Atomistic calculations of free energy were performed for several symmetric tilt grain boundaries over a wide temperature range in the shape memory alloy NiTi. Contrary to expectation in a non-segregating alloy, it was found that the grain boundary excess free energy either increases with temperature or remains relatively constant, a consequence of a negative grain boundary excess entropy. This anomalous behavior can be associated with the martensitic transformation in NiTi. Some grain boundaries host martensite nuclei at their cores which have a lower vibrational entropy than the surrounding bulk austenite. Other grain boundaries, without apparent martensite cores, also exhibit negative excess entropy, leading to the hypothesis that a larger family of grain boundaries in NiTi can access vibrational modes associated with the martensite structure. Such negative excess entropy grain boundaries could aid in nucleation of the martensitic transformation, making them an important microstructural feature for engineering shape memory alloy properties.

Large-scale molecular dynamics (MD) simulations enabled by computationally efficient semiempirical potentials are an invaluable tool for materials modeling. In the case of metallic alloys, embedded atom method (EAM) and Finnis-Sinclair (FS) potentials are a reasonable choice based on their good balance of quality and computational cost. However, these semiempirical potentials are not suitable for simulating ionic systems, which prevents their use in studying many technologically relevant metal-oxide systems. The charge transfer ionic potential (CTIP), which can utilize EAM/FS potentials available in the literature together with a variable charge representation of electrostatic interactions, should be a reasonable choice for performing reliable and computationally efficient MD simulations of such systems. However, only a few such potentials are available in the literature, and their computational cost is much higher compared to EAM/FS potentials. In the present work, we have attempted to remedy these deficiencies by combining several modifications to the CTIP model proposed in the literature and efficiently implementing them into the widely used Large-scale Atomic/Molecular Massively Parallel Simulator MD code. Using these modifications, we have developed a new Ni-O CTIP parameterization, which has been tested in several different scenarios of interest. First, the early stages of Ni surface oxidation were simulated, demonstrating the nucleation and growth of a crystalline NiO film across the surface. Second, solidification and vitrification in the Ni-O system were investigated, demonstrating that the new CTIP parameterization provides reasonable agreement with the experimentally determined equilibrium phase diagram. Finally, we studied the interaction of dislocations in a Ni matrix with a NiO inclusion using a simulation cell with an unprecedented number of atoms for a variable charge MD simulation. Thus, the approach utilized in the present study is an efficient method to simulate large scale atomic mechanisms in metal-oxide systems.

The demand for metal alloys that can perform at extreme temperatures above 1100 °C while remaining manufacturable has sparked renewed interest in printable oxide dispersion strengthened (ODS) alloys. Recently, NASA developed an ODS alloy designed for additive manufacturing, known as GRX-810, which has demonstrated exceptional tensile and creep performance at temperatures of 1093 °C and higher. In the present study, tensile tests of GRX-810 are conducted up to 1316 °C and creep tests are performed in both the horizontal and vertical orientations, relative to the build direction. Thermal cycling is executed at 1100 °C, 1200 °C, and 1300 °C in air. The oxidation behavior of GRX-810 is compared to that of alumina forming single crystal Ni-base superalloys and chromia-forming wrought alloys such as superalloys 718 and 625. High resolution atomic-scale characterization and atomistic modeling are employed to explain the exceptional high temperature properties observed in GRX-810, particularly in relation to the unique, finer trigonal yttrium oxides produced during the additive manufacturing process. GRX-810, an oxide dispersion strengthened alloy, shows excellent structural performance above 1100°C and stability up to 1300 °C. Grain-size effects, additive manufacturing–induced anisotropy, and fine trigonal Y₂O₃ particles enhance creep resistance.

