Rapid Solidification Process Of Liquid Zr And Ti Based Alloys Investigated By Electrostatic Levitation

The rapid solidification mechanism of substantially undercooled metallic materials shows great significance for both theoretical research and industrial application.In this respect,electrostatic levitation(ESL)is an advanced technique to achieve the high undercooling state of high-temperature metallic alloys.This dissertation aims at a systematic study about the liquid state properties,solidification kinetics and microstructure formation mechanisms of binary Zr-Ni,Zr-Co and ternary Ti-Ni-Al alloys,which have been accomplished by ESL technique combined with liquid quenching(LQ),drop tube(DT)and molecular simulation(MD)methods.The achieved major conclusions were summarized as follows:1.Rapid solidification process of substantially undercooled Zr-Ni alloysThe thermophysical properties of liquid ZrNi and Zr2Ni intermetallic alloys together with Zr64Ni36 eutectic alloy were measured by ESL technique.Some inflection points appear on the experimentally measured density and viscosity curves of superheated liquid Zr2Ni alloy,which hinted the occurrence of liquid structure transformation.The solidification process of ZrNi alloy displayed two different pathways during ESL experiments:(1)Metastable phase with B2 crystalline structure primarily nucleated from undercooled liquid alloy when liquid undercooling was less than 105 K,and it subsequently transformed into stable phase with B33 crystalline structure in the solidification process of residual liquid;(2)When the undercooling of liquid ZrNi alloy exceeded 200 K,the stable B33 phase directly formed from the undercooled liquid.With the aid of high-speed photography technique,it was found that the solid/liquid interface morphologies and dendritic growth kinetics of B2 and B33 phases were quite different.The measured crystal growth velocity versus melt undercooling of ZrNi and Zr64Ni36 alloys exhibited power law functions,while it was a parabolic relation for Zr2Ni intermetallic alloys.Theoretical analysis indicated that the crystal growth process of Zr2Ni compound was diffusion-limited.The microstructures of ZrNi and Zr2Ni compounds both showed the"faceted phase→non-faceted phase" transformation with the increase of undercooling.The structural morphology of Zr64Ni36 eutectic alloy transformed from divorced eutectic to anomalous eutectic cluster with the enhancement of undercooling.2.Thermophysical properties of liquid Zr-Co alloys and single-phase growth mechanisms for their intermetallic compoundsThe densities of 14 liquid Zr-Co alloys within the composition range of 22—50 at%Co were measured by ESL experiments.In addition,the surface tensions and viscosities of ZrCo,Zr55Co45 and Zr60Co40 alloy melts were determined by droplet oscillation method.The densities of liquid Zr63CO37 and Zr72Co28 alloys displayed non-linear relationships versus temperature,while they decreased linearly with the increase of temperature for other Zr-Co alloys.The dendritic growth kinetics of ZrCo intermetallic compound within undercooled liquid ZrCo,Zr55Co45 and Zr60Co40 alloys exhibited power law relations versus liquid undercooling.Theoretical analyses indicated that the crystal growth of ZrCo compound within ZrCo alloy was controlled by interface kinetics,whereas it was dominated by solute diffusion within Zr55Co45 and Zr60Co40 alloys.Furthermore,the structural morphology of ZrCo phase in ZrCo alloy transformed from coarse dendrites into equiaxed grains,and it changed from irregular dendrite fragments to uniform spherical grains in Zr55Co45 and Zr60Co40 alloys versus the increased undercooling.3.Phase selection and microstructure evolution of highly undercooled Zr-Co eutectic type alloysThe microstructure formation mechanisms of undercooled liquid Zr65.7CO34.3 and Zr73.7Co26.3 eutectic alloys as well as Zr60Co40 and Zr70Co30 hypereutectic alloys were investigated by the combination of ESL and LQ techniques.Based on this,the rapid solidification mechanisms of 10 different Zr-Co alloys within the composition range of 25—50 at%Co were summarized systematically.Two distinct microstructures were formed during the rapid solidification of Zr65.7Co34.3 eutectic alloy under ESL condition.One was composed of Zr2Co and ZrCo phases,and the other included ZrCo,Zr3Co and Zr2Co phases.Moreover,the three-phase solidification behavior was also found in Zr-Co hypoeutectic and hypereutectic alloys within the composition range of 33.3—50 at%Co during ESL experiments.The microstructures of Zr-Co alloys within the composition range of 25—33.3 at%Co only consisted of Zr2Co and Zr3Co phases.4.Rapid solidification mechanism of ternary Ti-Ni-Al multiphase alloysA power law relationship was found for the kinetic relation between dendrite growth velocity and liquid undercooling of undercooled liquid titanium.The experimentally achieved maximum growth velocity of Ti dendrites was 122 m·s-1 corresponding to 352 K(0.18 Tm)undercooling,which was perhaps the practically attained highest dendrite growth velocity during the rapid solidification process of metallic materials so far.Theoretical calculation indicated that the crystal growth of Ti dendrites was governed by thermal diffusion.MD simulation showed that the dendrite growth velocity of Ti and Zr depended to a large extent on the atomic scale thickness of interface layer.The rapid solidification mechanism of undercooled liquid Ti67.6Ni23.4Al9 alloy could be classified into two types:single recalescence(SR)and double recalescences(DR).During SR mode,the structural morphology of Ti67.6Ni23.4Al9 alloy transformed from divorced eutectic to(β-Ti+Ti2Ni)anomalous eutectic with the increase of undercooling.The microstructure formed under DR condition was composed of β-Ti dendrites and(β-Ti+Ti2Ni)eutectic structures.The primary τ4-Ni2TiAl compound of Ni64Al19Ti17 alloy changed from coarse dendrites into refined grains under ESL condition as the undercooling increased,whereas it transformed from equiaxed dendrites into fine grains with the decrease of droplet diameter in DT state.The solid phase transformation of primary τ4-Ni2TiAl was suppressed in DT,while it still occurred under normal ESL state.The micromechanical property of τ4-Ni2TiAl phase showed an increasing dependence on the enhanced undercooling or the decreased droplet sizes,which was reasonably ascribed to the grain refinement effect.The dendrite growth mechanism of intermetallic Ni3Ti compound within undercooled Ni3Ti and Ni74Ti18Al8 alloys was studied by ESL and LQ techniques.The dendritic growth kinetics of primary Ni3Ti compound phase in both alloys has been comparatively measured,which indicated that the addition of Al element reduced the growth velocity of Ni3Ti dendrites.The Ni3Ti dendrites in both alloys transformed from faceted to non-faceted growth as the solidification condition changed from ESL to LQ approach.