Effect of high temperature treatments on microstructure of Nb-Ti-Cr-Si based ultrahigh temperature alloy

To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy, coupons were homogenized at 1 200-1 500 °C for 24 h, and then aged at 1 000 °C for 24 h. The results show that the heat-treated alloy is composed of Nb solid solution (Nbss), (Nb,X)5Si3 and Cr2Nb phases. With the increase of heat-treatment temperature, previous Nbss dendrites transformed into equiaxed grains, and petal-like Nbss/(Nb,X)5Si3 eutectic colonies gradually changed into small (Nb,X)5Si3 particles distributed in Nbss matrix. A drastic change occurred in the morphology of the Laves phase after homogenizing treatment. Previously coarse Cr2Nb blocks dissolved during homogenizing at temperature above 1 300 °C, and then much finer and crowded Cr2Nb flakes precipitated in the Nbss matrix in cooling. Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti, Hf and Al in Nbss, (Nb,X)5Si3 and Cr2Nb phases reduced.