材料期刊网

采用类金属丁二腈 (SCN) 和H₂O配制的SCN--5%H₂O (摩尔分数) 透明合金研究了不同过冷度和搅拌速率下半固态球晶生长的形态稳定性. 结果表明,随着搅拌速率的提高, 球晶形成的孕育期急剧减小. 在低搅拌速率下, 随着过冷度的提高, 球晶形成的孕育期明 显减小; 而在高搅拌速率下, 过冷度的作用并不明显. 随着搅拌速率的增大, 球晶的固相分数先增加、后减小. 当搅拌速率增加到一定值后, 球晶组织将完全消失. 球--枝转变存在一个临界的过冷度. 采用高搅拌速率得到稳定的孤立球晶后停止搅拌并保温, 可以发现当过冷度为2.9℃时,球晶可以一直生长到100 μm以上时才失去相对稳定性, 并发生球--枝转化; 但过冷度为1.9℃时, 球晶生长到一定尺寸后将不再长大, 而且这个尺寸远远小于100 μm. 根据这些生长行为, 可以通过控制搅拌速率和过冷度从而优化半固态过程的凝固组织形态.

With the intensive development of semisolid metal processing technology, an important near--net--shape processing technology, the interaction between melt flow and solidification microstructure becomes gradually one of the important fundamental research fields in materials science. The most important characteristic of semisolid processing is as follows: the solidification microstructure changes markedly under either mechanical stirring or electromagnetic stirring, from dendritic growth under traditional conditions to non--dendritic or globular growth. However, understanding and modeling of the nucleation and crystal growth during semisolid solidification are more difficult than in conventional casting processes due to complicated effects of strong convection. Hence, to date, the formation mechanism of this kind of globular microstructure has not yet been much studied. In the present work, morphological stability of globular crystal was experimentally studied using a succinonitrile--5%H₂O (molar fraction) transparent alloy under different undercoolings and stirring rates. Succinonitrile--5%H₂O transparent alloy was heated to 55℃ (5.1℃ above the liquidus temperature) and held for\linebreak 30 min. The melt was then cooled to a temperature below the liquidus temperature at a cooling rate of 0.1℃/min and a series of stirring rates.  In~situ observation was performed using a stereomicroscope and JVC video camera. The results show that the incubation time for the formation of globular crystal decreases rapidly with the increase of stirring rate. When the stirring rate is low, the incubation time for the formation of globular crystal decreases obviously with the increase of undercooling. When the stirring rate is high, the effect of undercooling on the incubation time for the formation of globular crystal is weak. With the increase of stirring rate, the solid fraction of globular crystal increases at first, and then decreases. When the stirring rate increases to a certain value, the globular crystal will completely disappear. There is a critical undercooling for the transition of growth behaviour of globular crystals. Under the present experimental condition, when the undercooling is larger than the critical value, the size of globular crystal can increase to above 100 μm without globular/dendritic transition. But when the undercooling is less than the critical value, globular crystal will grow to a definite size much smaller than 100 μm. According to the growth behaviors of globular crystal, the semi--solid microstructure could be refined well under an optimized stirring rate and undercooling.