Landau theory of structural transformations in titanium–nickel and gold–cadmium alloys

We present a rigorous symmetry-based Landau theory of the two martensitic transformations from the cubic B2 (β2) austenite parent phase to the rhombohedral R (ζ′2) product phase of P3 symmetry and to the orthorhombic B19 (γ′2) product phase of Pmma symmetry. Both are improper ferroelastic transformations; their primary order parameters are expressed in terms of the shuffle displacements that correspond to the two ζζ0 TA2 modes for ζ=1/3 and ζ=1/2, respectively. Numerical agreement with experimental data is exact for the B2–B19 transformation and good/fair for the B2–R transformation in AuCd/TiNi, respectively.