The Grain Structure of Castings

9521875 Hellawell这项为期两年的续期是为了完成对金属铸件中等轴晶发展的研究。颗粒凝固的最初来源被认为是在铸件早期糊状区内分离的树枝晶碎片。碎片通过热溶解对流输送到大块液体中。残存的碎片长大、下沉，最终阻塞柱状颗粒。对模拟过程中的步骤进行了较为详细的识别和分析，并认识到促进枝晶臂脱离的条件与瞬时生长条件有关。这些条件反映了铸件中柱状枝晶生长前沿不断减速，温度梯度同时下降的情况。使用透明的类似物和选定的金属合金，对该过程中的这一关键步骤进行了分析和实验研究。%本主题解决了在理解合金铸件微观组织发展方面的一个主要空白。这一复杂的过程应该通过所提出的实验和理论工作来阐明。***

9521875 Hellawell This two-year renewal provides for completion of research on the development of equiaxed grains in metal castings. The initial source for the grain solidification is thought to be dendrite fragments that have become detached within the earlier mushy region of the casting. The fragments are transported out into the bulk liquid by thermo-solutal convection. Surviving fragments grow, sink, and eventually block columnar grains. The steps in modeling the process have been identified and analyzed in some detail, and it is recognized that the conditions that promote dendrite arm detachment are associated with transient growth conditions. These conditions mirror those in a casting, where columnar dendritic growth front is decelerating continuously and the temperature gradient is falling simultaneously. This critical step in the process is examined analytically and experimentally using transparent analogues and selected metal alloys. %%% This topic addresses a major gap in understanding the development of the microstructure of alloy castings. This complex process should be clarified by the experimental and theoretical work proposed. ***