization. Deviations from a linear temperature dependence of the resistivity increase with increasing solute concentration. This deviation (i.e. as determined at room temperature) also depends on the stacking fault energy . We observe a -dependency of which is also influenced by the deformation temperature. During deformation, controls the possibility of storing deformation energy in the form of dislocations and deformation twins. In combination with the general trend of alloying elements to shift recrystallization to higher temperatures, the recrystallization behaviour of single-phase copper alloys has been described qualitatively.

Towards smooth and pure iron nanowires grown by electrodeposition in self-organized alumina membranes

2009-12-31

Abstract: Due to their high aspect ratio, magnetic nanowires are interesting for various applications. Fe nanowires are of particular interest due to their high magnetization, which suggests high shape anisotropy is possible. Here, we show how the electrodeposition process can be adapted to the constraints of a high aspect ratio template in order to approach an ideal behaviour of smooth and pure Fe nanowires. An adjustment of the Fe2+ concentration and the addition of H3BO3 result in smooth and continuous nanowires; saturation polarization is achieved, together with an anisotropy field of up to 70% compared to pure Fe. Mössbauer spectroscopy reveals a negligible amount of impurities. Together with magnetic measurements we show that shape anisotropy aligns the preferential magnetization axis along the wire axis.

Carbon supersaturation of ferrite in a nanocrystalline bainitic steel

F.G. Caballero, M.K. Miller, C. Garcia-Mateo — 2009-12-28

Abstract: The extremely slow transformation kinetics of a nanocrystalline bainitic steel allows the carbon content of the bainitic ferrite away from any carbon-enriched regions such as dislocations and boundaries to be determined by atom probe tomography as the bainite transformation progresses at 200°C. A high level of carbon, well above that expected from para-equilibrium with austenite, has been detected in solid solution in bainitic ferrite at the early stage of transformation. Results provide strong evidence that bainite transformation is essentially displacive in nature so that the newly formed bainitic ferrite retains much of the carbon content of the parent austenite.

Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading

M.L. Young, M.F.-X. Wagner, J. Frenzel, W.W. Schmahl, G. Eggeler — 2009-12-25

Abstract: An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1GPa) and unloading. Both macroscopic stress–strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19′ phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys.

Tensile and compressive behavior of tungsten, molybdenum, tantalum and niobium at the nanoscale

Ju-Young Kim, Dongchan Jang, Julia R. Greer — 2009-12-25

Abstract: In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of 〈001〉-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log–log slope of strength vs. nanopillar diameter, is −0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (−0.44). In tension, however, Ta and Nb show higher size effect slopes (−0.80 and −0.77) as compared with W and Mo (−0.58 and −0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.

A study of a new type of deviation from the Kurdjumov–Sachs orientation relationship in face-centered-cubic/body-centered-cubic transformation system

Y. Meng, X.-F. Gu, W.-Z. Zhang — 2009-12-28

Abstract: Slight deviations from the Kurdjumov–Sachs orientation relationship (KS OR) are often observed in the face-centered-cubic/body-centered-cubic (fcc/bcc) transformation systems, and some can be explained on the premise of the parallelism of either the close-packed planes (CPPs) or the close-packed directions (CPDs), as suggested in many models. This work investigates a new OR which deviates from the KS OR in both the CPPs and the CPDs. This OR is determined by requiring the habit plane to contain a single set of periodic dislocations along the invariant line lying in the CPPs. The angles of the double deviation, as a function of the lattice parameter ratio, are proved to be identical. The calculated results for a stainless steel are in good agreement with the experimental observations. The habit plane for the new OR is associated with slightly lower interfacial energy than those with the ORs of single deviation, which might explain the preference by nature in the present case.

Interfacial diffusion in Cu with a gradient nanostructured surface layer

Z.B. Wang, K. Lu, G. Wilde, S.V. Divinski — 2009-12-25

Abstract: A graded microstructure was produced in the surface layer of a pure Cu sample by means of surface mechanical attrition treatment (SMAT) [Wang K, Tao NR, Liu G, Lu J, Lu K. Acta Mater 2006;54:5281.]. The diffusion behavior of 63Ni in such a surface layer was investigated by the radiotracer technique at temperatures <438K. It is shown that the effective diffusivity in the top 10μm surface layer is more than 2 orders of magnitude higher than that along conventional high-angle grain boundaries (HAGB) in Cu of similar purity. The diffusion rate increases gradually with increasing depth up to 30–50μm, and then decreases with further increasing depth. The enhanced diffusivities reveal higher-energy states of various interfaces in the SMAT surface layer. The excess free energy of HAGB in this layer is estimated to be ∼30% higher than that of conventional grain boundaries. An apparent retardation of the effective diffusion rate in the top 25μm surface layer is induced by tracer leakage into numerous twin-boundary-like interfaces, while the gradual decrease in interface excess free energy correlates with the observed decrease in diffusivity in the subsurface layer at depths exceeding 50μm.

Extending cluster dynamics to concentrated and disordered alloys: The linear-chain case

F. Berthier, I. Braems, E. Maras, J. Creuze, B. Legrand — 2009-12-28

Abstract: Cluster dynamics is often used in the multi-scale procedure to link atomistic and macroscopic approaches, in particular when modelling precipitation processes. To extend its use to concentrated alloys, it is necessary to take into account accurately the exclusion zones and their overlap. By investigating the one-dimensional case, one obtains an exact formula for these zones, and its generalization to higher dimensions is proposed. By integrating cluster fragmentation/coagulation processes into cluster dynamics equations, a perfect agreement between atomistic simulations (Kinetic Monte Carlo) and cluster dynamics is reached on the whole range of concentrations for kinetics which govern microstructure in one dimension.

