MARTENSITIC TRANSFORMATION IN Au-49.5at%Cd ALLOY Takuya Ohba, Department of Materials Science and Engineering, Teikyo University, Toyosatodai, Utsunomiya 320, Tochigi, Japan

Martensitic transformation is the typical first order and diffusionless (displacive) transformation. In general, structure phase transformation may be divided into two, order-disorder and displacive type transformation. For the displacive type of the second order transformation, displacements are small comparing with the interatomic distances. In case of martensitic transformation, on the other hand, displacements are larger than that of the second order displacive type transformation. The mechanism of the martensitic transformation is the central issue in recent research. AuCd is the typical alloy which shows martensitic transformation. Near equiatomic composition the high temperature phase is well known to be CsCl type. The martensitic phase is called [[zeta]]2' phase. Although the crystal structure of the [[zeta]]2' phase was not solved for long time, recently it was successfully determined using a stress induced single crystal martensite. From the results of the structure determination of [[zeta]]2' martensite, a transformation mechanism was proposed. Phonon softening, which was expected from the proposed mechanism, was also observed using an isotope of Cd. The static approach was also made to reveal the characteristics of the martensitic transformation. Structure factors in the parent phase were measured at various temperature. The measurements reveal that they decrease with approaching transformation temperature. These characteristics of the martensitic transformation in AuCd alloy will be discussed.