It was noted that the Au7Cu5Al4 β electron phase transformed displacively from an L21 parent to a nominally body-centered tetragonal martensite phase with c/a < 1. Analysis of its X-ray diffraction spectra indicated that the martensite was modulated by a [110]/([110]/[¯110]) transverse shear wave which showed that it really belonged to the generic B19 structure type. The martensite was also twinned, and the probable twinning structure was investigated. A 15R(9,¯6) stacking sequence was deduced which, because of an L21 ordering inherited from the parent phase, had to be doubled in order to produce a notional 30R(9,¯6,9,¯6) martensite that repeated properly. Although the measured X-ray diffraction spectra could be largely explained by the derived structures, the martensite also probably had additional higher-order lattice modulations.

Formation, Modulation and Adaptive Twinning of Martensite in the Au7Cu5Al4 Shape Memory System. M.B.Cortie, F.C.Levey: Intermetallics, 2002, 10[1], 23-31