Martensitic transformations in titanium-tantalum alloys

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Abstract
The transformations produced by quenching from the equilibrium β-phase region have been studied in a range of titanium alloys with 5·8-53 w/o tantalum. The martensitic phase is h.c.p. (α′) with up to 22 w/o Ta, and orthorhombic (α″) with 23-53 w/o Ta. In alloys with up to 32 w/o Ta, the product is finely twinned on { 101}α′, or { 11}α″, but the orthorhombie structures at higher tantalum contents are untwinned. In addition to the transformation twins, which are class A in the Mackenzie and Bowles (1957) notation, some adjacent product regions in Ti-32 w/o Ta were found to correspond to type II twins with η1 = α″ and K 2 = α″; this mode is the conjugate of class A. The presence or absence of internal twinning correlates with the magnitude of the smallest principal strain of the lattice deformation. At a sufficiently high tantalum content, this principal strain is zero so that there is then an irrational plane of lattice fit and the lattice invariant deformation of the crystallographic theories vanishes.