Three-dimensional analysis of microstructures in titanium

Hemant Sharma, Stefan M.C. van Bohemen, Roumen H. Petrov, Jilt Sietsma — 2009-12-31

Abstract: Samples of Ti–4.3Fe–6.7Mo–1.5Al were isothermally annealed in the temperature range of 730–780°C for various times to study the β–α transformation. Serial sectioning in conjunction with both optical and EBSD analyses was applied to determine the three-dimensional (3-D) morphologies of primary α phase. The 3-D analysis proved to be essential for characterization of the complex morphologies of α grains and consequently for the identification of growth behavior. It showed that nucleation of α grains takes place at β–β grain boundaries and significant branching takes place after initial growth of α grains along β–β grain boundaries. Some branches grow inside the β grain interior. The branching behavior is shown to interact with β–β grain boundaries, leading to a zig-zag morphology. The presented 3-D analysis of α grains and their influence on β–β grain boundaries clearly show that 2-D observations of the microstructural morphologies are not sufficient to adequately represent the transformation characteristics.

In situ observations of solidification processes in γ-TiAl alloys by synchrotron radiation

2009-12-31

Abstract: In situ observations of phase transformations involving melts are performed using energy-dispersive diffraction of synchrotron X-rays on electromagnetically levitated γ-TiAl alloys containing Nb. The determined primary solidification modes, confirmed by microstructure analysis, delivered new reliable data about the boundary of the α(Ti) solidification domain, which differs in the various Ti–Al–Nb phase diagram descriptions. These data have been used for a reassessment of the thermodynamic database of the ternary Ti–Al–Nb system. The new description realistically reflects the experimental findings. Liquidus and solidus temperatures determined by the pyrometric method agree fairly well with the calculated values. Direct experimental information on the nature of the reactions along the univariant lines is provided.

An assessment of the contributing factors to the superior properties of a nanostructured steel using in situ high-energy X-ray diffraction

S. Cheng, Y.D. Wang, H. Choo, X.-L. Wang, J.D. Almer, P.K. Liaw, Y.K. Lee — 2010-01-04

Abstract: In contrast to most nanostructured materials, outstanding mechanical property has been demonstrated in a nanostructured metastable austenitic steel, owing to the new characteristics of deformation-induced martensitic transformation. In this paper, by employing an in situ high-energy X-ray diffraction technique, we explore these characteristics by examining factors from the load partitioning, Lüders banding, to texture development. It was found that the martensitic transformation was mainly driven through Lüders band propagation. Marked load transfer takes place from austenite to martensite as Lüders band propagates, and continues into the homogeneous deformation regime. The texture development is mostly contributed by martensitic transformation, but dislocation-based plasticity also plays a role. The effective load partitioning along with the deformability of martensite promotes sample ductility.

Effect of a high magnetic field on the Al–Al3Ni fiber eutectic during directional solidification

Xi Li, Yves Fautrelle, Zhongming Ren, Yudong Zhang, Claude Esling — 2009-12-31

Abstract: The effect of a high magnetic field on the morphology and preferred orientation relationship of the Al–Al3Ni fiber eutectic has been investigated during directional solidification. Results indicate that an application of a high magnetic field has decreased the eutectic spacing and caused the appearance of the band-like structure. Moreover, it has been found that a high magnetic field has changed the preferred orientation relationship of the Al–Al3Ni eutectic. Indeed, the application of a 12T high magnetic field has caused the preferred orientation relationship of the Al{331}//Al3Ni{001}; Al [110]//Al3Ni[010] in the case of no magnetic field to transfer into the Al{001}//Al3Ni{102}; Al[102]// Al3Ni[010]. Further, in order to investigate the nucleation and growing mechanism of the Al–Al3Ni eutectic, the growth and morphology relationships between the Al–Al3Ni eutectic and the primary Al and Al3Ni phases have been investigated during volume and directional solidification, respectively. It has been found that the growth and orientation of the primary Al3Ni phase and the heating flow extraction direction have determined the growth of the Al–Al3Ni eutectic. This implies that the Al–Al3Ni eutectic growth may be capable of being controlled by adjusting the alignment of the Al3Ni eutectic phase and the heating flow extraction direction. The above experimental results have been discussed from the effect of a high magnetic field on the diffusion process and the epitaxial growth of the eutectic.

Oxygen grain-boundary transport in polycrystalline alumina using wedge-geometry bilayer samples: Effect of Y-doping

H. Cheng, H.S. Caram, W.E. Schiesser, J.M. Rickman, H.M. Chan, M.P. Harmer — 2009-12-30

Abstract: Novel wedge-geometry, dual-layer alumina samples, both undoped and 500ppm Y3+-doped, were studied in the temperature regime 1250–1400°C to determine the effect of Y3+ on oxygen grain-boundary transport in alumina. The samples consisted of a wedge-shaped, single-phase alumina top layer, diffusion bonded to an alumina/Ni substrate containing a fine, uniform dispersion of Ni marker particles (0.5 vol.%). The extent of the alumina spinel oxidation layer was measured as a function of the wedge thickness for a series of heat-treatment conditions. Models of the transport behavior were used to derive values for the rate constants (k) in both the alumina top layer and the alumina/Ni substrate. It was found that the presence of yttrium slows oxygen grain-boundary diffusion in alumina by a factor of ∼5 (at 1300°C), and increases the corresponding activation enthalpy for oxidation from 407±20 to 486±34kJmol−1. Microstructural observations suggested that yttrium also slows Ni outward diffusion. A comparison of the different k values revealed that, at 1300°C, the presence of Ni alone enhances transport by a factor of ∼2 relative to undoped alumina.

