Growth dynamics study of the martensitic transformation in Fe-30 pct Ni alloys: Part II. Computer simulation of martensi
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I.
INTRODUCTION
IN the
previous paper, tll we presented the experimental measurements of the growth velocity in an athermal martensitic transformation using two independent techniques, namely, an acoustic emission method and a magnetic induction method. The results of the two experiments are very consistent, showing a large random scatter of the growth velocity over a wide temperature range in which the martensitic transformation takes place. These findings raise doubts about the conclusions of previous investigations, t2,3m The velocity scatter was explained by the present authors in terms of the effect of strain conditions on the effective driving force of the transformation in the vicinity of growing martensite plates. To further investigate the growth dynamics of an athermal martensitic transformation, computer simulations of the martensitic transformation in an Fe-30Ni alloy have been carried out. The detailed approach and results of the theoretical modeling are presented in this paper. There are two widely used techniques in computer simulations: the MD method and the Monte Carlo method. The former is especially suitable for studying the time evolution of a model system and its dynamic properties, while the latter is used to examine many different configurations of a model system without simulating molecular motion. In our study of martensitic growth, the dynamic process is the point of interest, and the MD method is obviously the best way to study the structural changes from one phase to another. In the MD simulations, one uses a computer to calculate the simultaneous classical trajectories of a model system (usually containZHEN-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
ing several hundred or thousand atoms) according to N e w t o n ' s law of motion. To monitor the progress of the transformation, the MD program used in the present study is capable of plotting the projection of all atom positions at any desired instant along with the displacement vectors from the starting points and also provides the radial distribution function (RDF), temperature (via the average kinetic energy), and potential energy at selected points in time. The RDF is a very useful tool in monitoring the changing neighbor distances between atoms through observation of the growth and decay of different peaks. Using this program, Clapp and Rifkin have successfully studied the nucleation of martensitic transformations in simple metals. [5,61 The program has been further modified in the present study to include the volume force arising from the conduction electron gas pressure. In the MD simulations, all the atoms are contained in a box with periodic b
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