Loading rate and test temperature effects on fracture of in situ niobium silicide-niobium composites

Arc cast, extruded, and heat-treated in situ composites of niobium silicide (Nb5Si3) intermetallic with niobium phases (primary-Nb-p and secondary-Nb-s) exhibited high fracture resistance in comparison to monolithic Nb5Si3. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb5Si3 and plastic deformation of the Nb-p and Nb-s phases, producing resistance-curve behavior and toughnesses of 28 MPa root m with damage zone lengths less than 500 mu m. The effects of changes in the Nb-p yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nb-p. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nb-p, while the Nb-s always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nb-p phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rare and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models.