Out-of-plane stresses arising from grain interactions in textured thin films

2010-01-04

Abstract: Face-centered-cubic thin films often have mixed (111)/(100) fiber texture. These orientations can have very different in-plane stiffnesses, leading to the possibility of significant stress inhomogeneities. Previous X-ray studies appeared to confirm this, reporting much higher stresses in (111)- than (100)-oriented grains. In those studies, the stress in the film normal direction was assumed to be zero everywhere, but Poisson effects suggest that out-of-plane stresses may be significant. Here, an X-ray data analysis that allows for out-of-plane stresses is presented and applied to X-ray data taken from a Cu film. The in-plane stress is shown to be homogeneous, and significant out-of-plane stresses arise. This analysis is shown to be more accurate and more consistent with the microstructure than previous methods. Consideration of inhomogeneous triaxial stress states is seen to be critical to understanding mechanical behavior of films with mixed fiber texture. Models for yielding and texture development are discussed.

On the mechanism of twin formation in Fe–Mn–C TWIP steels

H. Idrissi, K. Renard, L. Ryelandt, D. Schryvers, P.J. Jacques — 2010-01-04

Abstract: Although it is well known that Fe–Mn–C TWIP steels exhibit high work-hardening rates, the elementary twinning mechanisms controlling the plastic deformation of these steels have still not been characterized. The aim of the present study is to analyse the extended defects related to the twinning occurrence using transmission electron microscopy. Based on these observations, the very early stage of twin nucleation can be attributed to the pole mechanism with deviation proposed by Cohen and Weertman or to the model of Miura, Takamura and Narita, while the twin growth is controlled by the pole mechanism proposed by Venables. High densities of sessile Frank dislocations are observed within the twins at the early stage of deformation, which can affect the growth and the stability of the twins, but also the strength of these twins and their interactions with the gliding dislocations present in the matrix. This experimental evidence is discussed and compared to recent results in order to relate the defects analysis to the macroscopic behaviour of this category of material.

Effects of temperature on structure and mobility of the 〈100〉 edge dislocation in body-centred cubic iron

D.A. Terentyev, Yu. N. Osetsky, D.J. Bacon — 2010-01-04

Abstract: Dislocation segments with Burgers vector b=〈100〉 are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b=1/2〈111〉. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight 〈100〉 edge dislocation is investigated here by atomic-scale computer simulation for α-iron using three different interatomic potentials. At low temperature the dislocation has a non-planar core consisting of two 1/2〈111〉 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the 〈100〉 dislocation. It is concluded that the response of the 〈100〉 edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2〈111〉 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.

Synthesis of metallic glass composites using phase separation phenomena

H.J. Chang, W. Yook, E.S. Park, J.S. Kyeong, D.H. Kim — 2010-01-13

Abstract: Phase separation phenomena in metallic glass systems have been explored by thermodynamic calculation and experimental method in the present study. Miscibility gap and spinodal curve for liquid phase in Gd–Ti–Al–Co/Cu systems have been calculated using the CALPHAD method. Based on the calculation result, various types of phase separation phenomena have been observed experimentally. Basically two types of structure are observed: interconnected-type structure by spinodal decomposition; and droplet-type structure by nucleation and growth mechanism. Multi-step phase separation occurs during cooling from the liquid, forming a hierarchical length scale composite structure. Depending on the glass-forming ability of the separated liquid phase, amorphous/amorphous or amorphous/crystalline composite structure can be obtained. The present study shows that there are several factors such as critical temperature, asymmetry and composition range of the spinodal curve which affect scale and morphology of the microstructure after phase separation.

Microstructure evolution of 6061 O Al alloy during ultrasonic consolidation: An insight from electron backscatter diffraction

E. Mariani, E. Ghassemieh — 2010-01-04

Abstract: 6061 O Al alloy foils were welded to form monolithic and SiC fibre-embedded samples using the ultrasonic consolidation (UC) process. Contact pressures of 135, 155 and 175MPa were investigated at 20kHz frequency, 50% of the oscillation amplitude, 34.5mms−1 sonotrode velocity and 20°C. Deformed microstructures were analysed using electron backscatter diffraction (EBSD). At all contact pressures deformation occurs by non-steady state dislocation glide. Dynamic recovery is active in the upper and lower foils. Friction at the welding interface, instantaneous internal temperatures (0.5–0.8 of the melting temperature, Tm), contact pressure and fast strain rates result in transient microstructures and grain size reduction by continuous dynamic recrystallization (CDRX) within the bonding zone. Bonding occurs by local grain boundary migration, which allows diffusion and atom interlocking across the contact between two clean surfaces. Textures weaken with increasing contact pressure due to increased strain hardening and different grain rotation rates. High contact pressures enhance dynamic recovery and CDRX. Deformation around the fibre is intense within 50μm and extends to 450μm from it.

Mechanical properties at the nanometer scale of GDC and YSZ used as electrolytes for solid oxide fuel cells

M. Morales, J.J. Roa, X.G. Capdevila, M. Segarra, S. Piñol — 2010-01-04

Abstract: The Young’s modulus (E), hardness (H) and fracture toughness (KIC) of various compositions of gadolinia doped-ceria (GDC, GdxCe1−xO2−x/2, 0.1⩽x⩽0.2) and yttria-stabilized zirconia (YSZ, Y0.08Zr0.92O1.96) electrolytes were investigated by nanoindentation. All samples were produced by the sol–gel method, formed by uniaxial pressure and sintered at 1400°C. In order to determine the mechanical properties, a Berkovich diamond tip was employed at applied loads of 5, 10, 30, 100 and 500mN. The results were interpreted by the Oliver–Pharr method and values of KIC were determined using the method of Palmqvist cracks. The residual imprints were observed by field emission scanning electron microscopy. The results obtained showed that the H, E and KIC of GDC decreased with increasing gadolinia concentration, due to the oxygen vacancies generated by the dopant addition. As a result, the mechanical properties of GDC were significantly lower than those of YSZ electrolyte.

