Theoretical Research On Melting Properties Of Fe, Co, Ni And NiAl Alloy

The phase transitions and melting properties of the materials under high pressure and temperatureare basic issue in condensed matter physics, and also the most focused research topics in the high-pressurephysics. It plays a key role in understanding the nature of physics of materials under elevated pressure andhigh temperature. In this paper, the high-pressure melting properties of Fe, Co, Ni and NiAl are predictedby using the classical molecular dynamics (MD) simulations combining with several embedded atommodel (EAM) potentials. Furthormore, we presented a detail investigation on the thermodynamicproperties of NiAl alloy under high pressure by using first-principles density-functional theory (DFT) andquasi-harmonic Debye model. We hope these studies are helpful for understanding their high-pressurebehaviors. The obtained results are expected to be used to guide the further or the ongoing experiments.We firstly employed MD simulations to investigate the melting properties of iron under highpressure. Earth’s core is structured in a solid inner core, and a liquid outer core, mainly composed of iron.The melting of iron under extreme conditions can be employed to determine the temperature at the Earth’sinner-outer core boundary, which is crucial to construct the geophysical and geochemical models for theEarth’s core. However, there exist large discrepancies between experimental and theoretical works anddisagreements among the calculated results from different theoretical models. Even in the low pressureregion of50-60GPa, the discrepancies between different research teams have been still of the order of500K. What’s more, several thousand degrees of disagreements exist in extrapolating form laser-heateddiamond anvil cell (DAC) pressures below100GPa to shock wave (SW) pressure of300GPa, and the highpressure melting curve of Fe still remains inconclusive up to now. Thus, the investigations on the meltingproperties of iron own great scientific value and practical meanings. Using one-phase, two-phase and Zsimulations combining with the EAM potential developed by Ackland et al., we have investigated in detailthe melting curves of iron over a wide range of pressures, and found that the obtained results are very closeto each other at the applied pressures, consistent well with both DAC experiments at low pressure and highpressure, and shock measurements at high pressure. In addition, depending on the one-phase approach, wehave also predicted the entropy of melting and solid-liquid interfacial energy of iron under compression, which may provide theoretical references for future measurements.Co and Ni are two important transitional metals. They play an important role in the catalysisindustry. For instance, the grapheme substrates doped with Co or Ni atoms have high catalytic activity forCO oxidation. Therefore, the non-noble-metal atoms can serve promising candidates for replacing theexpensive noble metal-based catalysts. The cobalt and nickel elements, which are adjacent to iron in theperiodic table, is potentially important for us to understand the melting properties of the Earth’s core, whichis believed to consist of iron-dominated alloys with Co or Ni as minor components. Knowledge of themelting properties of Co and Ni is of fundamental importance for constructing the new model of thetemperature distribution of the Earth’s core. In this paper, the melting curves of cobalt under highcompression were investigated from classical molecular dynamic simulations using two EAM potentials(Pun’s EAM potential and Zhou’s EAM potential). It is found that the Zhou’s EAM potential can producesatisfying results, in good agreement with the DAC experiment at low pressure. Since there are littleexperimental data for Co about high-pressure melting temperatures, we hope this piece of data can providea useful benchmark on the melting properties of high-pressure Co. Based on the Zhou’s potential, we havealso investigated successfully the structures of solid and liquid, melting entropy and solid-liquid interfacialenergy of Co, and obtained satisfying results. As for Ni, its melting properties were investigated by usingthree EAM potentials and two simulation techniques. We found the Mendelev’s EAM potential canreproduce a satisfying melting curve, consistent well with both DAC experiment at low pressure and shockmelting at high pressure. Due to lack of high-pressure experimental melting data for Ni as well as Co, theobtained results can provide data for high-pressure experimental measurements of Ni. Moreover,dependence on the Mendelev’s potential, we also studied the structures of solid and liquid, entropy offusion, and solid-liquid interfacial energy of Ni, and found that the calculated physical properties of nickelat zero pressure are in good agreement with the experiments.The NiAl alloy is a very important functional material within industrial sectors, and widely usedto aircraft industry and automobile industry because of good performances, such as high melting point, lowdensity, high thermal conductivity and good environmental resistance. Unfortunately, the melting curve ofNiAl under high pressure is not known. On the basis of Pun’s EAM potential, we have investigated itshigh-pressure melting properties in detail. To assess the qualities of the Pun’s potential, we first investigated the room temperature lattice constants and elastic stiffness constants by using energy-volumemethod and direct approach. The obtained results are in favorable consistent with the experimental values.Second, we calculated the melting curves of pure Ni, pure Al and NiAl alloy with single-phase andtwo-phase simulations, respectively. At ambient pressure, the melting point of NiAl is in excellentagreement with the experimental value. The melting curve of Ni obtainded with this EAM potential isconsistent well with the experiment. However, the larger discrepancies between theoretical andexperimental results of Al are observed. The discrepancies in the predicted melting temperatures for Alwith the measured values most likely arise from inadequacies in this potential. Through investigations onmelting properties of Fe, Co, Ni and NiAl alloy, we found the one-phase (hysteresis), two-phase(solid-liquid coexistence) and Z approaches all can reduce the superheating effectively, and their obtainedresults are in the close proximity at various pressures. The one-phase and Z methods are good alternative tothe two-phase approach.We investigated the thermodynamic properties (such as thermal equation of state (EOS), thermalexpansivity, specific heat, Debye temperature, and so on) of NiAl alloy at high pressure by using thedensity functional theory combining with the quasi-harmonic Debye theory. The theoretical results showthe theoretical EOS is consistent well with that of experiment. As for thermal expansion, at high pressure,the quasi-harmonic Debye model can be applied to high temperature.In light of the classical molecular dynamic simulations and first-principles calcualtions, byinvestigations on the melting properties of Fe, Co, Ni and NiAl alloy, and thermodynamic properties ofNiAl alloy under high pressure, we hope that our results may provide powerful guidelines for futureexperimental investigations, and such results can also contribute further understanding of thethermodynamic properties of NiAl alloy under high pressure.