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)₅Si ₃ 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.