EXPERIMENTAL ASSESSMENT OF CRACK-TIP DISLOCATION EMISSION MODELS FOR AN AL67CR8TI25 INTERMETALLIC ALLOY

A potential explanation for the cleavage fracture of intermetallic all oys 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 emissio n from crack tips. In the present study, an intermetallic alloy with t he L1(2) crystal structure, Al67Cr8Ti25, has been used to experimental ly assess the predictions of the Rice-Thomson dislocation-emission mod el. The assessment is performed in two ways. First, model predictions of a fracture made transition at elevated temperature are compared wit h experimental results. Bend tests performed at temperatures in the ra nge of 293 to 1061 K reveal that the fracture mode of Al67Cr8Ti25 chan ges from predominately cleavage fracture at room temperature to a mixe d mode of cleavage and intergranular fracture at intermediate temperat ures and then to predominately intergranular fracture at high temperat ures. The observed cleavage-to-intergranular fracture transition tempe rature is approximately 800 K, in good agreement with the model predic tion. Second, model predictions of the effect of grain orientation on the fracture mode are compared with experimental results. Electron bac kscatter patterns and fractographic techniques were used to analyze th e grain orientations and fracture modes of grains on the fracture surf aces of specimens fractured at four temperatures in the range 439 to 1 061 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.