Melting And Solidification Behaviour In The Mushy Zone Of Directionally Solidified Sn-Ni Peritectic Alloy

In this study, Sn-Ni peritectic alloys exhibiting peritectic reactionL+Ni3Sn2Ni3Sn4in which the primary Ni3Sn2phase and peritectic Ni3Sn4phaseare intermetallic compounds with narrow solubility, have been chosen forinvestigation. The melting and solidification process in the stationary mushy zonesand moving mushy zones of Sn-Ni peritectic alloys are studied in steep temperaturegradient through Bridgman-type directional solidification apparatus.During the thermal stabilization of Sn-36at.%Ni peritectic alloys, a stationarymushy zone where the solid and liquid coexist is formed between the non-moltensolid zone and complete liquid zone. This stationary mushy zone can be dividedinto the primary stationary mushy zone and peritectic stationary mushy zone. Theinterface between the former stationary mushy zone and the complete liquid zone isthe initial solid/liquid interface of directional solidification. With the increase of thetime of thermal stabilization of stationary mushy zone, the volume fraction of liquidin the mushy zones gradually decreases, and the solid phases arrange parallel to thedirection of temperature gradient. Meanwhile, the initial solid/liquid interfacemoves downward to positions with lower temperatures and becomes planargradually. The primary phase/peritectic phase interface at the peritectic reactiontemperature TPevolves from “flat”�'“zigzag”�'“flat”. The reason is thattemperature gradient lead to the TGZM(temperature gradient zone melting)mechanism, thus melting/solidification occur at different positions in the mushyzone.The solute concentration in liquid ahead of the initial solid/liquid interface ofSn-36at.%Ni peritectic alloys is calculated based on the law of conservation ofsolute. Comparison with other alloy systems shows that the difference in soluteconcentration between the complete liquid zone and the concentration of alloy isrelatively larger in Sn-Ni peritectic alloy. This can be attributed to relatively largerfreezing range and difference in solute concentration between solid phases andliquid of Sn-Ni peritectic alloy. In addition, as compared to intermetalliccompounds with nil solubility, when the solubility of both primary and peritecticphases are taken into consideration, the calculation is more close to theexperimental results. Moreover, during the thermal stabilization of Sn-36at.%Niperitectic alloys, a solute Ni depleted zone is formed in liquid ahead of thesolid/liquid interface. With the increase of thermal stabilization time, this depletedzone disappears due to solutal convection, and distribution of solute concentration in liquid ahead of the solid/liquid interface is more and more uniform.The influence of thermal stabilization on following directional solidificationmicrostructure is investigated in Sn-36at.%Ni peritectic alloys. The results showthat: for Sn-36at.%Ni peritectic alloys, the initial solid/liquid interface ofdirectional solidification is not planar in the range of common thermal stabilizationtime. Besides, the solidification sequence and growth mechanism of the phases doesnot change. At the growth rate of1μm/s, if depletion/enrichment of Ni/Sn atoms aremore obvious, dendrite morphology is more developed, and the microstructure ismore ordered. At the growth rate of10μm/s, if depletion/enrichment of Ni/Sn atomsare more obvious, dendrite morphology is more developed, and the microstructureis less ordered.Obvious coarsening phenomenon can be observed in the moving mushy zonesof directionally solidified Sn-36at.%Ni peritectic alloy; meanwhile, the restrictionon coarsening process by peritectic reaction has also been found. A coarseningmodel which takes into account of both peritectic reaction and coarsening process isproposed. And this model can well describe the coarsening process in peritecticalloy. The secondary dendrite arm spacing λ2and the specific surface of dendritesSVare used to characterize the coupling effects of above factors. Comparisonbetween calculation and experimental results shows that SVis more applicable tocharacterize the melting/solidification process as compared with λ2.Simultaneously, the melting/solidification process caused by the TGZMmechanism which is induced by temperature gradient can also be observed. Besides,in the presence of the tertiary dendrite arms, a “sawtooth” like morphology can befound on the secondary dendrites. This morphology is caused by different soluteconcentration on secondary dendrites with and without tertiary dendrites. A modelis proposed to describe this morphology, and can well characterize this “sawtooth”morphology. This morphology is more obvious at higher growth rate and morecomplete peritectic reaction.The “divorced” peritectic reaction can be observed in the mushy zone due to themelting/solidification in the mushy zone. A thermodynamic model considering thecoupling influences of peritectic reaction, Gibbs-Thomson effect and TGZM effectis proposed to interpret this “divorced” peritectic reaction. On the basis of thismodel, based on the law of conservation of solute, a model is proposed to describemicrosegregation between secondary dendrite arms, and the effects of the solidback-diffusion, peritectic reaction, Gibbs-Thomson effect and TGZM effect are alltaken into consideration. Comparison between calculation and experimental resultsshows that microsegregation between secondary dendrite arms can be reasonably interpreted through this model. Besides, the melting/solidification process inducedby the TGZM effect is dominant in presence of a steep temperature gradient. In thiscase, the retard of coarsening process by peritectic reaction is restricted. The rangeof the reaction constant f characterizing the completeness of peritectic reaction isnot constant but ranges from0.3to0.7with increasing growth rates. The rangedetermined in the present work is larger than that determined without considerationof the TGZM effect while the scopes of them are close to each other.The transfer of the leading phase is found in the moving mushy zones of Sn-22at.%Ni peritectic alloy. The Ni3Sn2phase which is the leading phase dudringequilibrium solidification is replaced by peritectic Ni3Sn4phase. A model isproposed to describe the solute redistribution during planar solidification ofintermetallic phase with narrow solubility. Transfer of leading phase discussedabove can be explained reasonably well with this model. It is found that the soluteconcentration in liquid ahead of the solid/liquid interface decreases polynominallywith the increase of solidification distance, which is distinct from solid solutionphases and intermetallic phases with nil solubility. Besides, as the initialconcentration of alloy can not be the same as concentration of intermetallic phasewith narrow solubility, steady-state boundary layer will not exist in the liquid aheadof the solid/liquid interface.