Remarkable fracture toughness of hypereutectic Nb-16Si-20Ti-2ZrC composites with fine primary γ-(Nb,X)5Si3 regulated by TiB2

To meet the requirements of the new generation of aeroengines, Nb-Si-based alloys have been widely studied as candidate materials for the new generation of blade materials; however, the lower ambient fracture toughness limits their applications. In this study, the effects of various contents (0, 0.5, 1, 3 at%) of TiB2 on the microstructure and ambient fracture toughness of the hypoeutectic composite of Nb-16Si-20Ti-2ZrC (base) composite were investigated. Ingots were manufactured via arc melting in an Ar atmosphere. Results demonstrate that the ingots were composed of Niobium based solid solution (Nbss) and γ-(Nb,X)5Si3, and TiC phases precipitate with a TiB2 content of 3 at%. A moderate amount of TiB2 (1 % TiB2 and below) markedly refined the microstructure, and the average phase sizes of Nbss and γ-(Nb,X)5Si3 decreased to 12.5 and 9.8 µm in the Nb-16Si-20Ti-2ZrC-1TiB2 composite, respectively. TiB2 promotes the movement of the eutectic point to the low Si side in the Nb-Si-based composite, transforming from a hypoeutectic (base) alloy to a eutectic alloy and a hypereutectic alloy. The ambient fracture toughness reached 18.6 MPa·m1/2, increased by 22 %. Many microcracks near the primary crack generated by the fine eutectic colony passivate the primary crack, and the deformation ability of the small-sized Nbss phase is enhanced, resulting in many dimples. The ambient fracture toughness of the Nb-16Si-20Ti-2ZrC-1TiB2 composite in this study is the highest among the reported as-cast Nb-Si-based composites, which can promote Nb-Si-based composites to meet the blade material requirements.