Microstructure evolution and preferred growth direction of laves phase Cr₂Nb-20Ti alloy during directional solidification

The Cr₂Nb-20Ti (atomic fraction) prepared by directional solidification with the withdrawal rates of 5, 10, 20, 100 μm/s were investigated to understand the microstructure evolution of the Laves phase Cr₂Nb/Ti alloy. OM, XRD, and SEM apparatus were employed to determine the solidified phases and microstructure. The results indicate that the microstructures of directionally solidified Cr₂Nb-20Ti alloy are all consisted of C15-Cr₂Nb dendrite and interdendritic β-Ti. The formation of metastable β-Ti instead of stable α-Ti is induced by the stabilization of Cr element. With the increase in withdrawal rate and perturbation at the liquid/solid interface, the C15-Cr₂Nb dendrites are refined and the dendritic morphology evolves from the petal-like pattern at 5 μm/s to the polygon-like pattern at 100 μm/s; simultaneously, the growth direction of the C15-Cr₂Nb dendrite gradually deviates from the heat flow direction and transits to the preferred growth direction. From the XRD experimental result, it shows that the C15-Cr₂Nb dendrite exhibits the (220) preferred growth plane due to the fact that the plane (220) has the maximal bare bond energy, leading to the fast growth kinetic and elimination of other growth planes.