The absence of steady-state flow during large strain plastic deformation of some Fcc metals at low and intermediate temp
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INTRODUCTION
I T is desirable to determine the links between the low temperature and high temperature deformation of metals. One means of achieving this goal has been to use the steadystate flow stress concept developed at high temperatures in the interpretation of low temperature deformation, l'1 According to this concept, a metal deformed at a given strain rate and temperature will approach a recovery-controlled steady-state flow stress (and structure) which remains invariant with strain and time in the steady-state. The steadystate flow stress and structure are uniquely determined by the temperature and strain rate. To obtain steady-state, the recovery processes must be sufficiently rapid to balance the hardening processes. Diffusional processes and thermally activated dislocation movements, which promote recovery, are enhanced at high temperatures. Thus, a steady-state condition is achieved more easily (i.e., at smaller strains) at high temperatures than at low temperatures. Accordingly, small strain tests (eef f < 0.5 to 1.0), which include tension and compression tests, have been used to determine steady-state properties at high temperatures, while large strain tests (~eff > 0.5 to 1.0), such as torsion, have been used at low temperatures. A different perspective of steady-state flow can be developed when deformation is studied over large strains at both high and low temperatures. There appear to be several processes that may intervene to prevent the development of a recovery controlled steady-state. Among these processes are continued strain hardening (at low temperatures), re-
D.A. HUGHES is with the Materials Department, Sandia National Laboratories, Livermore, CA 94550. W D NIX is Professor, Department of Materials Science and Engineering, Stanford Umversity, Stanford, CA 94305. Manuscript submitted November 16, 1987.
METALLURGICAL TRANSACTIONS A
crystallization, strain softening caused by either dynamic recovery or shear banding, and geometric instabilities. Because the concept of steady-state flow is so central to the understanding of plastic flow, it is important to identify conditions under which steady-state can be reached, as well as those under which a steady-state is not reached. In the present paper we adopt the strict view that the flow stress must remain invariant with strain at constant strain rate and that the microstructure must also remain unchanged in the course of steady-state deformation. In some cases a useful phenomenological description may be established on the basis of flow behavior that only approaches, but does not satisfy, the conditions described above. A strict view of steady-state is taken here, in the hope that this will lead to a better understanding of the factors that affect the flow of metals at large strains. The objective of this study is to explore the conditions under which recovery controlled steady-state deformation is achieved in fcc metals and solid solutions, using large strain tests at both low and high temperatures as the primary experimental tool. Stea
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