Effect of Sc on the microstructure and room-temperature mechanical properties of Nb-Si based alloys

Abstract

The microstructure and room-temperature mechanical properties of directionally solidified and heat-treated Nb‑16Si‑23Ti‑4Cr‑2Al‑2Hf‑0/0.3/0.5Sc (at.%) alloys were investigated. All alloys consisted of Nb-based solid solution (Nbss), α‑Nb5Si3 and γ‑Nb5Si3 with preferred 〈001〉 orientation for the first two phases, and Sc was enriched in γ‑Nb5Si3 phase. Sc-added alloys showed a more continuous Nbss matrix in which the finer Nb5Si3 phase distributed uniformly. The volume fraction of both α‑Nb5Si3 and γ‑Nb5Si3 phase was decreased by Sc addition. Sc alloying reduced the Young's modulus and nano-hardness of Nbss phase. 0.50 at.% Sc addition enhanced the fracture toughness and tensile strength from 9.88 MPa m1/2 and 430.3 MPa to 25.95 MPa m1/2 and 1119.7 MPa. The improvement in fracture toughness was due to the better deformability of Nbss phase. The high tensile strength was attributed to the dispersed and refined Nb5Si3 phase, which was not degraded by the increased Nbss volume fraction since dislocations were not open within most Nbss phase. Besides, the strong interface cohesion was preferable for the strength of Sc-added alloys. Keywords: Microstructure, Growth orientation, Young's modulus, Nano-hardness, Fracture toughness, Tensile strength