Effects of homogenizing and aging treatments on the microstructure and microhardness of an Nb-silicide based ultrahigh temperature alloy

To investigate the effects of homogenizing and aging treatments on the microstructure and microhardness of an Nb-silicide based ultrahigh temperature alloy, coupons were homogenized at 1 300 - 1 600°C for 50 h, and then aged at 1 100 °C for 50 h. The results show that the phase constituents of the heat-treated alloys are still Nb solid solution and silicides, a-(Nb,X)5Si3 phase forms after heat-treatments at temperatures above 1 500 °C. With the increase in heat-treatment temperature, the original big polygonal and plate-like silicides gradually break into small ones, and the amounts of retained typical lamellar and rod-like eutectic colonies reduce clearly. The variation of partitioning ratios of alloying elements between Nb solid solution and silicides causes the change in microhardness of the alloy. 1 500 °C/50 h + 1 100 °C/50 h is an optimum heat-treatment process.