Microstructurs analysis of directionally solidified Nb Si based composite with a high content of Cr

An advanced ultrahigh temperature Nb-Si based composite with a high content of Cr was prepared by directional solidification. The microstructure and phases were investigated under the condition of different melting temperatures or different solidification rates. The results reveal that the microstructure is composed of Nb_ss, α-Nb₅Si₃, NbCr₂ phases and a small amount of (Nb_ss+ Nb₅Si₃) eutectic. The composites phase composition has not been changed by directional solidification. The Nb_ss phase appears like grain and the Nb₅Si₃ phase is randomly distributed in the block or short lath at 1550 ℃. With the increase of the melting temperature, the Nb_ss phase changes from large grains to dendrites, while the Nb₅Si₃ phase undergoes an evolution from block to batten morphologies. The composite microstructure is fined obviously and the directional effects enhance as the increase of the solidification rates. The excellent directionally solidified microstructure is obtained when the solidifying rate is 18 mm/min, and the Nb₅Si₃ phase is column shape with good orientation.