Shape memory alloys are used in many applications which require them to undergo numerous transformation cycles. Generally, an important property for use in such cyclic applications is a small hysteresis, which is linked to functional fatigue resistance. Leveraging microstructural features which promote martensite nucleation is one strategy to achieve reduced hysteresis. In the austenite phase of NiTi, the Σ9(114)[110] symmetric tilt grain boundary has been recognized as one such feature. We have performed a series of molecular dynamics simulations to characterize this grain boundary and its relationship to the martensitic transformation. Upon thermal equilibration, even above the transformation temperature, the grain boundary spontaneously forms a twinned martensite structure at its core, which serves as a nucleus during the martensitic transformation. When the grain boundaries are near one another, the energetic barrier to the transformation is reduced and a small hysteresis results. In polycrystalline microstructures, the added constraints lead to an expanded transformation window and retained austenite upon cooling. Based on these results, grain boundary engineering could be an effective strategy to produce shape memory alloys with improved performance in cyclic applications.

Recent developments in oxide dispersion strengthened (ODS) alloys have revealed significant improvements in creep and oxidation properties compared to their non-ODS counterparts. The mechanistic origin of such property improvements is not well understood, particularly with regards to creep performance where ODS alloys can exhibit significantly different responses compared to their non-ODS counterparts. Investigation of such effects through simulation has recently been enabled through the development of new computationally efficient charge transfer ionic potential for the Ni-O system. Using this newly developed capability, we performed molecular dynamics simulation to investigate the mechanism of creep strengthening in ODS alloys by simulating oxide-dislocation interactions. These simulations demonstrated that the bypass of the oxide is controlled by the direct dislocation-particle interaction at the oxide-metal interface, rather than repulsion of the dislocation as required for Orowan looping. Therefore, the dislocation interactions must be governed by effects acting on the dissociated dislocation core over the Ni-NiO interface. The ODS strengthening effect is likewise observed to be relatively insensitive to temperature variation, which is in line with experimental measurements demonstrating ODS strengthening at a wide range of temperatures. Finally, this improved understanding of the fundamental oxide strengthening mechanisms indicates that local structure at the metal-oxide interface is critical for strengthening of the ODS alloys.

Precipitation of secondary phases is a common strategy used to control both the structural and functional properties of shape memory alloys. It can be used to promote nucleation of the martensitic transformation as well as improve cyclic stability. Less is understood about how precipitates affect the progression of an ongoing transformation, i.e., motion of austenitemartensite interfaces. In this study, we performed molecular dynamics simulations of the interaction of austenite-martensite interfaces moving in the NiTi alloy with Ni4Ti3 precipitates. It was found that the nanoscale precipitates obstruct interface motion until a sufficient undercooling is reached. The simulation results can be quantitatively explained with thermoelastic effects - elastic deformation of the precipitates acts to oppose the thermodynamic driving force favoring the transformation. A simple model is proposed to predict a more difficult transformation in shape memory alloys with higher concentrations of and/or harder precipitates. Additionally, simulations of cyclic transformations implicate inelastic deformation at the precipitate-matrix interface as one mechanism responsible for the cyclic drift in transformation characteristics. Deformation originated in a thin, amorphous interfacial layer and expanded with increasing cycles.

High energy stacking faults generated by lattice dislocations entering the strengthening precipitates of Ni-based superalloys are responsible for the unique mechanical properties of these structural materials. However, the question about stability of these faults has not received the attention it deserves. Using atomistic simulations, we show that the anti-phase boundary (APB) can spontaneously transform into super intrinsic stacking fault (SISF) and the complex stacking fault (CSF) can spontaneously transform into L12 lattice structure. The former transformation explains the experimentally observed presence of isolated SISFs and super extrinsic stacking faults (SESFs) in the precipitates. Finally, multiple studies were focused on finding alloying additions which increase the APB and CSF energies. We demonstrate therefore that alloying additions which increase stacking fault energies may conversely decrease their stabilities.

