The interaction of borides and longitudinal twinning in polycrystalline TiAl alloys

In this paper the occurrence of twinning parallel to the lamellae during compression at 700 degrees C of a polycrystalline nearly lamellar commercial gamma-TiAl alloy, Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB2, has been studied and shown to lead to the formation of cracks at colony boundaries. However, the occurrence of this longitudinal twinning mode was less common by at least a factor of ten in tests at room temperature. Furthermore, the debonding of colony boundaries caused by the shear strain of longitudinal twinning is exacerbated when the same gamma-TiAl variant favourably oriented for twinning occurs repeatedly in the lamellar structure. This effect was caused by the preferential nucleation of certain gamma-TiAl variants in the presence of TiB2 boride reinforcement It is shown that the boride additions increase the probability of double, triple or even higher order multiply stacked gamma-variants. This increases the resulting shear strain that must be accommodated and hence the probability of crack nucleation. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd.