Grain boundary segregation and intergranular fracture in a gamma-based titanium aluminide intermetallic

Abstract

The ongoing interest in the development of gamma-based titanium aluminides has stimulated renewed efforts to improve the understanding of fracture mechanisms in the possible application temperature range between 700 and 1,000 C. The early work by Lipsitt and co-workers established the occurrence of a transition from cleavage to intergranular fracture at temperature in the ductile-to-brittle-transition-temperature (DBTT) range between 650 and 700 C. They attributed this transition to the nucleation of grain boundary microcracks by interacting dislocations near the grain boundaries. However, no clear evidence of grain boundary microcracking was presented in their paper. Their dislocation-based arguments also do not explain the occurrence of intergranular fracture in the DBTT regime where increased grain boundary compatibility is expected as a result of the thermal activation of multiple slip systems. Experimental evidence of the role of grain boundary segregation in the inducement of intergranular fracture in Ti-48Al is presented in this paper. This includes in-situ Auger data that reveals the presence of S on intergranular and interlamellar facets on the fracture surfaces of impact specimens that were fractured under high vacuum in an Auger chamber of 700 C, i.e., in the absence of possible contamination by the test environment. The occurrence of intergranularmore » fracture at 700 C, i.e., in the absence of possible contamination by the test environment. The occurrence of intergranular fracture at 700 C is thus attributed to the reduced cohesion of the grain boundaries by S atoms that segregate to the grain boundaries as a result of thermally activated diffusion. The implications of the results are discussed for the future processing of gamma-based titanium aluminides.« less