DEPENDENCE OF PRIMARY PHASE AND ITS GROWTHDIRECTION ON SOLIDIFICATION PROCESS INDIRECTIONALLY SOLIDIFIEDTi-46A1-2Cr-2Nb ALLOY

Abstract

GE alloy Ti-(46一48)Al-2Cr-2Nb(atomic fraction) is well known for its high strength and improved ductility.The primary phase and its growth direction are important in controlling the lamellar direction of GE alloys.However it is greatly affected by solidification conditions.In this work,primary phase and its growth direction have been investigated by carrying out Bridgman-type directional solidification with different growth lengths ranging from 5 to 30 mm on Ti-46Al-2Cr2Nb alloy.It is found that the primary phase is /? at the beginning of directional solidification with constant temperature gradient(G=18 K/mm) and growth rate(^=20 "m/s).With the increase of growth length,A1 gradually concentrates in the liquid between primary dendrites,which leads to the peritectic reaction L+β a.With further increase of the growth length,growth competition between primary(3 phase and peritectic a phase is promoted,leading to gradual transition of primary phases from(3 phase to a phase.The growth direction of primary phase in different stages of solidification has been characterized by EBSD analysis.The results indicate that primary β phase has a growth direction parallel to its preferential growth direction 100β at the initial stage of solidification.By comparing the growth directions of the grains formed from primary β phase and peritectic a phase,it is found that peritectic a phase related to primary β phase by the {110}β /{0001}a orientatiorelationship.Therefore,as the primary phase has transformed to a phase,the growth direction deviatefrom its preferential growth direction 0001a at an angle of 45.9°.The growth direction of phase formed after the primary phase transformation is determined not only by the kinetic factorof solidification,but also by the β phase exiting at the beginning of directional solidification.Thesresults provide fundamental references for understanding and controlling the lamellar orientation oGE alloys.