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Unravelling the lamellar size-dependent fracture behavior of fully lamellar intermetallic γ -TiAl

A. Neogi, R. Janisch.

Acta Materialia, 227, 117698, (2022)

The fracture toughness of lamellar TiAl decreases with decreasing lamella thickness, until a critical spacing below which the crack tip mechanisms change.

Abstract
Strengthening of metals by incorporating nano-scale coherent twin boundaries is one of the important breakthroughs of recent years in overcoming the strength-ductility trade-off. To this effect, also twin boundaries in nano-lamellar lightweight Ti-Al alloys promise a great potential, but their contribution to the deformation and fracture behavior needs to be better understood for designing optimal microstruc- tures. To this end, we carry out linear elastic fracture mechanics informed large-scale atomistic sim- ulations of fully lamellar microstructures consisting of the so-called ”true twin” boundaries in γ -TiAl. We find that nano-scale lamellae are not only effective in improving the fracture toughness and crack growth resistance, but also that the lamellar size controls the crack tip mechanisms. We identify a critical lamella thickness in the region between 1.64 and 3.04 nm, above which the crack tip events are primar- ily dislocation-based plasticity and the critical fracture initiation toughness exhibits an increasing trend with decreasing lamella size. Below the critical thickness, a decline in fracture toughness is observed and the crack tip propagation mechanisms are quasi-brittle in nature, i.e. the cleavage of atomic bonds at the crack tip is accompanied by plasticity events, such as twin-boundary migration and dislocation nucleation. A layer-wise analysis of the unstable stacking fault energy, the energy barrier for dislocation nucleation, that the critical thickness is of a similar value as the distance from the twin boundary at which bulk properties are restored.

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