Deformation and fracture behavior of a directionally solidified Ni 3Al alloy

Abstract

The {111} slip behavior of a directionally solidified (DS) Ni 3AlZrB alloy has been investigated in tension in the temperature range of 773–1373 K, and in-situ transmission electron microscope (TEM) observation of the fracture behavior of the same alloy has also been carried out. The dependence of deformation rate on yield stress (at 0.2% plastic strain) is found to be exponential above the peak temperature. The dependence of deformation rate on flow stress (at 3.8% plastic strain) obeys a power-law relation above the peak temperature, with the stress exponent n = 4.89 in the temperature range of 1073–1173 K, suggesting a deformation mechanism of edge dislocation climb, and n = 2.70 in the temperature range of 1273–1373 K, suggesting a deformation mechanism of viscous screw dislocation glide. The dislocation structure develops with increasing stress and strain, the dislocation density ρ is related to the applied stress σ as ρ σ σ 1.7, and no steady-state dislocation density is achieved up to 3.8% strain. With increasing temperature, the activation volume decreases below the peak temperature, and stays constant at a low level above the peak temperature, reflecting two different processes of thermal activation or unpinning of the Kear-Wilsdorf (KW) locks. Room-temperature in-situ TEM straining tests revealed that, near a crack tip edge, dislocations glide away quickly and leave behind many long straight screw dislocations pinned by the KW locks. The large number of pinned screw dislocations reduce the mobility of dislocations and induce cleavage fracture. Additionally, for the first time, an athermal unpinning process has been observed for screw dislocations around a crack tip.