X-ray study of the premartensitic phenomena in AuCd
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I.
INTRODUCTION
THEREhas arisen renewed interest in thermoelastic martensite transformations observed in so-called Hume-Rothery alloys in connection with shape memory effects exhibited commonly in these materials. These alloys undergo thermoelastic martensite transformations from a bcc or a CsC1 type phase to martensitic phases which are basically given by particular stacking of hexagonal basal planes such as 2H type, 9R type, etc. Among the various interesting phenomena concerning thermoelastic martensite transformations, the appearance of premartensitic 'phases' just above T, (martensite starting temperature) is particularly important, since the understanding of the premartensite structure seems to give insight to the mechanism of martensite phase transformation itself. Extensive works have been performed to investigate precursor phenomena of martensite transformations within the high symmetry/3-phase, including tweed patterns observed by electron microscopy, ~'2 anomalous incommensurability detected by X-ray diffraction, 3 thermal hysteresis in resistivity,4'5 soft phonon modes measured by neutron inelastic scattering, 6'7's etc. The existence of anomalous phenomena above T, is sometimes interpreted as evidence for the existence of a new phase and in some materials intermediate phases were really discovered. However, the premartensitic phenomena seem to be a more general characteristic of the /3-phase itself. In this context, we use the expression 'premartensitic state' and 'premartensitic phenomena' instead of the conventional expression 'premartensitic phase'. Recently, Yamada et al. 9.to proposed a new model of the premartensitic state based on the analysis of anomalous 'incommensurability' of the diffraction pattern observed in a NiTi(Fe) alloy. 3 They described the premartensitic state and the process of phase transformation as follows: The premartensite state is characterized by a dense random distribution of embryos of martensite which are dressed by MLR's (modulated lattice relaxations) within the/3-matrix. At sufficiently high temperatures, embryos which are created with a high density by thermal excitation have a lifetime 'remb shorter than the time to introduce MLR around them (ZMLR). Therefore they stay 'bare' without showing Y. NODA, Research Associate, and T. NAKAGAWA, Student, are with Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan. M. TAKIMOTO, formerly with Osaka University, is with the Nomura Computer System Company, Limited, Tokyo, Japan. Y. YAMADA is Professor, Institute for Solid State Physics, The University of Tokyo, Minatoku, Tokyo, Japan. Manuscript submitted April 14, 1987. METALLURGICAL TRANSACTIONS A
any macroscopic effect. As the temperature is lowered and hence 'Temb becomes longer than ZMLR,they get dressed by MLR, which gives rise to interaction between neighboring embryos. Due to these interactions, cooperative ordering of embryos takes place at Ts, which is nothing but the formation of the macroscopic martensite phase. One of the crucial tests to
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