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I. I N T R O D U C T I O N W O R K hardening has long been a major topic in mechanical and physical metallurgy. An enormous number of studies on this issue, since Taylor's work over 50 years ago, m have produced many theories to explain the work hardening phenomena. However, there are still many questions or ambiguities remaining unanswered or unclarified. As Cottrell pointed out in his famous book: t2] "It (work-hardening) was the first problem to be attempted by dislocation theory of slip and m a y well prove to be the last to be solved." In fact, the only hypothesis on which all present work hardening theories agree is that work hardening is due to dislocation interactions. L3~ One of the questions which needs more work is latent hardening: the phenomenon in which one (inactive) slip system is hardened by dislocation accumulation and movements on another, active slip system. Particularly, latent hardening produced by cyclic deformation has hardly been explored, and its understanding will be important to m a n y issues associated with crack nucleation and propagation. The aim of the present study is to explore latent hardening in cyclic deformation. A brief review of monotonic latent hardening is first provided, and the experimental results on cyclic deformation are then presented in the light of monotonic issues. II. R E V I E W O F L A T E N T H A R D E N I N G IN MONOTONIC DEFORMATION To investigate latent hardening behavior a conventional method involves, first, a monotonic tension (or compression) test being carried out with a single crystal, and then the predeformed crystal being cut into new specimens with different orientations on which a secondary monotonic test is performed. In other words, the ZHIRUI W A N G , Research Associate, and C A M P B E L L LAIRD, Professor, are with the Department of Materials Science and Engineering at the University of Pennsylvania. W I L L I A M J. R O M A N O W is a Supervisor at the Materials Processing Center, Laboratory for Research on Structure of Matter, University of Pennsylvania, Philadelphia, P A 19104-6272. Manuscript submitted March 30, 1988. METALLURGICAL TRANSACTIONS A

original primary slip system in the first test is subjugated in the second test, and a new slip system is chosen as the primary system. By measuring and comparing the flow stresses in the different tests, depending on the orientations used, one will obtain information about hardening behavior on various slip systems, i.e., information about self-hardening and latent hardening. This method has been widely employed by many workers in monotonic deformation, [4-91and some essential results have been obtained. As early as 1925, Taylor and Elam t1~ noticed that plastic flow on one set of octahedral planes caused either the same or a slightly greater hardening on an inactive set when deforming aluminum crystals. From then on, similar phenomena, later called latent hardening, were observed and reported from time to time. But systematic work into the details of latent hardening was not carried out until