Fine Structures of the Pyramidal Slip Dislocations in Ti 3 Al Single Crystals at Temperatures of the Yield Stress Anomal
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ABSTRACT The (1 121) pyramidal slip in Ti3AI shows the so-called yield stress anomaly. A marked change in dislocation configuration is observed in the temperature range concerned; dislocations observed at the lowest temperature of the range, -70'C, are ordinary pairs of edge c+a/2 superpartial dislocations coupled by antiphase boundaries (APBs) while at the other extreme temperature 700'C, pairs of unlike superpartials coupled by APBs are observed. The change in dislocation configurations is interpreted as resulting from immobilization of edge c+a/2 superpartial segments by climb dissociation onto the basal plane. INTRODUCTION Ti3AI orders as the D0 19 superlattice structure and is one of the intermetallics which has been extensively studied. However, as with many other intermetallics the complete absence of ductility at room temperature has been posing the major challenge in developing a structural material. The ductility measurement of polycrystalline binary Ti3A1 reported [1] shows that the absence of ductility extends to as high as 700'C. Transmission electron microscope (TEM) study of dislocations in deformed polycrystalline materials identified primarily 1/3 (a) dislocations and also 1/6 (c+a/2) dislocations but to a very limited extent only in grains oriented in such a way that the stress axis coincided with the c axis of each grain [2,3]. Since a dislocations alone cannot accommodate the strain along the c axis, c+a/2 dislocations that give rise to this kind of strain play an important role in deformation of polycrystalline materials with the D019 structure. In spite of the importance of the pyramidal slip dislocations, however, the nature of these dislocations has never been systematically studied because of the difficulty of their activation in polycrystalline specimens. With this in mind, we have grown single crystals of Ti3Al and measured the temperature dependence of the critical resolved shear stresses (CRSSs) for major slip systems, prismatic, basal and pyramidal slips, clarifying the relative ease of each slip among these slip systems [4]. We also found that the CRSS for the pyramidal slip increases with increasing temperature [5]. We have performed a detailed TEM study of c+a/2 pyramidal dislocations to get a better understanding of the yield stress anomaly in Ti3A1 and found strange behavior these dislocations show near the peak-yield stress temperature. The purpose of the present paper is to describe some characteristic features peculiar to c+a/2 dislocations observed in a higher temperature range of the yield stress anomaly in comparison with the features in a lower temperature range. The change in the dislocation configurations is interpreted as resulting from the immobilization of edge c+a/2 superpartials by climb dissociation onto the basal plane. EXPERIMENTAL METHOD Crystals used are the same as in the previous study [61, and of the same chemical composition Ti-24.Oat%AI-0.25at%O-0.007at%N. For other experimental details the reader is referred to our preKK8.17.1 Mat. Res. Soc. Symp. Proc. Vo
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