Explicit anharmonicity, defined as the vibrational contribution beyond the quasiharmonic approximation, is qualitatively different between the group V and group VI bcc refractory elements. Group V elements show a small and mostly negative anharmonic entropy, whereas group VI elements have a large positive anharmonic entropy, strongly increasing with temperature. Here, we explain this difference utilizing highly accurate anharmonic free energies and entropies from ab initio calculations for Nb and Ta (group V), and Mo and W (group VI). The numerically calculated entropies are in agreement with prior experimental data. The difference in behavior between the two sets of elements arises not from their high-temperature behavior but rather from the 0K quasiharmonic reference state. We understand this by analyzing the 0K and the high-temperature phonon density of states and the electronic density of states. The qualitative difference disappears when the anharmonicity is instead referenced with a high-temperature effective harmonic potential. However, even for an optimized effective harmonic reference, the remaining effective anharmonicity is significant. The reason is that the anharmonicity in the bcc systems - carried by asymmetric distributions in the nearest neighbors - can never be accounted for by a harmonically restricted potential.

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Shape-memory Alloys (SMAs) exhibit tremendous mechanical properties owing to their reversible phase transformation between the austenitic and the martensitic phase. Out of these, equi-atomic nickel-titanium (NiTi) alloys are the most widely used SMAs for various applications since the transformation occurs close to room temperature. Advances in SMA engineering can be propelled by understanding the fundamental behavior of these materials. Atomistic scale computational techniques provide an efficient way of doing this. The accuracy of such techniques is based on the underlying model that is used in these simulations. With advancements in computational power, the scope for building more accurate material models and simulating bigger and more realistic systems have increased. This thesis is focused on understanding NiTi behavior at an atomistic scale using such accurate models. The work done as a part of this thesis is twofold. Firstly, previously existing interatomic potential models are used to perform molecular dynamics (MD) simulations to describe various phase transformation phenomena in NiTi SMAs including pseudo-elasticity and shape-memory effect, thereby comparing the performance of these existing models. Secondly, a new NiTi interatomic potential is developed, using a database generated by performing Density Functional Theory (DFT) calculations, the performance of which is better than existing models. @en

The field of atomistic simulations of multicomponent materials and high entropy alloys is progressing rapidly, with challenging problems stimulating new creative solutions. In this Perspective, we present three topics that emerged very recently and that we anticipate will determine the future direction of research of high entropy alloys: the usage of machine-learning potentials for very accurate thermodynamics, the exploration of short-range order and its impact on macroscopic properties, and the more extensive exploitation of interstitial alloying and high entropy alloy surfaces for new technological applications. For each of these topics, we briefly summarize the key achievements, point out the aspects that still need to be addressed, and discuss possible future improvements and promising directions.

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The unique and unanticipated properties of multiple principal component alloys have reinvigorated the field of alloy design and drawn strong interest across scientific disciplines. The vast compositional parameter space makes these alloys a unique area of exploration by means of computational design. However, as of now a method to compute efficiently, yet with high accuracy the thermodynamic properties of such alloys has been missing. One of the underlying reasons is the lack of accurate and efficient approaches to compute vibrational free energies—including anharmonicity—for these chemically complex multicomponent alloys. In this work, a density-functional-theory based approach to overcome this issue is developed based on a combination of thermodynamic integration and a machine-learning potential. We demonstrate the performance of the approach by computing the anharmonic free energy of the prototypical five-component VNbMoTaW refractory high entropy alloy.

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One of the effective potentials that has proven to be very versatile and useful for describing metals is the modified embedded atom method (MEAM) potential. The reference-free version of the MEAM (RF-MEAM) potential provides more flexibility for fitting than the 2NN-MEAM because it also describes the pair potential as an explicit function. In this work, we present a methodology to fit RF-MEAM potentials to DFT data. We then evaluate the performance of the fitted potential by comparing MD simulations with experimental and DFT data. As an example, the methodology is applied to a binary and a quaternary alloy, namely NiTi and NbMoTaW. In the case of the equi-atomic NiTi shape memory alloy, our attention focuses on designing a potential that properly captures its mechanical behavior, given that the existing potentials fail to predict elastic constants in agreement with experiments. To reach our aim, we included the stress tensors of different high temperature NiTi configurations in the fitting database. The obtained RF-MEAM potential outperforms existing EAM and MEAM potentials in predicting the lattice and elastic constants of austenitic and martensitic phases as well as the corresponding transformation temperatures. To demonstrate the suitability of this methodology also for more complex systems, a RF-MEAM potential is fitted to model the multi-component NbMoTaW high-entropy alloy. Validation is achieved through comparison between observables obtained through the MD output and ab initio data. The article also reports key improvements to the optimization code MEAMfit v2 and the freely-available LAMMPS implementation of the RF-MEAM formalism. Most notably, resorting to analytic derivatives of the objective function with respect to the potential parameters rather than derivatives through finite differences, the time necessary for fitting has decreased by an order of magnitude.

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Pseudoelasticity in NiTi shape memory alloy single crystals depends on the loading direction. Here, we present a comprehensive study in which molecular dynamics simulations of austenitic bulk single crystals under strain-controlled tensile and compressive loading along the 〈110〉,〈111〉 and 〈100〉 directions are performed, and the mechanical response of the crystals are contrasted. All simulations are performed using the MEAM interatomic potential proposed by Ko et al. (2015). The transformation strains and the Young's modulus of the initial austenitic and the final martensitic phases are compared with values obtained from the lattice deformation model and experimental results from the literature. Results show that depending on orientation the transformation occurs either through the formation of martensitic Lüders bands or through the transient formation of a multivariant martensite which, upon reorientation, becomes a dominant final single variant. Simulations are also performed to assess the orientation-dependent behavior of nano-wires subjected to bending, since the flexibility of the wires is orientation dependent.

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A comparison of the EAM-Finnis-Sinclair and the MEAM potential, two of the recently developed potentials to model NiTi, is carried out. The potentials are compared by studying the pseudo-elastic behavior in bulk NiTi for one specific crystallographic orientation. To this end we perform, for the first time, simulations where the transformation occurs not only under compressive but also under tensile loading along 〈100〉_B2 using both potentials. Results indicate that in both cases the MEAM potential captures the pseudo-elastic behavior more accurately. By using a lattice deformation model, it is demonstrated that the inaccurate transformation strains predicted by the EAM-Finnis-Sinclair potential are a direct consequence of its inability to predict experimental values of the lattice constants. Similarly, it is shown that the more precise values of the Young's modulus of the initial austenitic and the final martensitic phase estimated by the MEAM potential are the result of its ability to predict elastic constants more accurately than the EAM-Finnis-Sinclair potential. As a result, it is concluded that the MEAM potential is better suited to study the overall pseudo-elastic behavior in NiTi.

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The influence of grain boundaries and grain misorientation on the nucleation and growth of martensite in an equi-atomic nickeltitanium (NiTi) shape memory alloy (SMA) is investigated by performing molecular dynamics (MD) simulations on bicrystals with a modified embedded atom method (MEAM) interatomic potential. Stress-induced martensitic transformations are simulated in bicrystals with mixed grain boundaries and the behavior of the bicrystal is compared to that of individual single crystals. Here, a particular bicrystal with < 110 > and < 111 > oriented austenite grains is chosen as an example. Results indicate that the mixed grain boundary in the austenite bicrystal acts as a nucleation site for stress-induced martensitic transformation in the grains. The deformation behavior and the transformation strain of the bicrystal fall in between those of the two corresponding single crystals.@en