The Solidification Behavior Of Undercooled Co-xat.%Sn Alloys

Co-Sn alloys(20at.%Sn?22at,%Sn?24at.%Sn?26at.%Sn?28at.%Sn)were undercooled by the molten glass fluxing technique in combination with cyclical superheating methods,and the thermal history was monitored in real-time by means of high-precision double-color infrared thermometer during the melting and solidification of Co-Sn alloys in the undercooled process.The competitive nucleation behavior of undercooled Co-24at.%Sn eutectic alloy was analyzed by classical nucleation theory along with transient nucleation theory,and the phase composition of undercooled Co-Sn alloys with different undercooling was analyzed by X-ray diffraction and EDS,and the law of phase selection of undercooled Co-24at.%Sn eutectic alloy was explored by competitive nucleation theory combined with phase analysis.The microstructure evolution of undercooled Co-Sn alloys was discussed by metallographic microscope and scanning electron microscope,and crystal growth behavior of undercooled Co-24at.%Sn eutectic alloy was discussed by BCT model and J-H theory.The mechanism formation of irregular eutectic of undercooled Co-24at.%Sn eutectic alloy was discussed in detail by microstructure analysis and theoretical calculation,and the eutectic symbiosis zone of Co-Sn alloy was determined based on the microstructural analysis of Co-Sn alloy solidified at different compositions and undercooling degree in this work.The solidification behavior of undercooled Co-Sn alloy was systematically described in this work.The results indicated that:The undercooled Co-Sn alloys were realized by molten glass purification combinated with cyclic overheating technique.When the tin content is 20at.%,22at.%,24at.%,26at.%and 28at.%,the maximum undercooling degree of Co-Sn alloys is 225K?190K?265K?206K and 228K,respectively.There are only ?-Co phase and ?-Co3Sn2 phase in the microstructure of undercooled Co-Sn alloys,and no other metastable phases are found in the obtained range of undercooling degree.The nucleation theory and growth model calculation show that ?-Co3Sn2 phase preferrs to nucleate for undercooled Co-24at.%Sn eutectic alloy in the obtained range of undercooling degree,and a-Co phase and ?-Co3Sn2 phase are coupled to grow into regular lamellar eutectic.The regular lamellar eutectic gradually transforms into irregular eutectic with increase of undercooling,and the critical undercooling degree at the beginning and completion of irregular eutectic transformation are 53K and 228K,respectively.The calculation results of BCT model show that the component supercooling has always been dominant among the four kinds of undercoolings.The microstructure evolution of hypoeutectic alloy and hypereutectic alloy is similar.The primary phase is first precipitated in the melt with decrease of temperature,and then the lamellar eutectic is formed around the primary phase.The primary phase and lamellar eutectic gradually decrease until disappearance with increase of the undercooling degree,and the solidification structure gradually transforms into irregular eutectic.The formation mechanisms of irregular eutectic are as follows.Firstly,the solidification stress produced during the rapid solidification extruded the eutectic lamellar in the adjacent grains,which result in dislocation and fragmentation of the eutectic lamellae.At the same time,a large amount of latent heat released from the undercooled alloy melt during rapid solidification results in the eutectic lamellae remelting.Determination of Co-Sn eutectic alloy eutectic symbiosis zone:both hypo-eutectic alloy and hyper-eutectic alloy have primary phase at low undercooling degree.The primary crystal disappears with increase of undercooling degree,and the alloy enters eutectic symbiosis zone.When the tin content is 20at.%,22at.%,24at.%,26at.%and 28at.%,the undercooling degree of Co-Sn alloys entering eutectic symbiotic zone are 175K?102K?OK?96K and 202K,respectively.