LaMg11 with a giant unit cell synthesized by hydrogen metallurgy: Crystal structure and hydrogenation behavior

2010-01-04

Abstract: We have successfully applied a hydrogen metallurgy synthesis to yield LaMg12−x intermetallic at 500°C. During a vacuum thermal desorption from the LaH3–MgH2 nanocomposite prepared by reactive ball milling in H2, hydrogen desorption from the La dihydride occurred via a mechanism of cooperative phase transformation at temperatures being 400° lower than the desorption from pure LaH2. The crystal structure of the intermetallic was determined by synchrotron X-ray diffraction (SR XRD). The orthorhombic LaMg12−x has a giant unit cell, the volume of which exceeds 8000Å3. In situ SR XRD studies showed the fine details of the hydrogen desorption process with several parallel and consecutive transformation steps.

Comparison of residual microstructures associated with impact craters in Al–Sc and Al–Ti alloys

Yicong Ye, Peijie Li, L.S. Novikov, V.S. Avilkina, Liangju He — 2010-01-06

Abstract: Al–Sc and Al–Ti semi-infinite targets were impacted by high-speed projectiles with velocities of 0.8, 2 and 4kms−1, respectively. The results show that deep columned craters with hemispherical bottoms were formed in the Al–Ti target, while near-hemispheroidal or relatively shallower craters formed in the Al–Sc alloy. It is concluded that different microstructures of Al–Sc and Al–Ti alloys, including different grain sizes and secondary particles precipitated in the matrix, result in their having greatly different capabilities of resisting impact. Residual microstructures of different samples are further discussed. It is possible that, due to the very large amount of energy imported into the target by high-speed impact, secondary Al3Sc lost its coherency and consequently recrystallization occurred.

Dynamic grain growth and particle coarsening in Al–3.5Cu

O.V. Rofman, P.S. Bate — 2010-01-04

Abstract: Grain growth and particle coarsening in Al–3.5Cu at a temperature of 450°C has been studied. Plastic deformation of this Zener-pinned system at strain rates of 10−3 and 10−4s−1 led to an increase in both the grain growth and particle coarsening rates. The results of mechanical tests and metallography, including in situ studies, showed that the material was deforming primarily by intragranular slip. The dynamic grain growth was ascribed to the geometric effect of deformation on the Zener pinning, and the rate sensitivity of the growth to the dynamic particle coarsening. The principal effect of deformation on particle coarsening was concluded to be increased diffusion due to the dislocation content.

Crystallographic orientation and stress-amplitude dependence of damping in the martensite phase in textured Ti–Nb–Al shape memory alloy

T. Inamura, Y. Yamamoto, H. Hosoda, H.Y. Kim, S. Miyazaki — 2010-01-07

Abstract: The low-frequency damping (tanδ) of a strongly textured β-titanium shape memory alloy (SMA) was investigated by a dynamic mechanical analysis (DMA) in a tensile mode together with a tensile test and a crystallographic analysis of the transformation. In addition to the high background of tanδ in martensite, a broad tanδ peak with a relaxation-like character was found in the martensite phase similar to the Ti–Ni SMAs. There was a critical stress for the broad peak that corresponds to the stress to complete the martensite variant reorientation, whereas the high background of tanδ had no critical stress. This implies that the broad peak is generated through the long-range motion of twin boundaries. A guideline to estimate the orientation dependence of damping in textured SMAs is also proposed using the interaction energy between the external stress and the transformation.

Effect of equal-channel angular pressing routes on high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy

Yang Gon Kim, Young Gun Ko, Dong Hyuk Shin, Sunghak Lee — 2010-01-07

Abstract: The effect of equal-channel angular pressing (ECAP) route on the high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy was investigated. The 8-pass ECAPed specimens deformed via three different routes consisted of ultra-fine grains 0.5μm in size, and contained a considerable amount of second-phase particles, which were fragmented and distributed in the matrix. In the torsion tests, the maximum shear stress significantly increased with increasing number of ECAP passes, while the maximum shear stress and fracture shear strain were lowest in the specimen deformed via route A among the three 8-pass ECAPed specimens. Observation of the deformed area beneath the fractured surface revealed the adiabatic shear bands of 100μm in width in the specimen deformed via route A, which minimized the maximum shear stress and fracture shear strain, whereas they were hardly formed in the specimens deformed via route B or C. The formation of adiabatic shear bands was explained in terms of critical shear strain, deformation energy required for void initiation, and microstructural homogeneity related to ECAP routes.

The effect of the orientation of the basal plane on the mechanical loss in magnesium matrix composites studied by mechanical spectroscopy

A.S.M.F. Chowdhury, D. Mari, R. Schaller — 2010-01-11

Abstract: In metal matrix composites, thermal stresses are relaxed by either interface debonding, crack propagation or dislocation motion. The present paper shows that in the case of a magnesium matrix, interface thermal stresses are relaxed by dislocation motion in the basal plane (001) of the hexagonal structure. Different specimens were processed by a gas pressure infiltration method and the metallic matrix was oriented with respect to the interface by the Bridgman technique. Mechanical spectroscopy experiments show that the damping as well as the evolution of the relative shear modulus depend strongly on the orientation of the basal plane. The results are in good agreement with the theoretical model. The calculations show that the orientation of the glide plane influences the coupling between the thermal stress and the applied mechanical stress.

