\Dependency of microstructure and microhardness on withdrawal rate of Ti-43Al-2Cr-2Nb alloy prepared by electromagnetic cold crucible directional solidification

The intermetallic Ti-43Al-2Cr-2Nb (at.%) alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates (V) ranging from 0.2 to 1.0 mm.min(-1), at a constant temperature gradients (G=18 K.mm(-1)). Macrostructures of the alloy were observed by optical microscopy. Microstructures of the alloy were characterized by scanning electron microscopy (SEM) in back-scattered electron mode and transmission electron microscopy. Results showed that morphologies of macrostructure depend greatly on the applied withdrawal rate. Continuous columnar grains can be obtained under slow withdrawal rates ranging from 0.2 to 0.6 mm. min(-1). The microstructure of the alloy was composed of alpha(2)/gamma lamellar structures and a small number of mixtures of B2 phases and blocky. phases. The columnar grain size (d) and interlamellar spacing (lambda) decrease with an increasing withdrawal rate. The effect of withdrawal rate on microhardness was also investigated. The microhardness of the directionally solidified Ti-43Al-2Cr-2Nb alloy increases with an increase in withdrawal rate. This is mainly attributed to the increase of B2 and alpha(2) phases as well as the refinement of lamellae.