Abstract
In this paper the occurrence of twinning parallel to the lamellae during compression at 700 °C of a polycrystalline nearly lamellar commercial γ-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 γ-TiAl variant favourably oriented for twinning occurs repeatedly in the lamellar structure. This effect was caused by the preferential nucleation of certain γ-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 γ-variants. This increases the resulting shear strain that must be accommodated and hence the probability of crack nucleation.
Highlights
Though g titanium aluminides possess a higher specific strength than nickel superalloys [1] at temperatures of the order of 700 C, they suffer from a sufficient lack of ductility that a threshold approach to fatigue lifing is necessary [2]
Its nature is very different to that observed in polysynthetically twinned (PST) crystals
Unlike the many thin twins [7] of the PST crystals, these longitudinal twins were initiated from the lamellar interfaces and grew across the full width of the lamella
Summary
Though g titanium aluminides possess a higher specific strength than nickel superalloys [1] at temperatures of the order of 700 C, they suffer from a sufficient lack of ductility that a threshold approach to fatigue lifing is necessary [2]. Much of the work on the deformation modes of TiAl has employed polysynthetically twinned (PST) crystals [3,4] to study dislocation glide and twinning, where the twin-plane is transversal [5] to the lamella interface, transverse twinning. In these materials, twinning parallel to the lamellae, longitudinal twinning, occurs by the formation of parallel sided twins only ~100 nm in thickness. Lamellar domain boundaries were found to impose sufficient constraint to significantly increase the twinning shear stress The effect of this thicker morphology is to make it difficult to identify a longitudinal twin, as they have straight boundaries and often progress to the completion of a domain. Identifying such longitudinal twins requires that the orientations of the material are known both before and after it has been deformed
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