Seynergistic effect of Mo and Zr additions on microstructure and mechanical properties of Nb-Ti-Si-based alloys additively manufactured by laser directed energy deposition

The synergistic effect of Mo and Zr additions on microstructure evolution, room-temperature fracture toughness and microhardness of Nb-22Ti-15Si-xMo-yZr (x = 4,8, y = 3,6) alloys manufactured by laser directed energy deposited (L-DED) have been investigated. The major phases in as-deposited 4Mo-3Zr alloy are Nb solid solution (Nbss), Nb3Si and gamma-Nb5Si3. The Nb3Si phases disappear with increasing Mo content to 8 at.% or increasing Zr content to 6%. gamma-Nb5Si3 precipitates formed in the Nbss of as-deposited xMo-yZr alloys due to the cyclic re-heating, and the gamma-Nb5Si3 precipitates and Nbss matrix exhibits orientation relationship (OR) of [001](Nbss)//[112](gamma) and (110)(Nbss)//(010)(gamma) in as-deposited 4Mo-6Zr alloy. The microstructure of xMo-yZr alloys became relative homogeneity and the Nbss matrix showed the better continuity after the heat treatment (1400 degrees C/30 h). The Nb3Si phase has transformed into Nbss and alpha-Nb5Si3 with the OR have been determined as {100}(Nbss)//{100}(alpha) and{111}(Nbss)//{111}(alpha) in the heat-treated 4Mo-3Zr alloy. When Mo content increase to 8at.%, a part of gamma-Nb5Si3 phase also transformed into the alpha-Nb5Si3 phase with the OR have been determined as {110}(gamma)//{110}(alpha) and {100}(gamma)//{111}(alpha) after heat treatment. Among the as-deposited alloys, the 4Mo-6Zr alloy has the highest fracture toughness K-Q with the lowest hardness. The average K-Q of 4Mo-6Zr and 8Mo-6Zr alloys increased by 28% and 30% after the heat treatment, and reached 13.61 and 11.43 MPa.m(1/2), respectively. The hardness of Nb-22Ti-15Si-xMo-yZr alloys is significantly decreased after the heat treatment. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.