EUTECTIC MICROSTRUCTURE EVOLUTION OFDIRECTIONALLY SOLIDIFIED Nb-Ti-Si BASEULTRAHIGH TEMPERATURE ALLOY

Abstract

Nb-Ti-Si base ultrahigh temperature alloys that possess higher melting points, relatively lower densities and attractive high temperature strength have received worldwide attention for their potential applications as next-generation turbine blade materials.In this work,integrally directional solidification of an Nb-Ti-Si base ultrahigh temperature alloy was conducted at different withdrawing rates(2.5,5,10,20,50 and 100μm/s) with a constant melt temperature of 2000℃.Effect of solidifying rate on the integrally directionally solidified eutectic microstructure and solid/liquid interface morphology of this alloy has been investigated by XRD,SEM and EDS,and its directional solidification behavior has been discussed.The results show that the directionally solidified microstructure is mainly composed of petal-like Nb_(ss)/α(Nb,X)_5Si_3 eutectic colonies(EutecticⅠ) and coupled grown lamellar Nb_(ss)/γ(Nb,X)_5Si_3 eutectic(EutecticⅡ) which distributed in the intercellular area.The EutecticⅠand EutecticⅡare both aligned straight and uprightly along the growth direction. When the solidifying rate increases from 2.5μm/s to 100μm/s,the microstructure becomes finer and finer,and the petal-like eutectic colonies evolve from round morphology to tetragonal morphology. Either silicides or fine eutectics locate in the centers of round eutectic cells,while cross-like Nb_(ss) locates in the centers of tetragonal eutectic cells.EutecticⅡexhibits a well-aligned lamellar structure on longitudinal-section.The solid/liquid interface of the alloy undergoes an evolution from cellular dendrite,dendrite and finally to cellular dendrite morphologies.