Effects of drawing on the tensile fracture strength and its reliability of small-sized metallic glasses

Y. Wu, H.H. Wu, X.D. Hui, G.L. Chen, Z.P. Lu — 2010-01-07

Abstract: The effects of drawing on the structure and mechanical properties of a Co-based metallic glass under tension were thoroughly investigated. Surface changes induced by drawing, including removal of surface flaws, surface chemical homogenization and generation of compressive residual stress tend to increase the fracture strength, whilst open volumes created during drawing, particularly nano-voids, are likely to soften the wires. Initially, the surface changes are decisive factors, but as drawing proceeds, the open volumes gradually become dominant, yielding a maximum fracture strength in the wires with an area reduction ratio of 22%. Moreover, it was found that the fracture strength reliability was enhanced by the drawing, which is due not only to the surface perfection but also to the increase of plastic deformation capability, manifested by the decrease in the activation energy of individual shear transformation zones. Our results imply that the drawing technique could be a promising approach to continuously producing small-sized glassy wires with improved overall properties.

Effect of switching stresses on domain evolution during quasi-static crack growth in a ferroelastic single crystal

Peter Neumeister, Hannes Kessler, Herbert Balke — 2010-01-14

Abstract: The present paper demonstrates the effect of switching stresses on domain evolution and fracture toughening during quasi-static crack growth in elastically isotropic ferroelastic single crystals with transversally isotropic ferroelastic strains. With a simple switching algorithm and crack propagation procedure, domain evolution is simulated in an exemplary material with semi-infinite crack under mode I loading, starting from a mono-domain configuration. Domain reorientation is found to be strongly affected by switching stresses, which therefore have to be considered in the context of domain evolution modelling and fracture toughening. Before the onset of crack growth a needle-like domain is formed at the tip of the stationary crack, but this does not effect the crack tip stress intensity factor. Elongation of this domain during the onset of crack growth causes a large increase of the fracture toughness. Domain separation in a later stage results in toughness reduction. The subsequent domain evolution indicates a periodic formation of needle-like domains as observed in soft ferroelastic materials.

Phase transformation in an yttrium–hydrogen system studied by TEM

K. Wang, J.R. Hattrick-Simpers, L.A. Bendersky — 2010-01-04

Abstract: Phase transformations in Pd-capped epitaxial yttrium films grown on (0001) sapphire substrates covered with a Ti buffer layer and hydrogenated for different times were studied using transmission electron microscopy (TEM). For short hydrogen charging times, the phase transformation from α-Y to β-YH2 is associated with the nucleation and growth of two orientational variants, which after coalescence form twin-related lamellae of the β-YH2 phase with twin interfaces parallel to the substrate plane. Shockley partial dislocations are present at the twin boundaries; their glides during phase transformation are responsible for the formation of the twin lamellae. Superlattice reflections were observed for β-YH2, and the existence of a new long-range ordered superstoichiometric YH2+x phase was suggested. A structural model of the ordering based on the occupation of octahedral interstitial sites by H in a doubled cell of Y-face-centered cubic was offered. For samples hydrogenated for longer times, β-YH2-to-γ-YH3 phase transformation was accompanied by cracking along the twin boundaries, which eventually developed into a network of pores and caused significant swelling of the films. No γ-YH3 phase was observed directly in TEM because of its unstable nature under the high vacuum of a microscope column. The fully transformed YH3 films have over a 60% increase in its thickness, which is mostly accounted for by the high volume fraction of pores.

A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

M. Gendre, A. Maître, G. Trolliard — 2010-01-18

Abstract: Zirconium oxycarbide powders with controlled composition ZrC0.94O0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (Ea) from the densification rate law. It was concluded that at low macroscopic applied stress (25MPa), an intergranular glide mechanism (n⩽2) governs the densification process, while a dislocation motion mechanism (n⩾3) operates at higher applied load (100MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687–774kJmol−1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification.

Strain mapping in a deformation-twinned nanocrystalline Pd grain

H. Rösner, N. Boucharat, K.A. Padmanabhan, J. Markmann, G. Wilde — 2010-01-13

Abstract: Strain in a deformation-twinned nanocrystalline Pd sample of about 24nm grain size was mapped by means of geometric phase analysis based on an individual high-resolution transmission electron microscopy image. The in-plane components of the strain tensor were calculated and charted. Strains with magnitudes of about 0.8% were found in the grain interior. Twins and matrix were significantly distorted relative to each other (by about 3° on average) and showed a strong rotation gradient from top to bottom, revealing that the whole grain is bent. An estimate of the strain energy stored in the Pd grain yielded a value of . Based on the strain distribution observed, a temporal deformation scenario has been developed. In our judgement, deformation twins had formed first and subsequently dislocations were activated, most likely by the misfit strain/stress concentrations generated by the twins themselves. The interaction of the dislocations with the twin boundaries left behind Shockley partials and this accounted for the strain concentrations finally observed along the twin boundaries. It is concluded that in the temporal evolution of deformation in a nanocrystalline material, twinning must be by far the fastest deformation mode, which accounts for the abundance of deformation twins, especially at high strain rates.

The origin of stresses in magnetron-sputtered thin films with zone T structures

R. Daniel, K.J. Martinschitz, J. Keckes, C. Mitterer — 2010-01-13

Abstract: In order to understand the origin of the residual stress state in thin films and its thickness dependence, the structure–stress relation of magnetron-sputtered Cr and CrN layers with thicknesses ranging from 100nm to 3μm was investigated in detail. Based on correlations between the layer-thickness-dependent grain size, texture and morphology and the magnitude of the intrinsic and thermal components of the residual stress, a model is proposed that explains the origin of internal stresses of thin polycrystalline films with zone T structures. The model was further extended for the CrN/Cr dual-layer system, where the CrN top layer is epitaxially aligned with the underlying highly (200)-oriented Cr interlayer. It is shown for the first time that both the intrinsic and thermal stress components are thickness-dependent, which is associated with the layer microstructure.

