Nucleation and growth in martensitic transformations of ordered Fe 3 Pt alloys
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INTRODUCTION
IT is well known that many ferrous alloys transform from fcc austenite to bcc or bct martensite when temperature decreased. Among the ferrous alloys only the well-ordered Fe3PtilJ alloy and the disordered Fe-Pd alloys within a certain narrow composition range t2~ show an fcc-fct transformation besides the ordinary fcc-bct transformation. The fcc-fct transformation, which occurs not only in Fe3Pt or Fe-Pd alloys but also in several other alloys such as V3Si, Nb3Sn, and In-Tl,13] is described as a weak first order transformation and its behavior can be understood by extrapolating the scheme for second order transition; that is, the transformation is associated with such a mechanical instability that one of the elastic constants, C ' ( = (Cll -C12)/2), corresponding to the homogeneous (1i-0)/{110} shear strain almost vanishes. As far as the fcc-bct (bcc) martensitic transformations of ferrous alloys are concerned, they are typical first-order phase transformations with appreciable thermal hystereses, and the soft mode as a precursor of the transformations has not been observed. Ling and Owen r41measured the elastic constants of ordered and disordered Fe3Pt alloys by an ultrasonic pulse superposition technique. They reported that C' and C44 in a disordered Fe3Pt alloy did not change between 473 K and Ms (273 K), and the T~ mode in the (110) plane was also unchanged. This indicates that the fcc-bcc transformation does not need lattice softening. In an ordered Fe3Pt alloy (S = 0.6) C ' was unusually soft, and on cooling all the elastic constants softened, but at the Ms temperature (93 K) the resistance to the Bogers-Burgers shears remained much larger than that to the Burgers shears in the beta phase alloys. Finally, they concluded that the reduction in the resistance to the transformation shears by the development of elastically soft modes is not an essential factor in the nucleation of bct martensite in an Fe3Pt alloy. However, the (110)/{110} shear, which leads the fcc-fct transition, is closely related to the Bain strain which is very effective in the fcc-bct transformation. Hence, the lattice softening will have some influence on the nucleation of bct martensite. s. MUTO, Graduate Student, R OSHIMA, Associate Professor, and E E. FUJITA, Professor, are with the Department of Material Physics, Facultyof EngineeringScience, Osaka University,Toyonaka,Osaka 560, Japan. Manuscript submitted February20, 1988. METALLURGICALTRANSACTIONSA
In the preceding paper, tSj the present authors have systematically investigated Fe-Pt alloys with various platinum concentrations and degrees of long range order by means of transmission electron microscopy. They found that the transformations did not always occur successively like fccfct-bct, and concluded that the fcc-fct transformation and the fcc-bct transformation are independent from each other. In this paper we focus on the behavior occurring at the very initial stage of the thermoelastic transformation to bct martensite in Fe3Pt alloys. The correlation between the
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