Investigation On Microstructure Evolution And Micro-mechanical Properties Of Directionally Solidified Sn-9at.%Co Peritectic Alloy

Sn-Co peritectic alloy is an ideal lead-free solder,and a peritectic reaction(L+CoSn?CoSn₂)occurs during solidification.The growth behavior of CoSn and CoSn₂intermetallic compound phases generated during solidification is worthy of further study.Researchers have performed thermodynamic assessments and first-principle calculations on the Co-Sn system and summarized the phase diagram of Co-Sn binary alloy.However,the research on Sn-Co peritectic alloy is still very rare.Since there is almost no systematic research and report on the solidification characteristics of Sn-Co peritectic alloy,the application and development of Sn-Co peritectic alloy are greatly limited.In this work,directional solidification method was used to investigate the growth characteristics and mechanical properties of intermetallic compounds during solidification of Sn-9at.%Co peritectic alloy,the main research contents are as follows:(1)In this work,the transition of the leading phase during solidification is observed in all samples.By the application of the Interface Response Function,the calculated growth velocity range for the transition of the primary phase by the peritectic phase ranges from 0.501?m/s to 398?m/s.Moreover,the phase transition of CoSn and CoSn₂intermetallic compounds does not initiate from the initial growth interface,and the distance required for the occurrence of the phase transition is found to increase with increasing growth velocity;(2)The three-dimensional morphologies of CoSn and CoSn₂intermetallic compound phase was observed in the directional solidified samples after deep etching.It was found that although the two phases are both intermetallic compound phases with nil solubility,their growth behaviors are quite different.After calculating the dissolution entropy(?S^SL),it was found that the dissolution entropy of the CoSn₂phase was lower,so the CoSn₂phase tended to grow on a non-faceted way.In addition,the metastable CoSn₄phase was found in the directionally solidified Sn-9at.%Co peritectic alloy samples after deep etching.Combined with the three-dimensional morphologies of the metastable CoSn₄phase in the directional solidification samples,it is found that the growth of the metastable CoSn₄phase requires a high cooling velocity and a high Sn concentration in the melt.The metastable CoSn₄phase mainly grows up in a spiral way,which usually exhibits the laminated plate-like morphology or micro-columnar morphology perpendicular to the plate.In addition,there is a specific orientation relationship between the CoSn₂phase and the CoSn₄phase;(3)The continuous stiffness measurement(CSM)technique of nanoindentation was used to measure the hardness and Young's modulus of the two phases.It is found that the Young's modulus of the two phases is almost constant while the hardness increases with increasing growth velocity.The hardness of the CoSn₂phase is lower than that of the CoSn phase.