Body-centred tetragonal martensite formed from Au7Cu5Al4 beta phase

The Al-Au-Cu system contains a ternary beta electron compound with a nomina l stoichiometry of Au7Cu5Al4. At elevated temperatures this compound has a predominantly B2 structure, with some additional next-nearest neighbour ord ering of the L2, type. The high-temperature phase transforms displacively a nd reversibly to martensite, with nominal M-s and A(s) temperatures of 23 a nd 79 degreesC, 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 martens ite is fundamentally different from the classic 9R and 18R-type martensite of copper-base <beta> phase alloys, and also from the trigonal martensites formed in AuCd and TiNi, and appears rather to be of the same type as the t etragonal martensites of Ni2MnGa and La2AgIn. (C) 2001 Elsevier Science B.V . All rights reserved.