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 sil icide (Nb5Si3) intermetallic with niobium phases (primary-Nb-p and sec ondary-Nb-s) exhibited high fracture resistance in comparison to monol ithic Nb5Si3. In toughness tests conducted at 298 K and slow applied l oading rates, the fracture process proceeded by the microcracking of t he Nb5Si3 and plastic deformation of the Nb-p and Nb-s phases, produci ng resistance-curve behavior and toughnesses of 28 MPa root m with dam age zone lengths less than 500 mu m. The effects of changes in the Nb- p yield strength and fracture behavior on the measured toughnesses wer e investigated by varying the loading rates during fracture tests at b oth 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to deter mine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nb-p. Specimens tested at either higher loading rates or lower tes t temperatures consistently exhibited a greater amount of cleavage fra cture in the Nb-p, while the Nb-s always remained ductile. However, th e fracture toughness values determined from experiments spanning six o rders of magnitude in loading rate at 298 and 77 K exhibited little va riation, even under conditions when the majority of Nb-p phases failed by cleavage at 77 K. The changes in fracture mode with increasing loa ding rare and/or decreasing test temperature and their effects on frac ture toughness are rationalized by comparison to existing theoretical models.