Crystal plasticity analysis of the effect of dispersed β-phase on deformation and fracture of lamellar γ+α2 titanium aluminide

A finite element analysis based on the crystal plasticity theory was carried out to investigate the deformation and fracture behavior of polycrystalline lamellar γ-TiAl+α2-Ti3Al material containing 10 vol.% of b.c.c. β-phase precipitates. The effects of both the stable β-phase precipitates which deform by slip and the metastable β-phase precipitates which deform by a combination of stress-induced martensitic transformation and slip are studied. To model the cracking along the grain boundaries and the matrix–precipitate interfaces, the grain boundaries and interfaces were modelled using a cohesive zone approach. All the calculations were carried out using the commercial finite element code ABAQUS. The results obtained suggest that incompatibilities in the plastic flow between the adjacent grains give rise to a large build-up in hydrostatic stress in the region surrounding certain three-grain junctions which, in turn, leads to the nucleation of grain boundary cracks and ultimate failure. The stable β-phase precipitates located at the three-grain junctions help accommodate the incompatibilities in plastic flow doubling the strain to failure. The lattice expansion, which accompanies martensitic transformation in the metastable β-phase precipitates, further delays nucleation of the grain boundary–interface cracks giving rise to an additional increase in the fracture strain.