The interaction between slip and twinning systems and the influence of twinning on the mechanical behavior of fcc metals
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D EFORMATION
twinning has long been considered as irrelevant to the deformation mechanisms of fcc metals. The study of fcc of twinning has only started with the pioneer's discovery of twinning in pure copper by Blewitt and coworkers in 1957.1 The study of deformation mechanisms has long been restricted to single crystals of pure metals which unfortunately do not twin readily even at low temperatures because of their high stacking fault energy (SFE) and this was only recently extended to alloys. When comparing with the paramount number of studies on martensitic transformations since the discovery of Hadfield steel at the end of the last century, one may imagine the eventual development of research on twinning if it has been realized earlier that fcc twinning was actually responsible of the high mechanical properties of this steel. 2,3 From recent works it turns out that twinning if not as important as slip for fcc crystals, occurs actually in a number of fcc industrial alloys either single phased or precipitate-strengthened in current conditions, which will be emphasized when discussing the mechanical behavior. One may plot the critical shear stress for twinning as determined from single crystal data 4~ or as estimated from experiments with polycrystals, 7,s as a function of the intrinsic SFE as in Fig. 1, using either nodes' data for low SFE values. 9,~~or weak-beam data for high SFE values. 11-~3The resulting curve (with the adjustment suggested by Venables 4 actually demonstrates that for sufficiently high stresses twinning can occur in a wide range of SFE values. In addition the study of the interactions between slip and twinning, has progressed late in the fcc crystals since they do not exhibit brittle fracture as bcc crystals L. RI~MYis at Centre des Mat6riaux de l'Ecole des Mines de Paris, ERA CNRS 767, BP 87, 91003 Evry Cedex, France. This paper is based on a presentation made at a symposium on "The Role of Twinning in Fracture of Metals and Alloys" held at the annual meeting of the AIME, St. Louis, Missouri, October 1519, 1978, under the sponsorship of the Mechanical Metallurgy Committee of The Metallurgical Society of AIME. METALLURGICALTRANSACTIONSA
where the simultaneous occurrence of cleavage and twinning 14has stimulated a large interest about these interactions. It is actually not by chance if it is for the bcc crystals that Sleeswyk and Verbraak proposed a general method for studying twin-slip and twin-twin interactions which has been widely used since for a number of other crystallographic structures. 15Sleeswyk and Verbraak considered the transfer of the shear of an incident twin into an obstacle twin in terms of dislocation reactions, taking into account that bcc twinning can be described as resulting from the propagation of partial dislocations 1/6 (111) on every (112) plane. These authors showed by a geometrical analysis that a slip dislocation can always penetrate into an obstacle twin but in most cases the incident slip dislocation must dissociate into a slip dislocation in the twin and a pa
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