Influence of grain size and stacking-fault energy on deformation twinning in fcc metals
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I. INTRODUCTION
DEFORMATION twinning has been observed to strongly influence the mechanical properties of metals.[1,2,3] It is largely believed that the stress required for formation of deformation twins is controlled mainly by the value of the stacking-fault energy (SFE) of the metal. In particular, from an analysis of data from single crystals of low-SFE metals, Venables[4] concluded that there exists a parabolic relationship between the SFE and the twin nucleation stress. It should be noted, however, that there is considerable scatter in the data presented by Venables. Furthermore, the different alloys considered by Venables had different solute concentration levels, and, therefore, the solid-solution strengthening contributions for these different metals can be expected to be significantly different from each other. In a previous study,[1] the use of (s 2 s0)/G was suggested (where s is the flow stress during deformation, s0 is the initial yield strength of the metal, and G is the shear modulus of the metal) as an indicator of the dislocation density that is built in the metal during the deformation process, while accounting for the different solid-solution strengthening contributions in the different metals. Furthermore, it was demonstrated that twin initiation occurred in simple compression of polycrystalline 70/30 brass and MP35N (35 pct Ni, 35 pct Co, 10 pct Mo, and 20 pct Cr) at about the same value of (stw 2 s0)/G (where stw is the flow stress at twin initiation), although the two metals have very different values of SFE (refer to the value of gSFE in Table II). This observation suggests that deformation twin initiation requires a critical dislocation density. This notion is consistent with the model presented by Mahajan and Chin[5] for the mechanism of the formation of deformation twins. We EHAB EL-DANAF, Research Assistant Professor, SURYA R. KALIDINDI, Associate Professor, and ROGER D. DOHERTY, Professor and Head, are with the Materials Engineering Department, Drexel University, Philadelphia, PA 19104. Manuscript submitted July 2, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
note that, in order to establish the role of dislocation density on the initiation of deformation twinning, it is necessary to extend our previous study[1] to include a number of other low-SFE metals. It has also been suggested in literature that the SFE of the metal will have to be less than some critical value for a metal to exhibit deformation twinning.[6,7] Re´my and Pineau[6] studied deformation mechanisms in Co-Ni alloys and reported that deformation twinning occurred in metals with a SFE in the range of 20 to 50 mJ/m2, while both strain-induced martensitic transformation and deformation twinning occurred in metals with a SFE below 15 mJ/m2. The experiments of Oh et al.[7] in Fe-19Mn-5Cr-(0-5.5)Al0.25C alloys indicate that deformation twinning occurred only in the alloys with a SFE lower than about 21 mJ/m2. It is possible that these observations can also be explained by the requirement of a critical dislocation density i
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