Microstructure evolution of Ti44Al6Nb alloy directionally solidified with cold crucible

Abstract

Ti44Al6Nb (atomic fraction, %) ingots with industry size were directionally solidified without contamination with electromagnetic cold crucible. Effects of pulling rates and powers on the surface quality, the S/L interface, grain morphology and phase selection were studied. The results show that the surface quality are improved by decreasing pulling rate or increasing power, they influence the surface quality by changing the superheat degree of the melt or the volume of the mushy zone. The S/L interface becomes concave and the grain width becomes big with increasing of pulling rate, the columnar grains grow discontinuously when the pulling rate is increased. The grain width decreases with the increase of the power. Columnar grain to equiaxed grain transition (CET) is easy to occurred when the pulling rate is lower or the power is higher. At the beginning of solidification, the initial phase is α phase when the pulling rate is 0.5 mm/min, whereas, it is β phase when the pulling rates are others.