The recently identified multiprincipal-element alloy (MEA), known as GRX-810, exhibits a dramatic improvement in the creep resistance at elevated temperatures. Initial studies have identified segregation atmospheres around the oxide-dispersion particles, which likely contribute to this phenomenon. However, the many details remain elusive, especially the co-segregation effects. Such effects include both substitutional and interstitial type segregation, leading to a complex alloying environment. To investigate these segregation effects, Monte Carlo/ab initio based molecular dynamics (MC/AIMD) hybrid simulations were performed. Coupled with an algorithm to identify potential interstitial sites, evaluation of the joint interstitial and substitutional segregation events was made possible. The simulations have revealed critical elements of carbon segregation near the oxide-dispersoid particles, where carbon atoms are repelled from the oxygen-terminated metal-yttria interface but attracted to the yttrium-terminated interface. This discovery identifies critical distinctions between yttrium-based oxide dispersoids, which control critical solute atmospheres.

Micro-twinningMicro-twinning is the major creepCreep deformation mechanismDeformation mechanisms in Ni-based superalloysNi-based superalloy at temperatures above 700 °C. Recent experiments suggest that superlattice stacking faultsStacking faults in γ′ phase may serve as the precursors to twin formation. SegregationSegregation of alloying elements to these precursors may have a significant effect on formation and extension of micro-twinsMicro-twins. Using atomistic modelingAtomistic modeling we investigate and explain the effects of Nb and Cr alloying additions on these processes. The simulation shows that Nb increases the creep resistanceCreep resistance which is mostly associated with impeding the reordering of the high energy double complex stacking faultStacking faults. Cr, on the other hand, promotes twin growth, degrading the high temperatureHigh temperature creep propertiesCreep properties. These results can help to understand the effects of elemental composition of the alloy on creep resistanceCreep resistance.

As the operating temperature of jet turbine engines increase, creep becomes the life-limiting property for turbine disks and blades. At intermediate temperatures, between 600-800°C, microtwinning contributes significantly to creep strain in these alloys. Therefore, understanding how microtwins form and grow is critical to improving the creep life of future Ni-base superalloy components. In addition, exploring the effect of different alloying elements, such as Nb and Ta, on the formation of microtwins is critical for future alloy development. Several mechanisms of microtwinning have been proposed among which the Kolbe mechanism, based on thermally activated reordering, is believed to be dominant. In this work we employ atomistic simulation to investigate the effects of Nb solutes on the Kolbe mechanism. The simulation demonstrates that Nb atoms significantly slow down the reordering processes, explaining the experimentally observed improvement in the creep resistance.

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.

Abstract 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.

Micro-twinning is the major creep deformation mechanism in Ni-based superalloys at temperatures above 700 °C. Recent experiments suggest that superlattice stacking faults in γ′ phase may serve as precursors for twin formation. Segregation of alloy elements to these precursors may have a significant effect on the formation and extension of micro-twins. Using atomistic modeling, we investigated and explained the effects of Cr on these processes. Our results indicate that depending on the site preference of Cr in Ni3Al γ′ phase, two drastically different deformation behaviors can be expected. Occupying Al sites, Cr significantly accelerates deformation twinning. Cr on Ni sites, on the other hand, suppresses twin growth and slows down the high temperature deformation creep. These results help to rationalize the experimentally observed puzzling effects of alloy composition on creep resistance.

Interface free energy is a fundamental material parameter needed to predict the nucleation and growth of new phases. The high cost of experimentally determining this parameter makes it an ideal target for calculation through a physically informed simulation. Direct determination of interface free energy has many challenges, especially for solid–solid transformations. Indirect determination of the interface free energy from the nucleation data has been done in the case of solidification. However, a slow on molecular dynamics (MD) simulation time scale atomic diffusion makes this method not applicable to the case of nucleation from the solid phase when precipitate composition is different from that in matrix. To address this challenge, we outline the development of a new technique for determining the critical nucleus size from an MD simulation using a recently developed method to accelerate solid-state diffusion. The accuracy of our approach for the Ni–Al system for Ni3Al (γ′) precipitates in a Ni–Al (γ) matrix is demonstrated well within experimental accuracy and greatly improves upon previous computational methods [Herrnring et al., Acta Mater. 215(8), 117053 (2021)].

