Transverse deformation of a lamellar TiAl alloy at high temperature by in situ microcompression

The distribution of strain in hard mode oriented lamellar stacks of the two-phase gamma-TiAl/alpha(2)-Ti3Al alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol% TiB2 was measured at several temperatures up to 633 degrees C by in situ micropillar compression, complemented by electron backscatter diffraction orientation mapping and digital image correlation strain mapping of a thermally stable surface Pt speckle pattern. Post-mortem transmission electron microscopy further identified the finest scale deformation structures. It was found that slip and twinning transverse to the lamellae operates within discreet bands that zigzag across the lamellar structure. The shear strain within each band is approximately constant across the pillar width. This is inconsistent with current energetic models for transverse twin formation in gamma-TiAl, which assume independent, non-interacting twins. This is explained using a mathematical formulation for the stress required to operate this transverse mechanical twinning as a function of strain. This study has elucidated how the multi-scale combination of several transverse twinning systems on different {111} planes in gamma-TiAl lamellae can relieve the elastic stresses generated at a lamellar interface by the primary (highest Schmid factor) twinning system. It is thought that the facilitation of this mechanism will promote the ductilisation of lamellar gamma-TiAl alloys. This is crucial for an increased damage tolerance and ease of component manufacture, leading to a more widespread use of gamma-TiAl alloys. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.