Tweed structures associated with Fcc-Fct transformations in Fe-Pd alloys
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I.
INTRODUCTION
IT has
been reported that lattice instability takes place in many martensite alloys before the transformation. A variety of precursor effects associated with the lattice instabilities have been observed as premartensitic transformation phenomena such as a softening of elastic constants, ~'2 electric resistivity anomalies, 3'4 diffuse and extra diffraction effects in diffraction experiments, 5'6 and characteristic contrast effects in transmission electron microscopy. 7'8 The contrast anomaly in transmission eledtron microscopy has been referred to as mottled or cross-hatch structure. The tweed microstructure is also used to represent the cross-hatched contrast effect. The tweed microstructure has been reported not only in martensitic transformation but also in other phase transitions of order-disorder and precipitation. A characteristic diffraction contrast induced by elastic shear strains in the initial stages of precipitation and ordering was discussed by Tanner. 9 Delaey et al. summarized the modulated microstructures of electron microscope images in beta-phase alloys which exhibited thermoelastic martensite transformations. "~ However, it is not easy to distinguish between the contrast effect arising from the martensitic transformation and that from other phase changes inducing fine transformation products. Actually, various metastable phases have been reported in beta-phase type alloys. Robertson and Wayman examined the tweed contrast associated with the phase transition of the Ni-A1 system. "'~2'j3 The tweed contrast was observed not only in a martensite phase with Llo structure, but also in other phases of the Ni-A1 alloy system, where the precipitation o f the gamma and NisA13 phase occurred. J2 Although they explained the tweed con-
R. OSH1MA, Associate Professor, and F.E. FUJITA, Professor, are with the Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan. M. SUGIYAMA, formerly a Graduate Student at Osaka University, is with the R & D Laboratories-l. Nippon Steel Corporation, Kawasaki, Kanagawa 211, Japan. This paper is based on a presentation made in the symposium "Pretransformation Behavior Related to Displacive Transformations in Alloys" presented at the 1986 annual AIME meeting in New Orleans, March 2-6, 1986. under thc auspices of the ASM-MSD Structures Committee.
METALLURGICALTRANSACTIONS A
trast by (110)/[1]-0] static displacement waves of a long wavelength, the origin of the perturbing field and the correlation between the tweed contrast and martensitic transformation have not been explained well yet. The fcc-fct thermoelastic martensite transformation in Fe-Pd alloys occurs in the fcc austenite phase with no ordering reaction and it is not accompanied with any precipitation. 14 The martensite phase diagram of the Fe-Pd system was established as shown in Figure 1 by the present authors.J5 Recently, Sato et al. reported the decrease of the energy of the TA phonon branch corresponding to the elastic constant C ' ( = 1/2(C~
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