Formation and evolution of the non-dendritic morphology in undercooling melt with lower shearing rate

With the development of the semisolid metal processing technology, many efforts have been made to understand the relationship of the microstructural evolution with the shear rate and the melt undercooling. The globular and rosette microstructures can be obtained under the different shearing rates and melt undercooling. Since the microstructure evolves rapidly under the general high shearing rate in the previous researches, it is difficult to reveal the formation process of semisolid microstructure in detail. In this work, the evolutions of globular and rosette crystals were studies by using a transparent organic alloy Succinonitrile-5%H₂O (molar fraction) with a small shearing rate. It is found that there is a critical value of the shearing rate in the semisolid process above which the morphology of the globular crystal could be stable and below which it would be destabilized to form equiaxed dendrite. In addition, when the interface lost the stability, the globular crystals will evolve to dendrite under a higher melt undercooling and to rosette under a lower undercooling. The continuous splitting of the tip of the perturbation cells in the globular crystal induces the formation of the rosette morphology.