Body-centred tetragonal martensite formed from Au7Cu5Al4 β phase

The Al–Au–Cu system contains a ternary β electron compound with a nominal stoichiometry of Au7Cu5Al4. At elevated temperatures this compound has a predominantly B2 structure, with some additional next-nearest neighbour ordering of the L21 type. The high-temperature phase transforms displacively and reversibly to martensite, with nominal Ms and As temperatures of 23 and 79°C, respectively. It is shown here that the martensite phase is body-centred tetragonal with c/a<1. The orthogonal lattice is, however, evidently affected by a small degree of additional modulation. This martensite is fundamentally different from the classic 9R and 18R-type martensite of copper-base β phase alloys, and also from the trigonal martensites formed in AuCd and TiNi, and appears rather to be of the same type as the tetragonal martensites of Ni2MnGa and La2AgIn.