Microstructural evolution and mechanical properties of as-cast and directionally-solidified Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys at room and high temperatures

Arc-melting (AC) and directional solidification (DS) techniques were used to prepare Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys (hereafter referred as to 2Cr and 14Cr alloys, respectively), and the microstructural evolution and mechanical properties, including Vickers hardness, room temperature fracture toughness and high temperature strength, of the two AC and DS alloys were compared. The results showed that with heat-treatment at 1350 °C for 50 h, the AC-2Cr alloy composed of Nb solid solution (Nb_SS) and α-Nb₅Si₃ silicide, while Laves C15-Cr₂Nb phase arose in the 14Cr alloy. With two-phase Nb_SS/α-Nb₅Si₃ microstructure, the AC-2Cr alloy showed excellent room-temperature fracture toughness (K_Q: 14.2 MPa m^1/2) and 0.2% yield strength at 1250 °C (σ_0.2: 315 MPa) and 1350 °C (σ_0.2: 294 MPa), better than the AC-14Cr alloy with tri-phase Nb_SS/α-Nb₅Si₃/C15-Cr₂Nb microstructure (K_Q: 9.4 MPa m^1/2, σ_0.2: 189 MPa at 1250 °C and 87 MPa at 1350 °C). The DS technique was found not to change the phase constituent of each alloy, but it made the microstructure slightly orient to the growth direction, resulting in a significant improvement in room-temperature fracture toughness (by ∼43%) and high-temperature yield strength σ_0.2 (by ∼55%), as compared with the AC samples.