STRUCTURE DETERMINATION OF THE ZETA-2' MARTENSITE AND THE MECHANISM OF BETA-2-]ZETA-2' TRANSFORMATION IN A AU-49.5 AT PERCENT-CD ALLOY

The crystal structure of the zeta-2' Au-49.5 at%Cd martensite, which has been controversial for 50 years, has been determined by using single crystal data collected by four-circle diffractometer with MoK-alpha radiation. The space group is P3, trigonal, which has no center of symmetry. This result is in disagreement with that (P31mBAR) reported by Vatanayon and Hehemann. The lattice constants are a = 0.8095(3) and c = 0.57940(6) nm and 18 atoms are present in the unit cell. The structure was refined by the full-matrix least-squares method to a final R factor of 7.8% (wR = 4.1 %). The transformation mechanism is also discussed and it is shown that the zeta-2' martensite is created by three <011><011BAR> transverse displacement waves introduced simultaneously. Simulations with displacement waves were also made and a satisfactory results was obtained.