Unraveling The Mechanisms Of Pure Aluminum And Al-Mg Alloy Solidification Under Hyper-gravity Condition From Molecular Dynamics Simulations

Gradient nano-metals can effectively avoid the performance mutation caused by the abrupt change in structural size,thus improving the overall properties and use behavior,due to the gradient distrabution of its structural unit size in space.Centrifugal casting is a preparation method of gradient nano-metals,and the high-speed rotating sample is in a special hyper-gravity environment under centrifugal force.In this paper,we performed molecular dynamics simulations to investigate the melt,nucleation,and grain growth stages of aluminum and Al-Mg alloy under hyper-gravity,discussed the formation mechanisms of the gradient grain tissue and illustrated the relationship between hyper-gravity and microstructure evolution.During the solidification of pure aluminum under hyper-gravity,the hyper-gravity induced gradient pressure is identified as the main factor making the melting point,nucleation,and grain growth unevenly in the model,which leads to the decreased grain size along the hyper-gravity direction.Three solidification behaviors are observed in one solidification,i.e.,homogenous nucleation initiates first at the bottom of the model,where it undergoes the highest pressure;then heterogenous solidification combined with homogenous solidification appear in the middle;finally,directional solidification dominates because the insufficient undercooling makes the nucleation difficult.Moreover,the gradient structure and grain refinement are further enhanced with the increasing gravity intensity,and the metastable body-centered cubic phase can be formed under high pressure.During the solidification of Al-Mg alloy under hyper-gravity,hyper-gravity can effectively refine the solidification structure.In addition to the gradient pressure,hyper-gravity can also cause changes in the composition,which has an important effect on the melt structure and solidification of alloy.With the increase of magnesium content,the icosahedral clusters with geometric frustration effect in space increases,thus inhibiting homogenous nucleation of single phase α-Al.Two solidification behaviors are observed in the solidification,i.e.,homogenous nucleation occurs at the bottom and middle of the model,while heterogenous solidification combined with homogenous solidification appear at the top.Finally,The hyper-gravity induced gradient pressure and element distribution are identified as the main factors making the decreased grain size along the hyper-gravity direction.In the present work,we proposed a novel hyper-gravity loading method and applied it to the solidification of pure aluminum and Al-Mg alloy using molecular dynamics simulation to explain the effect of hyper-gravity on the solidification on the atomic scale.