Flow and fracture behaviour of Ni3(Al·Ti) single crystals tested in tension

The deformation and the fracture behaviour of the [0 0 1] orientated Ni3(Al.Ti) single crystals were investigated to determine the relation between the positive temperature dependence of the flow stress in the γ′-phase and the malleability of nickel-base superalloys. The positive temperature dependence of the flow stress is observed in the [0 0 1] orientation below about 1000 K (Tp) and the failure occurs in a catastrophic and brittle manner after considerable plastic deformation. The room temperature fracture stress increases with increase in the angle θ between the [0 0 1] orientation and the tensile axis at 290 K, and it is well expressed by a crack propagation criterion only by considering the effect of the normal stress on the {1 0 0} cleavage plane. The cleavage fracture stress for the [0 0 1] orientation is nearly independent of temperature below Tp, while the elongation decreases with temperature in contrast to the yield stress. The cleavage fracture of Ni3(Al.Ti) single crystals is explained by the rapid decrease of the mobile dislocation density due to a dislocation pinning mechanism based on the cube cross-slipping of the screw superdislocations which causes the positive temperature dependence of the flow stress. The insufficiency of the malleability of nickel-base superalloys seems to be attributed to that of the γ′-Ni3 (Al.Ti) phase, and the hot working of nickel-base superalloys near Tp in the γ′-phase should be avoided.