Microstructure and microhardness of directionally solidified and heat-treated Nb-Ti-Si based ultrahigh temperature alloy

Directionally solidified (DS) specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at (1 500 °C, 50 h) and (1 500 °C, 50 h) + (1 100 °C, 50 h), respectively. The results show that the microstructures become uniform, the long and big primary (Nb,X)5Si3 (X represents Ti and Hf elements) plates in the DS specimens are broken into small ones, and the eutectic cells lose their lamellar morphology and their interfaces become blurry after heat-treatment. Meanwhile, the (Nb,X)5Si3 slices in the eutectic cells of the DS specimens coarsen obviously after heat-treatment. Homogenizing and aging treatments could effectively eliminate elemental microsegregation, and the segregation ratios of all elements in niobium solid solution (Nbss) in different regions tend to 1. After heat-treatment, the microhardness of retained eutectic cells increases evidently, and the maximum value reaches HV1 404.57 for the specimen directionally solidified with a withdrawing rate of 100 μm/s and then heat-treated at (1 500 °C, 50 h) + (1 100 °C, 50 h), which is 72.8 % higher than that under DS condition.