Microstructure And Lamellar Orientation During Directional Solidification Of Ti-50Al-5.6Nb-0.2B Alloy

Due to the low density, specific strength and execellent corrosion resistance,γ-TiAl-based alloys have become the most potential structural materials in theaerospace and automotive industries. Besides, the optimal mechanical properities inroom-temperature and high temperature can be achieved when the loading stress areparrelleled to the lamellar orientation. Now directional solidification with seedingtechnique or coercion of solidication routes are the common ways to achieve acolumnar grain material with the lamellar orientation aligned parallel to the growthdirection. And in this paper the alloy contents with addition of Nb element, one kindof γ-Ti stable alloy, was measured to control the primary phase. Nb is an essentialelement to improve the high-temperature strength, oxidation and creep resistancewith contens varying from5to10at.%. But the recent research shows that theeffects on TiAl binary diagrams and solidification routes are changed over thecontents of Nb, which certainly leads to the diversity of microstructures and lamellaroritations. Presently, little effort is done to research the microstructures and lamellaroritations of alloys with high contents of Al and Nb, and the evolution with varyingof solidification conditions is not clear either.Taking into account of the above factors, a series of directional solidificationexperiments and lamellar orientation research work have been carried out forTi-50Al-5.6Nb-0.2B alloy. In this paper, the evolution of structures and orientationof different lamellars with the growth rates, while, the effects of different ratio sizeon the solidification routes and microstructures have been talked. In addition, theeffects of growth rates on the oritation of lamellar and initial phase were alsoinvestigated. The conclusions can be drawn as follows:For Φ3mm solidifying samples, the primary phase was-Ti from1m/s to30m/s; while for Φ10mm samples, the initial phase could be different with thewithdrawing rates (V):-Ti when V<20m/s, and-Ti when V is changing from20m/s to100m/s, which of course leads to the peritectic reaction.The metal alloyNb has been solutionized in-phase when the initial phase is-Ti; while in themicrostructure of solidification via-phase, there has been a large amount ofsegregation rich of Nb due to the transformation, futher more, the volumepercent of segregation was proportional to the growth rate.To refine the lamellar size, alloying element B was added to the alloy. While itturns out to be effective to reduce the lamellar spacing, and the refinement extent is proportional to the growth rate too. During the solidification, B has been rejectedinto the liquid between the dendrites by the formation of TiB and the shape changingfrom neddles/tubes to bending stripes or dots with the increase of V.The results of lamellar orientation shows that the initial-phase growingdirection changes with the growth rate: the oritation turned out to be <331>_β whenV=20m/s, but it changed to be <221>_β at30m/s and <111>_β at50m/s. Inconclusion, the ascending of V leads to the increase of interface anisotropy andpromote the growth in <100>_β of-phase.