Effect of lamellar orientation on the strength and operating deformation mechanisms of fully lamellar TiAl alloys determined by micropillar compression

The aim of this study is to determine the influence of lamellar orientation on the strength and operative deformation mechanisms of a fully lamellar Ti-45AI-2Nb-2Mn (at.%) + 0.8(vol.%) TiB2 (Ti4522XD) alloy. With this aim, micropillars with lamellae oriented at 0, 45 and 90 with respect to the loading direction were compressed at room temperature. The results revealed a large plastic anisotropy, that was rationalized, based on slip/twin trace analysis, according to the relative orientation of the main operative deformation modes with respect to the lamellar interfaces. Loading at 45 resulted in the activation of soft longitudinal deformation modes, where both the slip plane and the slip direction were parallel to the interfaces, and therefore, little interaction of dislocations with lamellar interfaces is expected. At 0 degrees loading, deformation was mainly accommodated by harder mixed deformation modes (with an oblique slip plane but a slip direction parallel to the lamellar interfaces), although the lamellar interfaces seemed to be relatively transparent to slip transfer. On the contrary, 90 loading represented the hardest direction and deformation was accommodated by the activation of transverse deformation modes, confined to individual lamellae, together with longitudinal modes that were activated due to their softer nature, despite their very small Schmid factors. Finally, a thorough study of pillar size effects revealed that the results were insensitive to pillar size for dimensions above 5 Am. The results can therefore be successfully applied for developing mesoscale plasticity models that capture the micromechanics of fully lamellar TiAl microstructures at larger length scales. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.