Effects of changes in strain path on work hardening in cubic metals
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INTRODUCTION
W H E N the strain path is changed during plastic deformation of a metal, its flow strength and work-hardening rate diverge from those which are characteristic of monotonic deformation. The responses of different metals and alloys to changes in the mode of deformation have been investigated in two-stage mechanical tests involving an abrupt change in strain path. Early investigations of this kind, notably by Ghosh and Backofen m and Hutchinson et al.,m demonstrated the practical importance of changes in work hardening caused by changes in strain path in relation to stretch formability in sheet metal shaping operations. Salient features of investigations made in the period up to 1982 are summarized by Wagoner and Laukonis ~31 in their paper on the tensile behavior of an aluminum-killed (AK) steel after prestraining in plane strain. It was concluded from this early work that the changes in work hardening following a change in strain path are predominantly transient and that both positive and negative changes in hardening rate can occur in two-stage tests made on different materials. It was also concluded that the transient changes in plastic
S. PLATIAS, formerly Postgraduate Student, School of Metallurgy and Materials, University of Birmingham, is Technical Superintendent, Hellenic Ferroalloys S.A., Athens, Greece. P.S. BATE, formerly Research Fellow, School of Metallurgy and Materials, University of Birmingham, is with INCO Engineering Products Ltd., Birmingham BI6 OAJ, United Kingdom. M. ZANDRAHIMI, Postgraduate Student, D. PRICE, Senior Technician, D. BARRETT, Senior Technician. W.T. ROBERTS, Reader, and D.V. WILSON, Emeritus Professor, are with the School of Metallurgy and Materials, University of Birmingham, P.O. Box 363. Birmingham BI5 2TT, United Kingdom. Manuscript submitted October 3, 1988. METALLURGICAL TRANSACTIONS A
response can often be characterized as being one of two t y p e s , I3-61 viz., (1) type 1: lowered initial flow stress accompanied by an increased hardening rate and (2) type 2: increased initial flow stress accompanied by a reduced hardening rate. Using finite element modeling, Chung and Wagoner ~6j have shown how particular forms of each of these types of transient can influence uniform elongation and total elongation in uniaxial tensile tests. The prominent transients developed in ferritic lowcarbon steels, commercial purity (CP) aluminum, and some low-strength aluminum alloys are usually type 2.12-51 Transmission electron microscope (TEM) investigations of the microstructural behavior of aluminum I71 and low-carbon steels 18~ have shown that large changes in strain path can destabilize the dislocation cell structures established in prior deformation. In the early stages of the second deformation mode, the original cell structures are disrupted and appear to be partially dissolved, and then, in continued deformation, new dislocation cell structures are built up. Relative to workhardening rates developed in monotonic deformation, the first phase of such structural changes
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