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 γ-Nb₅Si₃. The Nb₃Si phases disappear with increasing Mo content to 8 at.% or increasing Zr content to 6%. γ-Nb₅Si₃ precipitates formed in the Nbss of as-deposited xMo-yZr alloys due to the cyclic re-heating, and the γ-Nb₅Si₃ precipitates and Nbss matrix exhibits orientation relationship (OR) of [001]_Nbss//[1¯112]_γ and (110)_Nbss//(011¯0)_γ 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 °C/30 h). The Nb₃Si phase has transformed into Nbss and α-Nb₅Si₃ with the OR have been determined as {100}_Nbss//{100}_α and{111}_Nbss//{111}_α in the heat-treated 4Mo-3Zr alloy. When Mo content increase to 8at.%, a part of γ-Nb₅Si₃ phase also transformed into the α-Nb₅Si₃ phase with the OR have been determined as {112¯0}_γ//{110}_α and {101¯0}_γ//{111}_α 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.