双辊薄带连铸凝固过程中柱状晶向等轴晶转变的广义稳定性判据

Generalized stability criterion for columnar to equiaxed grain transition during solidification upon vertical twin roll casting

为促进柱状晶向等轴晶转变(CET)获得均匀的等轴晶,大多数有效的理论聚焦于生长而忽略了形核的影响。为此,构建耦合细化剂异质形核和枝晶生长的理论框架,采用考虑双辊薄带连铸(VTC)凝固过程中热力学驱动力(?G)和动力学能垒(Q)的广义稳定性(GS)新概念,研究CET位置、晶粒半径和力学性能。细化剂的加入,尤其是Al-5Ti-1B细化剂,通过增加形核的?G和生长的GS使晶粒半径细化到19μm,柱状枝晶的厚度增大到5~6 mm,同时提高强度和塑性。遵循高?G-高GS准则,细化剂直径优化为0.3μm。

To promote the columnar to equiaxed grain transition(CET)to obtain uniform equiaxed grain,most of the available theoretical works mainly focus on the growth but ignore the nucleation.Regarding it,a systematical framework integrating nucleation catalyzed by refiners with dendritic growth,was constructed,where,a new conception of generalized stability(GS),considering evolving thermodynamic driving force(∆G)and kinetic energy barrier(Q)upon solidification of VTC,was adopted to study the CET position,the grain radius and the mechanical properties.By adding the refiners,especially the Al-5Ti-1B refiner,the larger thickness of 5-6 mm for equiaxed grain and finer grain radius of 19μm are exhibited by increasing the∆G for nucleation and the GS for growth,possessing simultaneously improved strength and plasticity.Following a criterion of high∆G-high GS,the refiner diameter is optimized to be 0.3μm.