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Abstract

Titanium aluminides are most suitable for high temperature structural materials with a combination of high strength and low density. However, TiAl alloys have limited ductility and low fracture and fatigue strengths at low temperature. The underlying microstructure plays a key role in determining their strength and can be optimised for specific applications. This essentially requires an understanding of microscopic mechanisms in deformation and fracture which are difficult to track experimentally, because this would involve a monitoring of dislocations and interfaces during deformation. Large scale atomistic simulations can provide a viable alternative in understanding nanoscale behaviour with the ability to simulate specific regions of a microstructure. The nanoscale behaviour of two phase lamellar TiAl alloys (which have optimum combinations of strength and ductility) is largely influenced by the number and size of lamellar phases, the L1_0 TiAl and the D0_19 Ti3Al phase. Some, but not all interfaces at these lamellae are known to obstruct dislocation motion as in several nanotwinned/ thin film metals. To clarify which interfaces permit slip transfer and which are blocking dislocations, we carry out large scale molecular dynamics simulations of the interaction of dislocation motion with the lamellar interfaces in different microstructures, in which we vary systematically the type and spacing of the boundaries. Furthermore, by simulating nanoindentation experiments, and closely monitoring defect formation and dislocation interaction, we reveal the effect of lamellae size and interface type on the nucleation stresses and mobility of dislocations within the phases.