Influence of heat treatment on microstructure and mechanical properties of Nb-Ti-Si-Cr-Ta ultrahigh temperature alloys

A series of multi-component Nb-22Ti-14.8Si-4Cr-xTa (x = 0, 2, 4, 8 at.%) alloys were prepared via vacuum non-consumable arc melting and subsequent two-step homogenization heat treatment (1300 °C/20 h + 1450 °C/50 h). The microstructural evolution upon heat treatment was analyzed, and the influences of Ta content on the microstructure and mechanical properties of the heat-treated alloys were systematically investigated. The as-cast alloys possess a hypereutectic microstructure, consisting of primary (Nb,X)₅Si₃ blocks, Nbss/γ(Nb,X)₅Si₃ eutectics, and Nbss/γ(Nb,X)₅Si₃/Cr₂(Nb,X) three-phase eutectics. After heat treatment, the three-phase eutectics have been completely eliminated, and a small amount of (Nb,X)₃Si is newly precipitated. Ta addition promotes the formation of α(Nb,X)₅Si₃ phase by enhancing its thermodynamic stability. Ta exhibits a significantly higher partitioning preference in α(Nb,X)₅Si₃ phase than in γ(Nb,X)₅Si₃ phase. Heat treatment effectively optimizes the microstructural homogeneity of the alloys, thus significantly enhancing their room temperature fracture toughness and high temperature compressive strength. Higher Ta contents (4 and 8 at.%) addition further elevates the high temperature (1250 °C) compressive strength of the heat-treated alloys, reaching a maximum value of approximately 616 MPa. Ta addition improves the nanoindentation hardness of both Nbss and primary α(Nb,X)₅Si₃ blocks through solid solution strengthening, and heat treatment further enhances the nanoindentation hardness of silicides.