The Columnar to Equiaxed Transitionand Dendrite Fragmentation

The shape and size of dendritic grain structures in engineering alloys is central to the resulting properties of the material. For example, equiaxed grain structures tend to provide more isotropic materials properties then materials with columnar grain morphologies. By contrast, in other applications, such as Ni-base superalloys, columnar, or single crystal grain structures provide superior high temperature or creep properties. It is thus of critical importance to understand and predict the factors affecting the formation of grain morphology during solidification. The processes controlling the formation of equiaxed and columnar grains during solidification will be investigated using two approaches. The first is to measure the grain structures of Al-Si alloys processed by the Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL) team that is sponsored by ESA, and the second is to examine the effects of solidification on the dendrite fragmentation process. By processing these samples on the International Space Station, both sedimentation of the dendritic fragments and buoyancy driven convection can be avoided. This will permit an understanding of the physics underlying these processes to be gained, and enable a careful comparison with simulation.