Transformation path dependence of the bain strain relaxation during decomposition of a hypereutectoid CuBe alloy
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I.
INTRODUCTION
D E C O M P O S I T I O N of a hypereutectoid/3(bcc) phase CuBe alloy into the two-phase [y(B2) + ~(fcc)] mixture 11,2~ represents an interesting class of phase transformations in which diffusional atomic reconfiguration (composition separation and bcc to B2 atomic ordering) is accompanied by a large change in the crystal lattice (bcc to fcc). The abbreviations bcc and fcc represent bodycentered cubic and face-centered cubic, respectively. Recently, the authors have performed a transmission electron microscopy (TEM) investigation t3] to study the relaxation of the Bain transformation strain associated with the bcc to fcc crystal lattice rearrangement with particular regard to potential synergistic interplay with diffusional atomic reconfiguration processes. Focus was placed on the as-quenched states in which many characteristic events are already occurring due to the remarkably rapid kinetics of the decomposition. A distinct mechanism of the transformation strain relaxation was observed which involves the development of the polytwin precipitate plates of a characteristic sawtooth morphology. The morphology was examined in terms of a formation mechanism and it was suggested that both diffusional and diffusionless atomic motions may be necessary for its formation. Many interesting questions remain to be answered concerning the thermodynamic and kinetic nature of the sawtooth morphology. In the present study, a few of these questions are explored by examining alloys prepared by two different heat treatments: air cooling and water quenching followed by aging. A problem that is addressed is whether the development of B. CHEONG, Postdoctoral Associate, and D.E. LAUGHLIN, Professor, are with the Department o f Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. K. HONO, Research Associate, is with the Institute for Materials Research, Tohoku University, Sendai 980, Japan. Manuscript submitted June 23, 1993. METALLURGICAL TRANSACTIONS A
the sawtooth morphology is peculiar to a certain transformation path or, to put it in more general terms, how a change in transformation path would affect the relaxation of the Bain transformation strain and the precipitate morphology. Based on experimental results, the problem of the stability of a polytwin plate of the sawtooth morphology is considered.
II.
THEORETICAL B A C K G R O U N D
A portion of the CuBe phase diagram I41 is reproduced schematically in Figure l(a) with relevant lines added and/or extrapolated. The critical line of the second-order A2 --> B2 ordering transition is depicted as a dashed line. The dotted lines demarcate the conditional spinodals [51 in which spinodal decomposition is thermodynamically possible contingent upon A2 ~ B2 ordering. A chemicalfree energy vs composition diagram is shown in Figure l(b) which is consistent with the phase diagram characteristics at an arbitrary temperature TA below the eutectoid temperature. The free energy of the y(B2) phase plotted in the diagram is assumed to satisfy the e
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