Formation, modulation and adaptive twinning of martensite in the Au7Cu5Al4 shape memory system

The Au7Cu5Al4 β electron phase transforms displacively from an L21 parent to a nominally body-centred tetragonal martensite with c/a<1. The compound is of interest because it has the potential to serve as an 18 carat shape memory alloy in jewellery. Analysis of its X-ray diffraction spectra indicates that the martensite is modulated by a [110]/[[110][1̄10]]transverse shear wave, showing that it belongs, strictly speaking, to the generic B19 structure type. The martensite is also twinned, and the probable twinning structure is explored. A 15R(9,6̄) stacking sequence is deduced, which for reasons of the L21 ordering inherited from the parent phase, must be doubled to produce a notional 30R(9,6̄,9,6̄) martensite that properly repeats. However, although the measured X-ray diffraction spectra can be substantially explained by the structures derived, the martensite probably also has additional, higher-order lattice modulations.