Micro-twinning is the major creep deformation mechanism in most Ni-based superalloys at temperatures above 700 degrees C. Nevertheless, many aspects of twin nucleation and growth remain unexplored. The Kolbe mechanism for micro-twinning is currently widely accepted in the community to explain these processes, based on post mortem TEM characterization and indirect theoretical estimations. However, this does not mean that other mechanisms cannot contribute to creep. Molecular dynamics (MD) simulations offer an alternative possibility to probe different creep mechanisms. In this work, we use MD simulations to demonstrate that a qualitatively different mechanism for nucleation and growth of twins can be competitive with the Kolbe mechanism in the intermediate temperature regime of 600 degrees C-800 degrees C. The proposed mechanism is closely related to the formation mechanism of super intrinsic stacking faults (SISFs) originally introduced by Condat and Decamps in 1987.

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 austenite-martensite 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.

There has been a long debate on the stable phase of iron under the Earth's inner core conditions. Because of the solid-liquid coexistence at the inner core boundary, the thermodynamic stability of solid phases directly relates to their melting temperatures, which remains considerable uncertainty. In the present study, we utilized a semi-empirical potential fitted to high-temperature ab initio data to perform a thermodynamic integration from classical systems described by this potential to ab initio systems. This method provides a smooth path for thermodynamic integration and significantly reduces the uncertainty caused by the finite-size effect. Our results suggest the hcp phase is the stable phase of pure iron under the inner core conditions, while the free energy difference between the hcp and bcc phases is tiny, on the order of 10s meV/atom near the melting temperature.

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.

Ni-based superalloys are used in the hot sections of jet turbine engines because of their high strength, stability and resistance to oxidation. The properties of these alloys can be further optimized by adding alloying elements. Therefore, a fundamental knowledge on the effect of different elements on properties of Ni-based superalloys is required. Molecular dynamics simulation could shed light here but its application is hindered by the absence of reliable and computationally cheap semi-empirical potential of the interatomic interaction for 4 and more element alloys. We will present a new Ni-Al-Cr-Nb Finnis-Sinclair (FS) potential specially designed to simulate the dislocation propagation from gamma to gamma' phase. In order to construct this potential, we designed a special algorithm to incorporate the data on element partitioning in the potential development procedure. For example, it is known from experiment, that Cr is mostly present in the gamma phase. Figure 1 shows a snapshot obtained after equilibration of the model of the Ni68Al17Cr15 alloy at T=1000 K using the hybrid Monte-Carlo (MC)/molecular dynamics (MD) simulation with the developed semi-empirical potential. One can clearly see that the Cr partitioning is in agreement with the experimental data. We will discuss the developed algorithm to incorporate the solute partition data in details. Using the developed semi-empirical potential, we first investigated the effect of anti-site defects in the gamma phase on the single dislocation propagation. It was found that the dislocation velocity increases with the increasing of the anti-site defect concentration. This effect was attributed to smaller number of Al-Al pairs forming during the dislocation migration in the presence of the anti-site defects. Next, we investigated the effect of Nb on the dislocation pair propagation in the Kolbe mechanism. It was found that the addition of Nb leads to considerable decrease in the dislocation propagation rate. This is in agreement with the experimental data on the effect of Nb on the creep resistance of the Ni-based superalloys. We will discuss the origin of this effect.

Micro-twinning is one of the dominant creep deformation mechanisms in Ni-based superalloys at intermediate temperatures. The Kolbe micro-twinning mechanism, based on thermally activated reordering, has been confirmed experimentally and analyzed theoretically. However, many aspects of the reordering process are still under debate. This work presents the first molecular dynamics (MD) simulation study of the Kolbe mechanism. We demonstrate that atomic diffusion-mediated reordering takes place in the immediate vicinity of the cores of twinning partials and proceeds in a highly concerted manner, directly influenced by propagation of the dislocations.