Abstract: Interatomic potentials for the Al-Ti, Al-Ta, Al-Zr, Al-Nb and Al-Hf binary systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The fundamental materials properties (structural, thermodynamic and elastic behaviors of different intermetallics) could be readily described with the potentials using molecular dynamic simulation (MD), in rational agreements with experimental or first principles data. The potentials are further utilized to develop an interatomic potential for the (TiZrNbHfTa)Al₃ high entropy intermetallic compound (HEIC), which open the door to understand atomic scale behavior of HEICs.

Abstract: Interatomic potentials for the Al-Ti, Al-Ta, Al-Zr, Al-Nb and Al-Hf binary systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The fundamental materials properties (structural, thermodynamic and elastic behaviors of different intermetallics) could be readily described with the potentials using molecular dynamic simulation (MD), in rational agreements with experimental or first principles data. The potentials are further utilized to develop an interatomic potential for the (TiZrNbHfTa)Al₃ high entropy intermetallic compound (HEIC), which open the door to understand atomic scale behavior of HEICs.

Abstract: We report on the first interatomic potentials for Zirconium Diboride and Hafnium Diboride. The potentials are of the Tersoff form, and are obtained by fitting to a first-principles database of basic properties of elemental Zr, Hf, B, and the compounds ZrB₂ and HfB₂. Two variants of the Zr-B potentials have been obtained, and one for Hf-B. The potentials have been tested against a variety of properties of the compound, with the conclusion that they are stable and provide a reasonable representation of the desired properties of the two diborides.

Abstract: Atomistic Green-Kubo simulations are performed to evaluate the lattice thermal conductivity for single crystals of the ultra high temperature ceramics ZrB₂ and HfB₂. Recently developed interatomic potentials are used for these simulations. Heat current correlation functions show rapid oscillations, which can be identified with mixed metal-Boron optical phonon modes. Results for temperatures from 300K to 1000K are presented.

Abstract: Chemical short-range order (CSRO) in high entropy alloys (HEAs) has attracted interests recently and is believed to be capable for tuning their mechanical properties. However, the characterization of CSRO in HEAs through experimental methods remains challenging. In this work, a modified embedded-atom method interatomic potential with good accuracy for studying CSRO in HfNbTaTiZr alloy system was developed. By employing the potential, molecular dynamic/Monte Carlo simulation was performed to investigate the CSRO in HfNbTaZr HEA. The results indicated that Hf-Zr and Nb-Ta atom pairs were preferred in the BCC solid solution of HfNbTaZr, and a new type of CSRO with topological B2 order was predicted, which can help to understand the mechanical properties of HfNbTaZr HEA. It was also found that forming of CSRO was an incubation process for the precipitation in HfNbTaZr, implying the significance of CSRO on the phase stability or precipitation behavior of HEAs. The findings in the present work can help in understanding CSRO and establishing its relationship with precipitates in HEAs, and more topics related to CSRO and phase stability in HfNbTaTiZr alloy system can be further investigated by atomistic simulation.

Abstract: Determining the structure-property relations of liquid and amorphous metal oxides is challenging, due to their variable short-range order and polyhedral connectivity. To predict chemically realistic structures, we have developed a Machine Learned, Gaussian Approximation Potential (GAP) for HfO₂, with a focus on enhanced sampling of the training database and accurate density functional theory calculations. By using training datasets for the GAP model at the level of Density Functional Theory-Strongly Constrained and Appropriately Normed (DFT-SCAN) level of theory, our results show that the topology of both the low viscosity liquid and the amorphous form are dominated by edge-shared chains and small corner-shared rings of polyhedra. This topology is shown to be consistent with the structure of other liquid and amorphous transition metal oxides of variable ion size, such as TiO₂ and ZrO₂. Current limitations of the ML-GAP modeling method for obtaining glass structures and future perspectives are also discussed.

Abstract: Understanding the structure and properties of refractory oxides is critical for high temperature applications. In this work, a combined experimental and simulation approach uses an automated closed loop via an active learner, which is initialized by x-ray and neutron diffraction measurements, and sequentially improves a machine-learning model until the experimentally predetermined phase space is covered. A multiphase potential is generated for a canonical example of the archetypal refractory oxide, HfO₂, by drawing a minimum number of training configurations from room temperature to the liquid state at ~2900°C. The method significantly reduces model development time and human effort.

Abstract: Metastable high entropy alloy (HEA) with excellent properties have attracted extensive attentions recently. However, as a consequence of limited experiments of high-resolution transmission electron microscopy (HRTEM) and the difficulties of molecular dynamic (MD) simulations for the phase transformation process, the detailed atomic deformation mechanisms in the HEA is not well understood. We carry out the in situ HRTEM observation of the martensitic transformation process and find surprisingly wide phase interface between the parent and the martensite in a typical high strength and high elongation metastable HEA. One specific interatomic potential is developed for the metastable HEA and large-scale MD simulation is carried out to investigate the martensitic transformation process from body-centered cubic to hexagonal close packed structures. The whole processes of the stress-induced martensitic transformation (nucleation, incubation, bursting and propagating of the new phase) are well reproduced in the MD simulations, suggesting its good agreements with the HRTEM observations. The width of the phase interface mainly depends on the competition between interfacial energy and lattice distortion energy during the martensitic transformation process. This wide phase interface acts as a buffer to coordinate the martensitic transformation induced strain and as a buffer storage for dislocation gliding and pile-up. As a result, the metastable HEA achieves a high strength combined with a large tensile elongation. The revealed atomic-scale deformation and corresponding interatomic potential should be useful to guide the design in the new series of high-performance metastable alloy.