Effects of heat treatments on the microstructure of Nb-Si based in-situ composites

The master alloy ingot was prepared by vacuum consumable arc melting. Heat treatments were carried out at 1400, 1450, 1500 and 1550°C for 24 hours, respectively. Both the arc-melted and heat-treated microstructures were investigated by optical microscope (OM), scanning electron microscopy (SEM), X-ray energy dispersive spectrum (EDS) and X-ray diffractometer. The results show that the heat-treated microstructure is still composed of (Nb, Ti)ss and (Nb, Ti)₅Si₃. With increase of temperatures for heat treatment, the hexagonal and strip-like (Nb, Ti)₅Si₃ blocks break gradually into small ones, and the (Nb, Ti)₅Si₃ slices in the eutectic microstructure change into particles. After heat treatment at 1450°C, the average radius of the sphere-like (Nb, Ti)₅Si₃ particles is the smallest one about 5 μm. After heat treatment at 1500°C, the microstructure is slightly coarser and several holes appear in the microstructure. Compared with the (Nb, Ti)₅Si₃ slices in the arc-melted eutectic colonies, the Nb content increase but the Ti content decrease in the spheroidized (Nb, Ti)₅Si₃ particles after heat treatments. The optimum heat treatment process is 1450°C for 24 h for the composite materials.