Microstructure and phase selection in directional solidification of Co-Sb alloy

The compound CoSb₃is one kind of thermoelectric material that has been received more attention due to its potential application in green refrigeration and power generation. The general way to prepare CoSb₃material is sintering and the solidification behavior of CoSb₃compound is rarely reported, since this compound is obtained through the peritectic reaction, and there exists phase competitive growth in solidification process. In this work, Bridgman directional solidification and laser rapid solidification experiments on Co-93.0%Sb (mass fraction) alloy were carried out. XRD, SEM and EDS were employed to determine the solidified phases and characterize the microstructure. The results showed that for Bridgman directional solidification, the solidification microstructure of Co-93.0%Sb alloy contained only the CoSb₃and Sb phases at the solidification rates of 2 and 5 μm/s, whereas at the solidification rates of 20, 50, 100 and 500 μm/s, the microstructure contained CoSb3, CoSb₂ and Sb phases. Furthermore, the volume fraction of CoSb₃phase decreased with increasing solidification rate. For laser rapid solidification, the solidification microstructure consisted of CoSb₃, CoSb₂ and Sb phases at the scanning rate of 5 mm/s. As the scanning rate ranging from 10 to 50 mm/s, the microstructure is composed of only CoSb₃ and Sb phases. The critical rate of peritectic phase CoSb₃instead of primary CoSb₂ solidified directly from the melt was theoretically predicted to 7.61 mm/s, agreeing well with the experiment result. In addition, the formation mechanisms of peritectic phase CoSb₃ at Bridgeman directional solidification and laser rapid solidification were analysed. The formation of peritectic phase at low solidification rates was due to the local solidification time available for the peritectic reaction, and at high solidification rates higher than 10 mm/s the peritectic phase CoSb₃ was obtained directly from the melt. Therefore to obtain a large volume fraction of peritectic phase CoSb₃, a low solidification rate is recommended.