Atom probe study of sodium distribution in polycrystalline Cu(In,Ga)Se2 thin film

E. Cadel, N. Barreau, J. Kessler, P. Pareige — 2010-01-14

Abstract: This article reports the first investigations of CuIn1−xGaxSe2 (CIGSe) polycrystalline thin films by means of atom probe tomography. Attention is focused on the distribution of Na atoms within the films. Both Na-containing and Na-free CIGSe thin films have been investigated. When Na is available during the CIGSe coevaporation, it is observed to be mainly segregated at the grain boundaries of the films; however, it is also detected within the grains of CIGSe at very low concentration.

Scaling the response of microscale compression molding of elemental Cu in the presence of dynamic recrystallization

Fanghua Mei, Ke Chen, J. Jiang, W.J. Meng — 2010-01-13

Abstract: Replication of metallic high-aspect-ratio microscale structures by direct compression molding is a promising microfabrication technique. In this paper, the micromolding response of elemental Cu is measured as a function of molding temperature. Companion tensile testing of macroscale Cu specimens was carried out at the corresponding molding temperatures. Microstructures of tensile tested macroscale Cu specimens and molded Cu specimens were characterized by combining focused ion beam (FIB) sectioning and imaging with ion-induced secondary electrons (ISEs). At all temperatures tested, measured tensile stress–strain curves for Cu show clear evidence for the occurrence of dynamic recrystallization (DRX) within macroscale tensile specimens. Additional FIB sectioning and imaging with ISEs of molded Cu specimens provide microstructural evidence for the occurrence of DRX during molding. The present results show that, even in the presence of DRX, an appropriate one-parameter scaling continues to reduce the molding response to a universal function independent of the molding temperature.

Double twinning in Ni–Mn–Ga–Co

K. Rolfs, M. Chmielus, R.C. Wimpory, A. Mecklenburg, P. Müllner, R. Schneider — 2010-01-13

Abstract: Magnetic shape-memory alloys tend to deform via magnetic-field-induced and stress-induced twin-boundary motion. The rather low martensite transformation temperature of ternary Ni–Mn–Ga limits the operating temperature for potential applications. By alloying 5at.% cobalt, the martensite transformation temperature and the Curie temperature was increased from 70 and 110°C respectively up to 160°C. In the single crystalline samples two non-modulated structures with tetragonal and orthorhombic lattices were found. The non-modulated orthorhombic structure has similar lattice parameters to the pseudo-orthorhombic 14M Ni–Mn–Ga phase. The single crystal specimen with the non-modulated orthorhombic structure exhibited a cyclic permutation of all three crystallographic axes in response to uniaxial loading. The parallelepiped-shaped sample was compressed repeatedly in all three directions. While maximizing work done by the load during deformation required three different martensite variants to result from deformation in three different directions, only two different martensite variants were found. The analysis of the sample shape revealed two variants mutually related through cyclic permutation of the lattice parameters, which cannot result from a single twinning event. The cyclic permutation is discussed in the light of Crocker’s double twinning mechanism.

The deformation of Gum Metal through in situ compression of nanopillars

E.A. Withey, A.M. Minor, D.C. Chrzan, J.W. Morris, S. Kuramoto — 2010-01-13

Abstract: The name “Gum Metal” has been given to a set of β-Ti alloys that achieve exceptional elastic elongation and, with appropriate preparation, appear to deform by a dislocation-free mechanism triggered by elastic instability at the limit of strength. We have studied the compressive deformation of these materials with in situ nanocompression in a quantitative stage in a transmission electron microscope. The samples studied are cylindrical nanopillars 80–250nm in diameter. The deformation pattern is monitored in real time using bright-field microscopy, dark-field microscopy or electron diffraction. Interesting results include the following: (i) nanopillars approach, and in several examples appear to reach, ideal strength; (ii) in contrast to conventional crystalline materials, there is no substantial “size effect” in pillar strength; (iii) the deformation mode is fine-scale with respect to the sample dimension, even in pillars of 100nm size; (iv) shear bands (“giant faults”) do form in some tests, but only after yield and plastic deformation; and (v) a martensitic transformation to the base-centered orthorhombic α′′ phase is sometimes observed, but is an incidental feature of the deformation rather than a significant cause of it.

Phase transformations and induced volume changes in a nitrided ternary Fe–3%Cr–0.345%C alloy

S. Jegou, L. Barrallier, R. Kubler — 2010-01-13

Abstract: Phase transformations during nitriding of a ternary carbon iron-based alloy Fe–3%Cr–0.345%C were studied, aiming for a better understanding of residual stresses generation and evolution. The relationship between the precipitation of Cr7C3 carbides and CrN nitrides, the induced volume change and the mechanical properties were investigated at three distinct depths of the diffusion zone. The relaxation of residual stresses arose through phase transformations according to the diffusion of nitrogen but also of carbon.

A closer look at the local responses of twin and grain boundaries in Cu to stress at the nanoscale with possible transition from the P–H to the inverse P–H relation

L. Yue, H. Zhang, D.Y. Li — 2010-01-15

Abstract: Nanocrystalline copper is considered to be a candidate for electrical contacts for dynamic systems because of its intrinsic conductivity and enhanced fretting resistance. However, the enhanced electron scattering at high-density grain boundaries significantly deteriorates the overall conductivity of nanocrystalline copper. Recent studies suggest that nanosized twin boundaries in copper might be a solution to such a dilemma. To better understand the general mechanical behavior of nanotwin boundaries, we conducted molecular dynamics simulation studies to investigate responses of both nanotwin and nanograin boundaries in copper to stress at the nanoscale, particularly in the critical range of 5–25nm where the inverse Petch–Hall relation (P–H) may occur in nanocrystalline copper. The obtained results suggest that the twin boundary blocks dislocation movement more effectively and the degree of emitting dislocations under stress is considerably lower than that of grain boundary, leading to superior mechanical behavior. The inverse P–H relation is not applicable to the nanotwinned system. It is also demonstrated that the inverse P–H relation occurring in nanograined materials does not necessarily result from grain boundary sliding.

