Influence of temperature and strain rate on slip and twinning behavior of zr
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INTRODUCTION
PLASTIC deformation in pure metals with cubic crystal structures is reasonably well understood on the basis of dislocation kinetics theory,t~41 since slip is the dominant deformation mode in single crystals or polycrystals under most deformation conditions. In contrast, our understanding of plastic deformation in noncubic structures in which twinning plays a significant part, indeed in certain cases more important than slip, remains less clear. Although extensive effort has been made to investigate the slip and twinning behavior of hexagonal close-packed (hcp) structures during plastic deformation,t5~1 the interplay between the two modes and its effects on the stress-strain response of hcp metals and alloys remains poorly defined. One of the main reasons for this situation is the lack of a thorough understanding of the mechanism of mechanical twinning; therefore, developing a detailed explanation of the correlation between slip and twinning during plastic deformation is difficult. The slip systems in hcp structures are more complex than in cubic structures due to anisotropy,t~~ The observed independent slip systems in the former are considerably fewer than in the latter and are usually less than the five required for sustained plastic deformation of polycrystals. This is one of the reasons that twinning is commonly seen to play a crucial role in the plastic deformation of hcp lattices.[12~ Although the importance of twinning in compensating for the insufficient independent slip systems in hcp structures to satisfy strain compatibility conditions at grain boundaries has long been understood,[9,~2] the contribution of twinning to the overall plastic deformation has been difficult to quantify and thus has often been underestimated. As a result, most interpretations of the stress-strain and work-hardening behaviors of the hcp structures are based solely on slip mechanisms.[5,8.~3] Previous work has shown that the contribution of twinning to plastic strain can be calculated using the equation ~-'twin -~- ~V/T~ S Vt, where V, is the volume fraction twinned and s is the twinning shear.tl4] Assuming half of the volume
S.G. SONG, Postdoctoral Fellow, and G.T. GRAY II1, Research Staff Member, are with the Materials Research and Processing Science Group, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted December 12, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
of a specimen is twinned, the strain caused by twinning on the three prominent twinning modes, i.e., those with composition (/(1) planes (1102), (1121), and (1122) in Zr, is calculated to be 5.9, 22.5, and 44.9 pct, respectively. In practice, the first twinning mode is predominant in Zr at ambient temperature and contributions from the other two modes are minor, t6,71A twinning volume fraction of 0.5 for a specimen which has been subjected to a strain of 20 pct appears realistic in pure Zr, since nearly all of the grains of such a specimen are observed to be heavily twinned. In addition, secondary twins are frequently seen
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