A Space Simulation Study Of Physical Properties And Crystallization Processes For Ni/Fe Based Alloys At High Temperatures

The thermophysical properties and crystallization processes of highly undercooled liquid alloys are the important subjects in the field of space applied physics.The in-depth study of these properties is beneficial to the control of both microscopic crystalline morphologies and macroscopic physical properties.In this work,the Fe–B and Ni–Si metal-metalloid system,together with the Ni–Zr and Ni–Sn metal-metal alloy system were selected as the study objectives.By using the electrostatic levitation?ESL?and drop tube?DT?techniques as space simulation conditions,in combination with the first-principles?FP?and molecular dynamics?MD?calculations,the high-temperature thermophysical properties and crystallization processes of these binary Ni/Fe based alloys were investigated systematically and the following main results were obtained.1.Electrostatic levitation stability and the active control for high-temperature liquid alloysBy using theoretical analysis and experimental measurements,the geometry parameters of the electrodes were found to be the key factors responsible for electrostatic levitation stability.A quantitative expression of the levitation force was established,and the levitation ability and stability of metallic materials were enhanced by improving the overall design of the ESL system.A modified pre-heating method before levitation was adopted for the charging process and stable levitation of alloy melts under high temperature.The active control of melting and crystallization processes of metals were accomplished and the crystal growth velocity was measured accurately.Furthermore,the infrared pyrometer and CCD imaging techniques were successfully realized for the non-contact measurement of physical quantities such as the temperature and volume.2.Thermophysical properties and crystallization processes of liquid Fe–B and Ni–ZrThe high undercooling and rapid crystallization of Fe–B and Ni–Zr alloys were achieved under ESL condition.The high-temperature thermodynamic properties,including the density,volume expansion coefficient?VEC?,and specific heat of these alloys were also measured.These properties vary linearly both at liquid and high-temperature solid states.The density increases by the amount of 5% during the crystallization process,and the VEC at liquid state is about twice as that at solid state.For Ni–Zr alloys,the density and VEC decrease with the increase of Zr atom content generally.However,there are some abnormal behaviors for the density of Ni Zr₂ alloy and VEC of Ni₃6Zr₆4 alloy.After the thermodynamic properties were determined,the crystallization processes of Fe B,Fe₂ B and Ni₇Zr₂ intermetallic compounds?IMCs?were investigated at ESL condition.The results reveal that the growth velocities of Fe B and Fe2 B crystals increase in the form of power law with enhanced undercooling,and the result of Fe B is a little lager than that of Fe₂ B.While the growth velocity of Ni₇Zr₂ crystal exhibits double-exponential behavior with undercooling.The obtained maximum growth velocity equals 0.45 m s^-1at the critical undercooling of 335 K,above which the growth velocity reduces gradually due to the decrease of atomic diffusion.Comparative studies under drop tube reveal that Ni₇Zr-2 crystal tends to grow in faceted manner at small undercooling,where?001?crystal plane has the smallest growth velocity.Whereas at high undercooling,the faceted growth was suppressed obviously.Theoretical analysis based on the experimentally measured thermophysical properties suggests that the crystal growth of these compounds are kinetic-controlled at large undercoolings.3.Rapid crystallization kinetics of liquid Ni–Sn and Ni–Si eutectic alloys under free fall conditionThe rapid crystallization of selected Ni–Sn and Ni–Si eutectic alloys under free fall condition were achieved using drop tube technique.With the enhancement of undercooling,the microstructures of Ni–14.15at%Sn hypo-eutectic alloy is refined obviously.Ni–19.23at%Sn eutectic alloy undergoes the transition of regular lamellar eutectic to irregular eutectic.While Ni–21.76at%Sn hyper-eutectic changes from coarse dendrite to refined equiaxed crystals.For Ni–18at%Si hypo-eutectic alloy,the primary ?Ni dendrite is refined evidently,while the Ni–21.4at%Si eutectic grows in the manner of refined irregular microstructure.Whereas the growth manner and crystallization morphology of Ni–23at%Si hyper-eutectic are both altered.The thermophysical properties including the enthalpy,specific heat,density and diffusion coefficients of the above alloys were calculated using MD simulations.The results indicate that the specific heat and volume expansivity change apparently with Sn or Si atom content.The crystallization kinetics of the eutectic alloys were analyzed theoretically by incorporating the MD calculated thermophysical properties.It is found that the concentration undercooling is the main factor that control the crystal growth at small undercooling.While at large undercoolings,the kinetic undercooling play important role in determining the growth of intermetallic compounds,but has less effect on the growth of solid solutions.4.Physical properties of Ni/Fe based alloys at solid stateFirst-principle calculations were performed to systematically investigate the structural,elastic,phonon and thermodynamic properties of the stable and metastable IMCs in Fe–B,Ni–Sn and Ni–Si system,together with the lattice constants and thermal expansions of Ni–Sn supersaturated solid solutions.By fitting the energy–volume relationships with equation of state,the equilibrium properties such as the volumes,energies and bulk moduli of the IMCs can be derived accurately.The elastic constants and hardnesses of Fe–B compounds are very high,for example,the maximum elastic constant of Fe B₂ exceeds 700 GPa and the Vickers hardness reaches up to 31.4 GPa,although the elastic anisotropy is very weak.Generally,the singlecrystal elastic constants and polycrystalline elastic moduli of Ni–Sn and Ni–Si system decrease with the increase of Sn or Si atom content.By incorporating the FP results with Debye or phonon models within quasi-harmonic approximation,the thermodynamic properties,including the Gibbs free enregy,entropy,enthalpy and specific heat of these IMCs can be predicted.The derived results for the stable IMCs,Fe B,Fe₂ B,Ni₃Sn,Ni₃Sn₂ and Ni₃Sn₄ under ambient condition agree very well with experimental or CALPHAD results.This method can be also used to predict the thermodynamic properties of metastable IMCs.Based on the FP calculated energy–volume relationships of IMCs,a new approach was proposed to establish the second nearest neighbor modified embedded atom method?2NNMEAM?potentials of alloys.The fitted potential parameters for Ni–Sn and Ni–Si alloy systems can be used for atomic simulations such as MD,and the structural and thermodynamic properties of the alloys were predicted accurately.Which extends the electronic scale calculation to atomic or molecular scale simulations.For the Ni–Sn supersaturated solid solutions,the lattice constant,lattice distortion,thermal expansion and linear expansion coefficient?LEC?were investigated.The obtained maximum solubility is 9.2% and the undercooling is larger than 200 K.The lattice constant rises linearly with the increase of Sn atom content in the rate of 0.01 ? per 1at% Sn.The lattice distortion is mainly confined to the first nearest neighbor atoms with the value of about 4%,but smaller than 1% for farther atoms.The thermal expansion of Ni–Sn solid solutions are obviously influenced by the low-temperature to high-temperature solid-state phase transition of Ni₃ Sn compound.However,the LEC almost remain unchanged with Sn atom content below the transition temperature.