Anomalous transformation-induced deformation in 〈1 1 0〉 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 〈1 1 0〉 direction, (1) no transformation was observed in crystals pulled in the 〈1 0 0〉 or 〈1 1 1〉 directions and (2) little or no transformation occurred in severely worked rods, which are polycrystals with very strong 〈1 1 0〉 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 {1 1 1.5}; a 〈1 1 0〉 tensile load significantly decreases the energy of this plate; loading along 〈1 0 0〉 or 〈1 1 1〉 is less effective; (2) while worked rods have a strong 〈1 1 0〉 axial texture, the perpendicular planes are severely distorted, increasing the elastic energy of α″ and inhibiting the transformation.