Achieving an appropriate strength-ductility synergy of Nbss/Nb5Si3 in situ composites governed by gradient annealing heat treatment

Abstract

The time-temperature gradient annealing heat treatment (TTGA) was performed on the directionally solidified Nb-16Si-23Ti-4Cr-2Al-2Hf-0.4Re (at.%) alloy to investigate the microstructure morphology, precipitated behaviors, room temperature mechanical properties and damage evolution. The Nb ss , αNb 5 Si 3 and γNb 5 Si 3 are all the ever-present constituent phases throughout the whole annealing process. After gradient annealing at 850 °C for 12 h, the TTGA induces another four precipitating behaviors: (i) bulk fcc-Ti precipitates along Nb ss /αNb 5 Si 3 interface; (ii) fcc-Ti partially transforms into hcp Ti; (iii) ultrafine fcc-Ti disperses inside Nb ss ; (iv) spherical γNb 5 Si 3 precipitates from internal of Nb ss . For conventional heat treatment, the room temperature fracture toughness and ultimate tensile strength of CHT alloy are 13.1 MPa·m 1/2 and 1046.7 MPa respectively. After TTGA at 850 °C for 12 h, the K Q in GA-1 alloy increases to 21.6 MPa·m 1/2 with higher of 64.9%. Owing to higher continuity and better deformability of Nb ss matrix, more toughening mechanisms as crack bifurcation, deflection and bridging, especially microcrack and Nb ss /αNb 5 Si 3 interface decohesion can be observed in GA-1 alloy. In addition to achieve roughly the equivalent UTS (1012.3 MPa) compared with other gradient annealed alloys, GA-1 alloy also exhibits obvious tensile plasticity. • TTGA at 850 °C for 12 h maximizes volume fraction of Nb ss matrix to 68.6%. • Ostwald ripening leads to maximal silicide size of 5.76 μm after longer-time TTGA. • TTGA at 850 °C for 12 h induces fcc Ti, hcp-Ti and γNb 5 Si 3 precipitating from Nb ss . • TTGA at 850 °C for 12 h maximizes K Q to 21.6 MPa·m 1/2 without sacrificing of UTS. • Deflection, bridging, microcrack and interface decohesion are generated by TTGA.