Growth dynamics study of the martensitic transformation in Fe-30 pct Ni alloys: Part I. Quantitative measurements of gro
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I.
INTRODUCTION
C O N S I D E R A B L E effort, both theoretical and experimental, has been expended in the past 40 years on understanding the nucleation, growth, and final morphology of martensitic transformations. Several competing theories f~-41 have been proposed to understand the heterogeneous nature of martensitic nucleation originating from lattice defects. The phenomenological crystallographic theories [5,6,7] have also been developed to predict some aspects of the final morphology of various martensitic transformations. However, little work has been done on either modeling ~r measuring the growth stage which is among the least understood aspects from a fundamental viewpoint today. Since the growth process is likely to have a substantial effect on the ultimate morphology of the martensite structure, which, in turn, can be expected to have a large impact on its mechanical properties, it is of great importance to bridge the gap in our understanding of the growth dynamics of martensitic transformations. The key to the growth dynamics of martensitic transformations is the growth velocity of the moving interface and how it is affected by various driving forces and resisting forces in connection with temperature or strain condition in the vicinity of the interface. Nishiyama [81 has classified the rate of martensitic growth into three different velocity regimes. The fastest ("umklapp") is of the order of mechanical twinning velocities and is often ZHEN-ZHONG YU, formerly Graduate Research Assistant with the Department of Metallurgy, University of Connecticut, is Research Scientist with Advanced Fuel Research, Inc., P.O. Box 18343, East Hartford, CT 06118. PHILIP C. CLAPP, Professor of Metallurgy, is with the Department of Metallurgy and Institute of Materials Science, University of Connecticut, Storrs, CT 06268. Manuscript submitted August 30, 1988. METALLURGICAL TRANSACTIONS A
associated with athermal martensitic transformations (v -~ 103 m / s ) . The intermediate Cschiebung") is of the order of dislocation velocities in slip deformation and is proportional to the degree of undercooling (v ~ 10 -~ to 10-1 m / s ) . The slowest is associated with thermoelastic growth and is proportional to the cooling rate (v ~ 5 x 10 -4 m / s at a cooling rate of 20 ~ It is not surprising that more substantial advances have been made in studying the growth dynamics of the slow or intermediate regimes, as compared to the fast velocity one. Reasonably accurate measurements have been made in the former regimes using a hot stage equipped with a movie camera, tg'l~ Recently, Grujicic, Olson, and Owen 1121made a systematic study, which provides a great deal of insight and information into the growth dynamics of slow-moving martensite, on the interface mobility of slow velocity, thermally activated martensitic transformations. However, the growth dynamics of umklapp martensitic transformations still remain essentially unknown, mainly because Of the difficulties involved in the direct and accurate measurement of extremely fast inter
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