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I.

INTRODUCTION

THISpaper presents results from a program of research on the role of parent phase microstructural features on martensitic transformation (MT). Given that MT arises from inherent lattice instability of the parent crystal structure as temperature is decreased, in real crystalline materials martensite crystallites are nucleated heterogeneously, and important qualities such as transformation kinetics are known to be greatly affected by microstructural features such as grain boundaries and dislocations. Yet the exact nature of martensite nucleation is not known, the precise physical effect of grain boundaries on martensite nucleation and growth has not been described, the role of lattice defects such as vacancies is appreciated only in an inexact way, and the effect of atomic ordering is not understood on a mechanistic level. If we consider what is currently known about these various aspects of parent phase stability ~ we can cite the following key points: (i) martensite nucleation: although the subject is still very much in question, the most accepted ideas indicate that martensite "nuclei" originate with certain dislocation configurations in the parent phase; (ii) grain size: it is well established that Ms decreases with decreasing parent grain size, indicating that fine grains, with more grain boundary barriers per unit volume, and/or with fewer potential nuclei per unit volume, tend to inhibit MT; (iii) dislocations: studies on the effect of cold work show two competing influences: additional nucleation sites are provided, tending to increase M , but plate growth may be inhibited, with the opposite effect on the transformation temperature range; (iv) vacancies: M~ tends to increase with increasing vacancy concentration, indicating that they tend to decrease the parent phase stability, promoting MT; (v) ordering: the effect of parent phase ordering varies from one alloy system to another, but for CuZnAI alloys it is well known that M~ tends to decrease if the parent phase is disordered, i.e., the parent phase is less stable if it is JEFF PERKINS is Associate Professor, Materials Science Group, Naval Postgraduate School, Monterey, CA 93440. Manuscript submitted August 11, 1981. METALLURGICAL TRANSACTIONS A

ordered. 2-8 Given that these are (except for (i)) the experimental observations, we are left with rather incomplete explanations of the mechanisms by which these parent phase microstructural features influence martensitic transformation. Therefore we have undertaken a research program to try to provide some of these explanations. In this paper we describe the initial results from studies on rapidly solidified CuZnAI alloys. The CuZnAI alloy system is, along with the CuA1Ni and TiNi system, one of the most widely studied shape memory systems. It therefore exhibits a thermoelastic type martensitic transformation, characterized by relatively low volumetric mismatch between the parent and martensite phases. Rapid solidification was employed in this study to create unique parent phase microstructures