Understanding the mechanical reinforcement of uniformly dispersed multiwalled carbon nanotubes in alumino-borosilicate glass ceramic

Amartya Mukhopadhyay, Bryan T.T. Chu, Malcolm L.H. Green, Richard I. Todd — 2010-01-20

Abstract: The improvement of mechanical properties of carbon nanotube–reinforced polycrystalline ceramic or glass matrix composites was limited in earlier studies by the difficulties in producing a good dispersion of carbon nanotubes. Additionally, a proper understanding of the reinforcing mechanisms, if any, affecting the mechanical properties of ceramics containing carbon nanotubes is still lacking. We report here the effects of a good dispersion of as much as 10wt.% multiwalled carbon nanotubes (MWCNTs) on the mechanical properties of dense alumino-borosilicate glass ceramics (ABS) prepared by an ultrasonication-assisted sol–gel technique followed by hot pressing (950°C; 2h; Ar atmosphere). The fracture toughness and flexural strength of the nanocomposites increased with increasing MWCNT content up to 10wt.%. The ABS–10wt.% MWCNT nanocomposite possessed nearly double the strength of the unreinforced ABS, accompanied by ∼150% improvement in fracture toughness. However, a further increase in MWCNT content to 15wt.% resulted in a modest deterioration of the mechanical properties due to agglomeration of the MWCNTs. The carbon nanotubes have been observed to bridge crack openings of the order of ∼100nm and the experimental evidence, along with theoretical analysis, showed that crack bridging provided the major contribution towards the improvement in fracture toughness. Debonding between the MWCNTs and the matrix appeared to occur in the matrix, away from the actual interface. However, the absence of significant pull-out of broken sections of the MWCNTs during fracture, due to failure of the bridging CNTs being predominantly at the crack plane, indicates that further toughening may be available if this mechanism can be activated.

Substitutional diffusion in multicomponent solids with non-ideal sources and sinks for vacancies

F.D. Fischer, J. Svoboda — 2010-01-21

Abstract: Based on previous work by the authors the diffusion equations for a multicomponent solid are derived. Generation and annihilation of vacancies are described by an evolution law which is directly coupled with an eigenstrain rate. Manning’s correlation factor f is used in the kinetic factor for diffusion of vacancies. In addition to presentation of the diffusion equations, a rigorous treatment of the boundary conditions, assuming no or an ideal source and sink for vacancies at the surface, is presented. As an instructive example development of the site fractions of the components and the eigenstress state are demonstrated for a multicomponent chemically inhomogeneous layer on a substrate. Both the role of nonlinear terms and the correlation factor f in the diffusion equations are studied. Computational procedures to calculate to the proper boundary conditions are also outlined.

Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings

F. Rovere, D. Music, J.M. Schneider, P.H. Mayrhofer — 2010-01-20

Abstract: The effect of Y incorporation into cubic Cr–Al–N (B1) was studied using ab initio calculations, X-ray diffraction and energy-dispersive X-ray analysis of sputtered quaternary nitride films. The data obtained indicate that the Y incorporation shifts the critical Al content, where the hexagonal (B4) structure is stable, to lower values. The calculated critical Al contents of x≈0.75 for Cr1−xAlxN and x≈0.625 for Cr1−x−yAlxYyN with y=0.125 are consistent with experimentally obtained values of x=0.69 for Cr1−xAlxN and x=0.68 and 0.61 for Cr1−x−yAlxYyN with y=0.02 and 0.06, respectively. This may be understood based on the electronic structure. Both Cr and Al can randomly be substituted by Y. The substitution of Cr by Y increases the phase stability due to depletion of non-bonding (anti-bonding) states, while the substitution of Al by Y decreases the phase stability mainly due to lattice strain.

Multi-scale modeling of elastic response of three-dimensional voxel-based microstructure datasets using novel DFT-based knowledge systems

Giacomo Landi, Stephen R. Niezgoda, Surya R. Kalidindi — 2010-01-22

Abstract: In this paper, we present a novel approach for predicting the elastic fields in three-dimensional voxel-based microstructure datasets subjected to uniform periodic boundary conditions. This new formalism has its theoretical roots in the statistical continuum theories developed originally by Kroner. However, in the approach described by Kroner the terms in the series were established by selecting a reference medium and numerically evaluating a complex series of nested convolution integrals. This approach is largely hampered by the principal value problem and exhibits high sensitivity to the properties of the selected reference medium. In the present work, we have recast the same series expressions into much more computationally efficient representations using discrete Fourier transforms (DFTs). The main advantage of the new DFT-based framework is that it allows easy calibration of Kroner’s expansions to results from finite element methods, thereby overcoming all of the main obstacles associated with the principal value problem and the need to select a reference medium. Consequently, the DFT-based approach presented here produces much more accurate predictions. In this paper the new mathematical formalism is first presented in a generalized framework, and its viability is then demonstrated for a selected class of two phase composites.

