Deformation twinning in difficult-to-work alloys during severe plastic deformation

We report deformation twinning formed at unexpected conditions its a means of microstructural refinement and additional source for plasticity in selected difficult-to-work alloys during severe plastic deformation 10 equal channel angular extrusion (ECAE). The selected materials were 316L austenitic stainless steel, a NiTi shape-memory alloy and Ti-6Al-4V. Extensive deformation twinning was observed in 316L stainless steel and Ti-6Al-4V at 800 degrees C during ECAE. The twinning activity at such high temperatures in stainless steel was attributed to the effect of high strength levels oil the partial dislocation separation. It was also speculated that a similar mechanism Could be effective in Ti-6Al-4V. In NiTi with 50.8 at.% Ni, very high Volume fraction of twin-related nanograins formed in B2 phase after one pass at room temperature. This was thought to be a consequence of a set of mechanisms including stress-induced martensitic transformation, deformation twinning in martensite and back transformation to B2 phase. The deformation twinning formation instead of well-known Type-I and Type-II transformation twinning in martensite was also because of high strength and shear levels applied during deformation. In all these cases, twin thickness was smaller than 100 run which is important for Hall-Petch strengthening. With the supporting evidence from recent Studies on nanocrystalline aluminum and copper, it was Concluded that deformation twinning can be one of the main modes of deformation in many metallic alloys in a wide range of temperatures when high strength levels are reached irrespective of the way in which they are achieved. (c) 2005 Elsevier B.V. All rights reserved.