The motion and growth behaviors of nucleuses in Al melt solidified under supergravity condition—molecular dynamics simulation

Abstract

Abstract That supergravity can refine grains is verified in many materials. However, the underlying mechanism is still an open question. Although some convincing theories have been proposed, including the ‘crystal rain’ theory and the dendrite fragmentation theory, there is a lack of solid evidence, especially from the atomic scale. Based on the presetting nucleuses method, this study investigates the motion and growth behaviors of nucleuses during the solidification process of Al melt under supergravity condition with molecular dynamics simulation. It is found that supergravity builds a gradient pressure in the samples along the direction of supergravity, and the gradient pressure results in the gradient distribution of sample density. The preset nucleuses move directionally along the direction of supergravity forming ‘crystal rain’, while their directional moving velocity decreases due to the increase of buoyancy, which is caused by the increase of melt density in the motion path of the nucleuses. The supergravity-induced pressure not only decreases the critical size of nucleuses but also increases the growth velocity of nucleuses. The research results also indicate that larger nucleuses grow much faster than smaller ones at the same pressure. Owing to the gradient distribution of pressure, the nucleuses grow much faster along the direction of supergravity than other directions and evolve into an ‘inverted cone’ shape. Therefore, these findings show that supergravity can change the nucleation, motion and growth of nucleuses by establishing a gradient pressure in the melt, thus affecting the microstructure of the casting. Our results provide solid support for the ‘crystal rain’ theory and the nucleation rate rising theory from atomic scale.