过冷熔体中球晶组织的形成规律

Formation mechanism of spherical particles in undercooled melt

研究LSPSF工艺条件下Al-20%(质量分数)Cu合金的初生α(Al)的形貌演变,结合微观组织模拟技术和M-S界面形态稳定性理论,分析过冷熔体中球晶组织快速形成的基本规律。结果表明:合金熔体中自由晶数目和浆料冷却速度共同决定初生固相的尺寸和形态;最早(凝固发生3 s内)观察到的初生α(Al)呈细小球形,并在整个演变过程中始终保持球形不变,没有枝晶组织出现;晶粒周围溶质扩散层的叠加可明显降低固?液界面前沿的浓度梯度,提高固?液界面的稳定性;均匀化晶粒周围溶质分布,抑制晶粒的择优生长,促进初生α(Al)保持球形;由溶质扩散层的叠加和界面能引起的抑制和粗化效应可促使失稳的等轴晶粗化成球晶。

The microstructure evolution of Al-20%（mass fraction）Cu alloy under LSPSF （Low superheat pouring with a shear field） conditions was investigated. The quick formation mechanism of spherical particles in an undercooled melt was discussed by microstructure simulation technique and M-S interface stability theory. The results show that the morphology of primary a（Al） is determined by both the number of free crystals and the cooling intensity of melt. The first observable primary a（Al） of Al-20%Cu alloy is spherical shape, and grows spherically in subsequently lower cooling. The overlapping diffusion fields from adjacent growing crystals can induce a stabilizing effect on the morphological instability at the solid-liquid interface and promote the globular growth of primary a（Al）. Restraining and coarsening caused by overlapping diffusion fields and Gibbs-Thomson effect are main dynamic conditions that promote the morphology transition of instabilized crystal from equiaxed shape into spherical shape.