Mechanical twinning and texture evolution in severely deformed Ti-6Al-4V at high temperatures
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We have investigated the deformation behavior and texture evolution of two-phase Ti-6Al-4V subjected to severe plastic deformation using equal channel angular extrusion (ECAE) at a high temperature (0.55Tm). Significant deformation twinning activity was observed after one and two ECAE passes in a 90 die at 800 C. Twinning activity at such a high temperature is a first-time observation in this material and is attributed to the high strain and stress levels imposed during ECAE. High stress levels and the stress state can affect the separation of twinning partials considerably. Resolved shear stress magnitudes on twin partials were found to be high during the ECAE process that helps the nucleation of mechanical twinning. The twinning mode was identified as the {1 0 over(1, ) 1} type using electron diffraction patterns which is one of the twinning modes observed in Ti at temperatures above 350 C. Although only one twinning variant was mainly evident after one pass, multiple twin variants of the same mode were observed after the second pass with a significant increase in twin volume fraction. ECAE processing aligned the basal planes of the hexagonal close-packed phase, initially having a random texture, with the ECAE shear plane. Texture evolution during ECAE was successfully predicted using a viscoplastic self-consistent crystal plasticity framework capturing the effect of the observed twinning mode on texture. Mechanical twins formed during ECAE and grain refinement led to a noteworthy improvement in flow stresses under tension and compression at room temperature. A strong directional anisotropy in yield strengths was also evident which cannot be explained only by crystallographic texture. It was speculated that the asymmetry of critical resolved shear stresses of deformation modes and the processing-induced deformation structure should play a role. With the supporting evidence from our previous works on the severe plastic deformation of other difficult-to-work alloys, it was concluded that mechanical twinning can be one of the main modes of deformation in difficult-to-work alloys in a wide range of temperatures when high strength levels are reached, irrespective of the way in which they are achieved. 2006 Acta Materialia Inc.
