Kear-Wilsdorf Locks and Mechanical Properties of L1 2 Alloys

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KEAR-WILSDORF LOCKS AND MECHANICAL PROPERTIES OF L12 ALLOYS GEORGES SAADA and PATRICK VEYSSItRE LEM*, CNRS-ONERA, BP 72, 92322 Chfitillon Cedex, France. ABSTRACT There is no generally accepted model of the atypical mechanical properties of L12 alloys such as the flow stress peak. Since its introduction in the early sixties, the so-called KearWilsdorf (KW) lock has played a central role in the analysis of the mechanical properties of this category of alloys. We analyze the mechanical stability of a split configuration, intermediate between the superdislocation fully extended in the primary slip plane and the KW lock. The influence of composition on mechanical properties is stressed, based on experimental determinations of the flow stress dependencies upon load orientation when composition is varied. INTRODUCTION More than 30 years of research has been devoted to the analysis of the atypical mechanical behavior of ordered intermetallic alloys and in particular to the flow stress peak in L1 2 alloys. There is much to learn from the available theoretical analyses of the mechanisms that control the positive temperature dependence of the flow stress (TDFS); however, the detailed origins of this atypical mechanical property and of related phenomena remain to be elucidated. On an experimental standpoint, it makes little doubt that every property that could shed light on the positive TDFS is now documented - quite extensively in many instances - but our understanding of the overall problem still suffers from uncertainties in the determination of several crucial parameters. In the domain of positive TDFS, deformation occurs on the octahedral planes and it is agreed that (i) the Schmid law, including the tension-compression asymmetry, is significantly violated, (ii) the flow stress is very little sensitive to the strain rate, (iii) there exists a microstrain level below which the TDFS is normal, (iv) the rate of strain hardening is anomalously large, (v) the flow stress is partly reversible, (vi) the microstructure is dominated by APB-coupled superdislocations which are elongated in the screw direction where they are dissociated in the cube plane, a configuration known as the Kear-Wilsdorf (KW) lock. This paper is not aimed at presenting a new model of the positive TDFS. Rather, we intend to reexamine some aspects of the phenomenon whose complexity may not have been fully appreciated in the past, such as - the importance of composition, based on measurements of the flow stress dependence upon orientation which point to the role of lattice friction on dislocation behavior. - properties of KW obstacles including the driving forces for transformation and KW stability, - the framework of a tentative model based on the fact that strain is essentially provided by non-screw segments and the consequences of this on deformation at a constant strain rate, flow stress reversibility, creep and on relaxation experiments. SOME CONSIDERATIONS ON COMPOSITION-RELATED EFFECTS Because of technical limitations in growing stoichiometric single cr

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