Computer simulation of reversible martensitic transformations
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INTRODUCTION
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IN prior work, v,21 we used linear elastic theory to construct a computer simulation model for martensitic transformations in simple solids. The model helps to show how the elastic energy that develops during the transformation controls the microstructure and determines the thermal resistance to the transformation, which is most crudely measured by the difference (AM) between the martensite start and finish temperatures (M~ and M~). For simplicity, the transformation was assumed irreversible; once an element of martensite formed, it could neither revert to the parent phase nor change its crystallographic variant. However, a martensitic transformation can always be reversed by heating, and in many cases, it can also be reversed by an applied stress that raises the elastic energy of the martensitic phase. The characteristics of the reverse martensite transformation are both scientifically interesting and technologically important. They govern the thermal hysteresis of the reverse transformation on heating and are responsible for such phenomena as thermoelasticity, pseudoelasticity, and the shape memory effect. They may also influence the microstructure that results from the transformation. We have, therefore, extended the computer simulation model to reversible transformations. In the present article, we report the results of computer simulation studies of reversible martensitie transformations in solids that are free of external stress. In further work, we shall describe how applied stresses change the results.
PING XU, is Senior Processing Engineer, Applied Materials, 3225 Oakmead Village Drive, MS 1266, Santa Clara, CA 95054. J.W. MORRIS, Jr. is Professor of Metallurgy, Materials Science and Mineral Engineering, University of California, Berkeley, and Program Leader, Structural Materials, Center for Advanced Materials, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Manuscript submitted February 8, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
BACKGROUND
It is convenient to discuss the thermal reversion of martensite in terms of two limiting cases: macroscopically reversible (thermoelastic) transformations and macroscopically irreversible (thermoplastic) transformations. In an ideally reversible transformation, the temperature-transformation behavior on heating (the TT curve) simply retraces that developed on cooling. Hence, A~ = MI and AI = Ms, where A, and A• are the start and finish temperatures for the reverse transformation and Ai < To, where TO is the temperature at which the two phases have equal free energy. The transformation is thermoelastic in the sense that the martensite volume fraction is a unique function of the temperature; the sample swells or shrinks by a predictable amount as the temperature is varied between Ms and M# The term "thermoelastic" was coined by Kurdjumov531 He and otherst~-9~have identified a number of systems that exhibit approximately reversible martensitic transformations, including alloys such as Au-Cd, Ag-Cd, Cu-Zn, Cu-Zn-AI, and Cu-A1-Nit
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