Anomalous transformation-induced deformation in 〈110〉 textured Gum Metal

Tensile tests on single crystals of Gum Metal (Ti–36Nb–2Ta–3Zr–0.3O (wt.%)) showed, anomalously, that while a stress-induced β(bcc)→α″(orthorhombic) transformation occurred in a crystal pulled in the 〈110〉 direction, (1) no transformation was observed in crystals pulled in the 〈100〉 or 〈111〉 directions and (2) little or no transformation occurred in severely worked rods, which are polycrystals with very strong 〈110〉 texture. Analysis of the energetics of the β→α″ transformation offers straightforward explanations: (1) an α″ precipitate has zero elastic energy if it forms as a thin plate with the habit {111.5}; a 〈110〉 tensile load significantly decreases the energy of this plate; loading along 〈100〉 or 〈111〉 is less effective; (2) while worked rods have a strong 〈110〉 axial texture, the perpendicular planes are severely distorted, increasing the elastic energy of α″ and inhibiting the transformation.