Anomalous Transformation-Induced Deformation in Gum Metal

Tensile tests of single crystals of Gum Metal (Ti-36Nb-2Ta-3Zr-0.3O (wt %)) showed, anomalously, that (1) extensive, stress-induced (bcc)”(orthorhombic) transformation occurred in a crystal pulled in the <110> direction, but no transformation was observed in crystals pulled in the <100> or <111> directions and (2) little or no transformation occurred in tensile tests of severely worked rods, which are polycrystals with very strong <110> texture. Analysis of the energetics of the ” transformation offers straightforward explanations for these results. (1) An ” precipitate has very low elastic energy if it forms as a thin plate with a habit near {11√2}. A <110> tensile load significantly decreases the energy of this plate, promoting the transformation; loading along <100> or <111> is much less effective. (2) While cold-swaged rods of Gum Metal have a strong <110> axial texture, their perpendicular planes are severely distorted, increasing the elastic energy of ” and inhibiting the transformation.