Mikhail I. Mendelev's research while affiliated with Iowa State University and other places

The formation and migration of austenite-martensite interfaces plays a key role in the reversible martensitic transformations of shape memory alloys (SMAs). How these interfaces interact with the SMA microstructure is a primary determining factor in important functional properties such as hysteresis and transformation span. Therefore, successful microstructural engineering of SMAs requires in-depth knowledge of interface behavior. The rapid nature of martensitic transformations makes experimental observations of moving interfaces challenging. Molecular dynamics (MD) simulation is a unique tool which can probe the atomic-scale details of austenite-martensite interfaces as they migrate and interact with different microstructural features. While MD simulations allow access to atomic-scale mechanisms, they are limited in time scale, typically to nanoseconds. This limitation creates problems when focusing on the entire transformation process in SMAs, specifically nucleation of new phases. To trigger nucleation on the nanosecond time scale, MD simulations must be performed so far from equilibrium that their relevance to experiment becomes questionable. Here, we demonstrate new MD simulation techniques to generate energetically preferred austenite-martensite interfaces in NiTi under near-equilibrium conditions. We then take advantage of this approach to probe interface behavior under conditions relevant to experiments. Our results demonstrate how austenite-martensite interfaces behave with dramatic differences in single crystals compared to more realistic microstructures containing features such as grain boundaries and precipitates. We identify trends in interface behavior which can be utilized to inform microstructural engineering approaches for SMAs.

Ni is the second most abundant element in the Earth’s core. Yet, its effects on the inner core’s structure and formation process are usually disregarded because of its electronic and size similarity with Fe. Using ab initio molecular dynamics simulations, we find that the bcc phase can spontaneously crystallize in liquid Ni at temperatures above Fe’s melting point at inner core pressures. The melting temperature of Ni is shown to be 700 to 800 K higher than that of Fe at 323 to 360 GPa. hcp, bcc, and liquid phase relations differ for Fe and Ni. Ni can be a bcc stabilizer for Fe at high temperatures and inner core pressures. A small amount of Ni can accelerate Fe’s crystallization at core pressures. These results suggest that Ni may substantially impact the structure and formation process of the solid inner core.

Martensitic transformations in shape memory alloys are often accompanied by thermal hysteresis, and engineering this property is of prime scientific interest. The martensitic transformation can be characterized as thermoelastic, where the extent of the transformation is determined by a balance between thermodynamic driving force and stored elastic energy. Here we used molecular dynamics simulations of the NiTi alloy to explore hysteresis-inducing mechanisms and thermoelastic behavior by progressively increasing microstructural constraints from single crystals to bi-crystals to polycrystals. In defect-free single crystals, the austenitemartensite interface moves unimpeded with a high velocity. In bi-crystals, grain boundaries act as significant obstacles to the transformation and produce hysteresis by requiring additional nucleation events. In polycrystals, the transformation is further limited by the thermoelastic balance. The stored elastic energy can be converted to mechanisms of non-elastic strain accommodation, which also produce hysteresis. We further demonstrated that the thermoelastic behavior can be controlled by adjusting microstructural constraints.

Ni is the second most abundant element in the Earth's core. Yet, its effects on the inner core's structure and formation process are usually disregarded because of its similar atomic numbers with Fe. Using ab initio molecular dynamics simulations, we find that the bcc phase can spontaneously crystallize in liquid Ni at temperatures above Fe's melting point at inner core pressures. The melting temperature of Ni is shown to be 700-800 K higher than that of Fe at 323-360 GPa. Phase relations among hcp, bcc, and liquid differ between Fe and Ni. Ni can be a bcc stabilizer for Fe at high temperatures and inner core pressures. A small amount of Ni can accelerate Fe's crystallization under core pressure. These results suggest Ni may substantially impact the structure and formation process of the solid inner core.

Plain Language Summary The structure of Earth’s solid inner core is a fundamental question in understanding the Earth’s interior. Fe is the major element in the Earth’s solid inner core, while its stable phase under inner core conditions is still under debate. The inaccuracy of present ab initio free energy calculations was too large to estimate the small free energy difference between different Fe phases, making this debate unsolved. In this paper, we developed a method to determine Fe’s melting temperatures from ab initio calculations. This was achieved by utilizing a potential fitted to high‐temperature ab initio data and performing a thermodynamic integration from classical systems described by this potential to ab initio systems. This method significantly reduces the uncertainty caused by the finite size effect in the ab initio calculations. Using this method, we calculated the free energy difference and melting temperatures of hcp and bcc Fe under inner‐core boundary and center conditions. We show that the hcp phase is the stable phase of pure Fe throughout the inner core condition. However, the bcc and hcp phases show a very small free energy difference that may be altered by other elements in the inner core.

Molecular dynamics (MD) simulations have long been unable to provide significant insights into large-scale processes involving diffusion. Here, we implement a kinetic Monte Carlo (kMC) algorithm to circumvent such issues. We have implemented this approach in the widely used MD simulation package, LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). Validation of the MD/kMC algorithm is achieved by replicating results predicted by diffusion equations. Comparisons are made between a traditional MC approach and the kMC algorithm for the case of L12 phase growth in Ni-Al alloys. These examples highlight the unique advantages provided by the kMC approach to unlock new capabilities for MD simulations.