Understanding the structural, mechanical, thermal, and electronic properties of MgCa bulk metallic glasses by molecular dynamics simulation and density functional theory calculation
SP Ju and CC Yang, COMPUTATIONAL MATERIALS SCIENCE, 154, 256-265 (2018).
This study uses molecular dynamics (MD) and the density functional theory (DFT) calculation to investigate the structural, mechanical, thermal, and electronic properties of Mg3Ca7, Mg5Ca5, Mg7Ca3 bulk metallic glasses. A fitting process for the Mg-Ca cross-element parameters of the second-nearest neighbor modified embedded-atom method (2NN MEAM) potential was performed via the particle swarm optimization (PSO) based on density functional theory (DFT) calculations with reference data present in ESI. Using these fitted potential prameters, the Mg3Ca7, Mg5Ca5, Mg7Ca3 structures were constructed with the simulated-annealing basin-hopping (SABH) method. X-ray diffraction results provided the angle distribution range, which indicates that the predicted Mg3Ca7, Mg5Ca5, Mg7Ca3 match those of Mg- and Ca- BMG5. This confirms that the fitted parameters of 2NN MEAM potential can indeed precisely represent the interactions of the Mg and Ca atoms in MgCa BMG. Results from the analysis of Honeycutt-Andersen (HA) index show the local icosahedral-like structures occupy 75.52%, 75.33%, 73.84% within Mg3Ca7, Mg5Ca5, Mg7Ca3. The chemical affinity indicated by Warren-Cowley chemical short-range-order (CSRO) parameters shows the Ca atom has a higher affinity to the Mg atoms while possessing less affinity to itself. The tensile simulation results indicate that the Young's moduli are about 9.77, 9.56, and 8.52 GPa, and the strengths are about 582, 552 and 507 MPa for Mg3Ca7,Mg5Ca5, and Mg7Ca3, respectively. For the thermal properties, the melting temperatures of Mg3Ca7, Mg5Ca5 , and Mg7Ca3 range from 750 to 810 K, which are located within the experimental melting temperatures of MgCa alloys. Einstein equation is utilized to calculate self-diffusion coefficients of Mg3Ca7 , Mg5Ca5, and Mg(7)Ca(3 )around melting temperatures, based on the MSD profiles at 500 ps time interval. Based on these diffusion coefficients, the diffusion barriers of Mg, Ca, and total atoms can be determined. The diffusion barriers of total, Mg and Ca for Mg7Ca3 are the lowest. The DFT calculation results show most Mg atoms in Mg3Ca7, Mg5Ca5, and Mg7Ca3 tend to possess negative charges, while most Ca atoms have positive charges. The corrosion mechanism of Mg3Ca7, Mg5Ca5, and Mg7Ca3 was explored by using the Fukui function (fk(-)) distribution for an electrophilic reaction. For Mg(3)Ca(7 )and Mg5Ca5, the surface Ca sites possess the larger fk(-) values, which are the better electron donating sites for H2O dissociation. For Mg7Ca3, both surface Mg and Ca atoms possess higher fk(-) values, indicating the reaction of H2O dissociation and the Mg7Ca3 corrosion could be initialized at either Mg or Ca sites.
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