Investigation On Microstructure Evolution And Mechanical Properties Of Directionally Solidified Cu-68at.%Sn Peritectic Alloy

Peritectic alloys are frequently encountered in structural materials and functional materials.At present,most of the investigations on the peritectic alloy are focused on the solid solution phases which have wide solubility ranges,and there is limited data on the growth characteristics of the intermetallic compounds with nil or narrow solubility ranges.However,the solubility range of the peritectic phase in some advanced functional materials is narrow or nil.Therefore,the study of the peritectic alloy system in which the the peritectic phases have nil or narrow solubility ranges is of great theoretical and practical value for enriching the solidification theory of the peritectic alloy and preparing advanced functional materials with the characteristics of the peritectic reaction.In this work,the Cu-68at.%Sn hyperperitectic alloy in which the primary Cu₃Sn phase and peritectic Cu₆Sn₅phase are intermetallic compounds with narrow solubility ranges is selected.The Bridgman-type directional solidification apparatus is used to investigate the microstructure evolution and phase selection of Cu-Sn alloy at different growth velocities.Meanwhile,the growth characteristics and mechanical properties of Cu₃Sn phase and Cu₆Sn₅phase are analyzed.At the intermediate velocities(5,10 and 20?m/s),the peritectic Cu₆Sn₅phase transforms into the kinetically stable phase and will precipitate from the melt firstly.The competitive growth of Cu₃Sn phase and Cu₆Sn₅phase is controlled by the highest temperature growth criterion,the calculated velocity range for the transition of the primary phase into the peritectic phase is 2.1 to 33.4?m/s by the application of the Interface Response Function.With the increasing growth velocity,the phase selection of the directionally solidified Cu-68at.%Sn alloys in this work follows the sequence of cellular growth of primary Cu₃Sn phase?dendritic growth of peritectic Cu₆Sn₅phase?cellular growth of primary Cu₃Sn phase.Different from the growth behavior of the solid solution phase when the phase transition occurs,the kinetically stable phase Cu₆Sn₅does not initiates from the initial growth interface.A certain freezing distance of Cu₃Sn phase is required for phase transition to occur,and this distance is found to increase with increasing growth velocity.Not only the interface temperature of peritectic phase should higher than primary phase,but also the liquid composition at the S/L interface reachesC_LP,can the phase transition of intermetallic compounds Cu₃Sn and Cu₆Sn₅whose solubilities are narrow occur.The peritectic Cu₆Sn₅phase exhibits a dendritic morphology after phase transition,which is related to the temperature dependence of the entropy of solution?S^?L.There are three substructures for the Cu₆Sn₅phase after deep etching:round scallop shape substructure,long prismatic substructure and fiber substructure.The round scallop shape substructure is formed when the Cu₆Sn₅phase is directly precipitated from the melt.The long prismatic substructure evolved from the scallop substructure.The fiber substructure is formed through the eutectic reaction.It is found that Cu₃Sn phase and Cu₆Sn₅phase exhibit creep behaviors during the nanoindentation experiment after the analysis of the load-displacement curves,and multiple displacement bursts are observed when the strain rate is 0.1 s^-1for Cu₆Sn₅phase.By the application of the continuous stiffness measurement,the hardness are measured to be 5.66-6.69 GPa for Cu₃Sn phase,6.14-6.79 GPa for Cu₆Sn₅phase.Corresponding Young's modulus are 117.11?3.52 GPa for Cu₃Sn phase,105.64?3.5GPa for Cu₆Sn₅phase.There is a positive correlation between the hardness and growth velocity,while modulus remains unchanged.The hardness of Cu₃Sn phase is lower than that of Cu₆Sn₅phase but Young's modulus shows reverse trend at the same growth velocity.