Microstructure of Nb–Ti–Cr–Si based ultrahigh temperature alloy processed by integrally directional solidification

Abstract

The Nb–Ti–Cr–Si based ultrahigh temperature alloy was integrally directionally solidified with the using of special ceramic crucibles. The effects of different withdrawing rates on the microstructure and the solidification path in a Nb–Ti–Cr–Si based ultrahigh temperature alloy were investigated. The results showed that the microstructure was composed of primary niobium solid solution (Nbss) dendrites, Nbss/(Nb,X)₅Si₃ (here X represents Ti and Hf elements) eutectic colonies and fine Ti rich phase layer between Nbss dendrites and Nbss/(Nb,X)₅Si₃ eutectic colonies when the withdrawal rates vary from 2·5 to 10 μm s⁻¹. However, the directionally solidified microstructures were composed of primary Nbss dendrites, Nbss/(Nb,X)₅Si₃ eutectic colonies and finer (Ti,Nb)ss/Nb₃Si/Cr₂Nb colonies colonies when the withdrawal rate was reached and exceed than 20 μm s⁻¹. The primary Nbss and Nbss/(Nb,X)₅Si₃ eutectic aligned the growth direction when the withdrawal rates vary from 2·5 to 20 μm s⁻¹; however, the discontinuous (Nb,X)₅Si₃ plates were present when the withdrawal rates vary from 50 to 100 μm s⁻¹. The solidification path was studied according to the microstructure of solid/liquid interface.