A Tem Study of the Relationship Between Dislocation Structure and the Anomalous Temperature Dependence of the Flow Stres
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A TEM STUDY OF THE RELATIONSHIP BETWEEN DISLOCATION STRUCTURE AND THE ANOMALOUS TEMPERATURE DEPENDENCE OF THE FLOW STRESS IN ORDERED Ni 3 (Al,1%Ta). M. J. MILLS*, N. BALUC** and H. P. KARNTHALER*** *
Institut de Genie Atomique, Ecole Polytechnique F&l6rale, 1015 Lausanne, Switzerland.
Present Address: Materials Department 8314, Sandia National Laboratories, Livermore, CA 94550, USA. ** Institut de Genie Atomique, ,cole Polytechnique FRddrale, 1015 Lausanne, Switzerland. "*** Institut fir Festf6rperphysik, University of Vienna, A-1090 Vienna, Austria.
ABSTRACT The anomalous increase in the yield strength of single crystals of Ni3 AI(I%Ta) as a function of temperature has been correlated with the post-deformation substructure using weak beam TEM techniques. At low temperatures (77K), there is evidence for abundant cross-slip between (111) and (1I1). With increasing temperature, this conventional cross-slip process is gradually replaced by the formation of KW locks--straight screw segments which have crossslipped and completely dissociated on (010). This change in the mode of cross slip corresponds with the onset of the yield strength anomaly. At still higher temperatures (544K and 715K), the KW locks become mobile and bow out on (010). The density of dislocations on the cube plane also increases sharply at higher temperatures, even though deformation occurs principally by glide on the primary (111) based on slip trace analysis. These observations are inconsistent with the widely accepted cross slip pinning model and suggest that deformation occurs primarily by the motion of the non-screw components. A dislocation model is introduced which attempts to account for the observed dislocation configurations while remaining consistent with the yield strength and work hardening behavior of these alloys. INTRODUCTION The theoretical models that have been proposed to explain the increase in the yield strength as a function of temperature for several intermetallic alloys with the L12 structure are based upon consideration of a thermally activated cross-slip of superlattice (SL) screw dislocations from {111 ) to (010). As first suggested by Flinn [1], the presumed driving force for this process is that the APB energy on 1010) is lower than on the (111). Takeuchi and Kuramoto (TK) [2] incorporated this concept into a deformation which is illustrated in Figure 1. They assume that SL screw dislocations move extensively on the ( 111 1. However, the free-flight motion is inhibited by a periodic distribution of pinning points along the dislocation line. The pinning points are locations where (010) cross slip has occurred. While the cross-slipped segments are sessile, the remainder of the dislocation moves forward on the ( 111) and bows locally near the obstacles. Under the dynamical break-away condition, dislocation motion can continue when the effective shear stress on ( 111 ) is sufficient to achieve the critical bowing angle. The distance between the pinning points, 1, varies inversely with the frequency of (010) cross-slip even
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