Microstructure and room temperature fracture toughness of directionally solidified Nb-Ti-Si based ultrahigh temperature alloy

The master alloy ingot with the nominal composition of Nb-20Ti-16Si-6Cr-5Hf-4Al-2B-0.06Y (at%) was prepared using vacuum consumable arc-melting. The integrally directional solidification of the alloy was conducted in a self-made high vacuum and ultrahigh temperature directional solidification furnace with the use of ceramic crucibles at the melt temperature of 2050°C. Room temperature fracture toughness of both arc-melted and integrally directionally solidified specimens have been measured by single-edge notched bending test. The directionally solidified microstructure, fractographs and crack propagation path of single-edge notched bending specimens of the alloy at different solidifying rate V (10, 20 and 50 μm/s) have been investigated by SEM and EDS analyses. The fracture mechanism of the single-edge notched bending test has been discussed. The results show that the integrally directionally solidified microstructure of the alloy is mainly composed of hexagonally cross-sectioned primary (Nb, X) ₅Si ₃ columns and coupled lamellar Nbss/(Nb, X) ₅Si ₃ eutectic colonies both aligned straightly and uprightly along the growth direction (here X represents Ti, Hf and Cr elements, Nbss denotes Nb solid solution). The directional solidification effects are remarkable. Both the average diameter of eutectic cells and lamellar spacing in them decrease with the increase in solidifying rate. The directional alignment of both Nbss and (Nb, X) ₅Si ₃ normal to the notch provides more resistance to the crack initiation and propagation, and therefore the room temperature fracture toughness K _Q is improved significantly by the integrally directional solidification. The maximum K _Q value occurs for the directionally solidified specimens with V=50 μm/s and is about 16.1 MPa · m ^1/2.