Characterization of microplasticity in TiAl-based alloys

Three different characteristic microstructures of Ti–44Al–8Nb–1B (at.%) have been studied by in situ loading, with acoustic emission and image correlation and ex situ by electron backscatter diffraction and transmission electron microscopy. Microyielding and pre-yield cracking occur at different applied stress levels; nearly fully lamellar material yielding at the lowest and duplex material at the highest stress. The early microyielding in the lamellar microstructures is explained by strain heterogeneity seen in the early stages during loading; the microyielding is higher in lamellae at 45° than in lamellae parallel or perpendicular to the loading direction. Duplex microstructures show no detectable strain heterogeneity until macroscopic yield occurs. The lower bound of the internal stress present in undeformed near lamellar and fully lamellar samples was estimated by observations of dislocation loops at alpha-2/gamma interfaces. The observations are discussed in terms of the influence of internal stress on the tensile and fatigue properties of the different microstructures.