[en] 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√m with damage zone lengths less than 500 microm. 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 rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models