Microstructure evolution and room temperature deformation of a directionally solidified Nb-Si-Ti-Cr-Al-Hf-Y alloy

An Nb-14Si-22Ti-4Cr-2Al-2Hf-0.15Y(at.%) alloy was prepared by directional solidification (DS) with liquid metal cooling, and the withdrawal rates selected were 1.2, 6, and 18 mm·min-1, respectively. The Influence of withdrawal rate and heat treatment on the microstructural evolution, fracture toughness and tensile strength at room temperature were investigated. Results show that the directionally solidified microstructure is composed of primary (Nb, X)ss dendrites and (Nb, X)ss/α-(Nb, X)5Si3 eutectic cells aligning with the growth direction. The formation of bulk Nb3Si is suppressed. With an increase in withdrawal rate, the dendrite arm spacing of (Nb, X)ss decreases, and the (Nb, X)ss/α-(Nb, X)5Si3 eutectic cells become finer and distribute homogeneously. Directional solidification can significantly improve the room temperature fracture toughness, especially the alloy with a withdrawal rate of 6 mm·min-1; its average value reaches 14.1 MPa·m0.5, about 34% higher than that of the alloy without directional solidification. The withdrawal rate has obvious effect on tensile strength, and the tensile strength is improved from 200 MPa to 429 MPa as the withdrawal rate increases from 1.2 mm·min-1 to 1.8 mm·min-1. After heat treatment, the primary (Nb, X)ss branches become coarser; both the room temperature fracture toughness and tensile strength of the alloys solidified at 1.2 and 6 mm·min-1 are somewhat lower than the corresponding values of the alloy without heat treatment, while they are higher than the corresponding values of the alloy without heat treatment when solidified at 18 mm·min-1.