Experimental assessment of crack-tip dislocation emission models for an Al67Cr8Ti25 intermetallic alloy

Abstract

A potential explanation for the cleavage fracture of intermetallic alloys with low or moderate critical resolved shear stress (CRSS) is the existence of an energy barrier for crack-tip dislocation emission, as described by models that analyze the energetics of dislocation emission from crack tips. In the present study, an intermetallic alloy with the Ll2 crystal structure, Al67Cr8Ti25, has been used to experimentally assess the predictions of the Rice-Thomson dislocation-emission model. The assessment is performed in two ways. First, model predictions of a fracture mode transition at elevated temperature are compared with experimental results. Bend tests performed at temperatures in the range of 293 to 1061 K reveal that the fracture mode of Al67Cr8Ti25 changes from predominately cleavage fracture at room temperature to a mixed mode of cleavage and intergranular fracture at intermediate temperatures and then to predominately intergranular fracture at high temperatures. The observed cleavage-to-intergranular fracture transition tem-perature is approximately 800 K, in good agreement with the model prediction. Second, model predictions of the effect of grain orientation on the fracture mode are compared with experi-mental results. Electron backscatter patterns and fractographic techniques were used to analyze the grain orientations and fracture modes of grains on the fracture surfaces of specimens frac-tured at four temperatures in the range 439 to 1061 K. Experimental results reveal a correlation between fracture mode and slip system orientation relative to the crack, in good agreement with dislocation emission model predictions.