Growth velocities and growth orientations in an undercooled melt of Ni₃₁Si₁₂ intermetallic compound

The melt of Ni₃₁Si₁₂ intermetallic compound was undercooled by a melt fluxing technique. The recalescence processes were in-situ observed by a high-speed camera and the solidification microstructures were analyzed by electron back-scattering diffraction. A sharp increase of the growth velocity was found at a critical undercooling ΔT^*≈128 K and a second β₂-Ni₃Si intermetallic compound was observed around the primary Ni₃₁Si₁₂ intermetallic compound when the undercooling ΔT>119 K, indicating that the Ni₃₁Si₁₂ intermetallic compound is actually not stoichiometric. With the increase of undercooling, the microstructure changes from coarse striped grains, coarse dendrite to refined equiaxed grains, while the growth orientation transits from <112¯0>, mixed <112¯0>&<11¯00> to <11¯00>. Fragmentation of <112¯0>-oriented dendrites with the reduction of interface energy and solute diffusion as its driving forces, is suggested to be the origin of grain refinement. Since fragmentation happens adequately upon the transition of growth orientation, no experimental evidences can be found for the formation of seaweed dendrite, which if occurs should not be an intermediate for grain refinement. The current work is not only helpful for understanding non-equilibrium solidification but also useful for controlling microstructures of intermetallic compounds.