The effect of platinum on Al diffusion kinetics in β-NiAl: Implications for thermal barrier coating lifetime

Kristen A. Marino, Emily A. Carter — 2010-01-22

Abstract: First-principles density functional theory calculations are used to study Al diffusion in β-NiAl. The activation energy and diffusion constant pre-exponential factors are calculated for five previously postulated Al diffusion mechanisms: next-nearest-neighbor Al jumps, the triple defect mechanism and three variants of the six-jump cycle mechanism beginning with an Al vacancy. We predict that the triple defect mechanism has the lowest activation energy and is the mechanism by which Al diffusion occurs in NiAl. In order to elucidate why Pt has a beneficial effect on thermal barrier coating lifetime, the effect of Pt on each of these mechanisms is also examined. In all cases, Pt decreases the diffusion activation energy, which should enhance Al diffusion in the coatings.

Polydimensional modelling of dendritic growth and microsegregation in multicomponent alloys

Sebastian C. Michelic, Jörg M. Thuswaldner, Christian Bernhard — 2010-01-25

Abstract: Various models for simulating dendritic solidification have been proposed in the past. However, those models based on cellular automata (adopting the virtual front-tracking (VFT) concept) are often only suitable for the consideration of one alloying element. As typical industrial alloys are constituted of numerous alloying elements, the application of these models to practical alloys is therefore rather limited. In order to overcome this drawback, a new, modified VFT model, which allows for the treatment of several alloying elements in the low Péclet number regime, is presented. By a new and effective approach, based on a functional extrapolation of the concentration gradient, we are able to study dendritic growth in multicomponent Fe–C–Si–Mn–P–S alloys. Comparisons with well-established analytical models confirm the correctness of the model; results for free and constrained dendritic growth effectively demonstrate the capabilities of this new model.

Transformation kinetics for surface and bulk nucleation

Elena Villa, Paulo R. Rios — 2010-02-01

Abstract: A rigorous mathematical approach based on the causal cone and stochastic geometry concepts is used to derive new exact expressions for transformation kinetics theory. General expressions for the mean volume density and the volume fraction are derived for both surface and bulk nucleation in a general Borel subset of . In practice, probably any specimen shape of engineering interest is going to be a Borel set. An expression is also derived for the important case of polyhedral shape, in which surface nucleation may take place on the faces, edges and vertices of the polyhedron as well as within the bulk. Moreover, explicit expressions are given for surface and bulk nucleation for three specific shapes of engineering relevance: two parallel planes, an infinitely long cylinder and a sphere. Superposition is explained in detail and it permits the treatment of situations in which surface and bulk nucleation take place simultaneously. The new exact expressions presented here result in a significant increase in the number of exactly solvable cases available to formal kinetics.

The thermal explosion synthesis of AlNi monitored by neutron thermodiffractometry

X. Turrillas, M.J. Mas-Guindal, T.C. Hansen, M.A. Rodríguez — 2010-01-29

Abstract: The synthesis of AlNi from thermally activated equimolar powder mixtures of aluminium and nickel was monitored in situ and acquired diffraction patterns every 2s or less. The analysis of diffraction patterns permitted establishment of its kinetics, which could be modeled according to an equation expressed as . From 1530 to an activation energy of 9±2kJmol−1 was estimated. Also, the crystallite size evolution on cooling was established to vary from nm at 1530°C to nm at 180°C. Finally, the reaction was found to occur through the melting of aluminium and the subsequent dissolving of nickel to form the polycrystalline single-phase product, AlNi.

Evolution of deformation mechanisms of Ti–22.4Nb–0.73Ta–2Zr–1.34O alloy during straining

Y. Yang, S.Q. Wu, G.P. Li, Y.L. Li, Y.F. Lu, K. Yang, P. Ge — 2010-01-28

Abstract: The plastic deformation behavior of Ti–22.4Nb–0.73Ta–2Zr–1.34O alloy was investigated by compression testing at room temperature. The multi-peak stress oscillations of the true stress–strain curve, characterized by a stress plateau, initial strain-hardening, followed by strain-softening and a second strain-hardening stages, is observed in a titanium alloy for the first time. The experimental results show that the above four-stage plastic deformation behavior is caused by a change in the dominant deformation mechanisms. At the stress plateau stage, the alloy deforms via multiple plastic deformation mechanisms. The initial strain hardening is caused mainly by tangling of dislocations. Subsequent strain softening is due to the formation of kink bands. The second strain hardening corresponds to the formation of shear bands. The above results suggest that the dominant deformation mechanisms of Ti–Nb–Ta–Zr–O alloys are related not only to the stability of the β phase, but also to the extent of plastic deformation.

X-ray diffraction study of the phase purity, order and texture of ductile B2 intermetallics

R.P. Mulay, J.A. Wollmershauser, M.A. Heisel, H. Bei, A.M. Russell, S.R. Agnew — 2010-02-01

Abstract: Representatives (AgY, CuY, AgEr, CuDy, MgY and MgCe) of the newly discovered family of ductile stoichiometric B2 intermetallic (metal–rare-earth element, MR) compounds were characterized by X-ray diffraction, to determine if their anomalous ductility is related to an exceptional level of phase purity, lack of chemical ordering or a strong crystallographic texture. Brittle NiAl served as an anti-type in this study. We found that all of the rare-earth compounds, except MgY, have a significant volume fraction (∼5–20vol.%) of second phases (M2R intermetallics and R2O3 oxides), which has not been reported in previous studies of these materials. The most ductile of observed MR compounds, AgY, is highly ordered. A moderate texture was observed in AgY, which may explain its higher ductility (using polycrystal modeling) as compared to other MR compounds. However, the intrinsic polycrystalline ductility of these compounds in the randomly textured state (like that observed in CuY) still has no specific, definitive explanation.

Corrigendum to “Phase-field theory of grain growth in the presence of mobile second-phase particles” [Acta Mater. 58 (2010) 272–281]

Srikanth Vedantam, Ashis Mallick — 2009-12-25

The authors regret the misspelling of N. Moelans in Refs. . The name was listed as N. Moleans and it should have been N. Moelans. The corrections are listed below: