Luis Sandoval

Finite-Size Effects in Fe-Nanowire Solid−Solid Phase Transitions: A Molecular Dynamics Approach

By means of classical molecular-dynamics simulations, we investigate solid-solid phase transitions in cylindrical iron nanowires. The interatomic potential employed has been shown to be capable of describing the martensite-austenite phase transition in iron. We investigate the dependence of the transition temperature on the wire diameter, the heating/cooling rate, and a tensile stress applied in axial direction. We observe that the phase transition temperature is inversely proportional to the wire diameter during heating and depends linearly on an applied axial tensile stress. The transition temperature becomes independent of the heating/cooling rate for the smallest rates investigated. The time the wire needs for completing the structural change is found to be independent of the diameter, the tensile loading, and the heating/cooling rate for the range of parameters considered. Finally, we find that there exists a maximum tensile stress above which the nanowire can no longer recover its initial structure after cooling.

The Bain versus Nishiyama–Wassermann path in the martensitic transformation of Fe

Using atomistic simulations and an embedded-atom interatomic potential, which is capable of describing the martensite (fcc ! bcc) transition in Fe, we compare the Bain and Nishiyama-Wassermann transformation paths. We calculate the minimum-energy paths for these two transformations at 0K. For finite temperature, we study the evolution of the free energy along the two paths, calculated via the method of metric scaling, which shows only small differences. However, the variation of the elements of the stress tensor, and of the atomic volume, show clear differences: the Bain path leads to by a factor of five higher compressive pressures compared to the Nishiyama-Wassermann path. This means that the Bain path requires additional work applied on the system in order to accomplish it, and gives atomistic evidence that the martensite transformation rather follows the Nishiyama-Wassermann path in reality.

Characterization of Fe potentials with respect to the stability of the bcc and fcc phase

By calculating free energies, several published interatomic interaction potentials for iron are investigated with respect to the stability of the low-temperature bcc phase and the high-temperature fcc phase. These are empirical many-body potentials for use in atomistic simulation. We find that in all of these potentials—except one—the bcc phase is the stable crystal structure for all temperatures up to the melting point. However, several potentials exhibit a metastable fcc phase in the sense that the fcc structure corresponds to a local minimum of the free energy.

Solid–solid phase transitions in Fe nanowires induced by axial strain

By means of classical molecular-dynamics simulations we investigate the solid-solid phase transition from a bcc to a close-packed crystal structure in cylindrical iron nanowires, induced by axial strain. The interatomic potential employed has been shown to be capable of describing the martensite-austenite phase transition in iron. We study the stress versus strain curves for different temperatures and show that for a range of temperatures it is possible to induce a solid-solid phase transition by axial strain before the elasticity is lost; these transition temperatures are below the bulk transition temperature. The two phases have different (non-linear) elastic behavior: the bcc phase softens, while the close-packed phase stiffens with temperature. We also consider the reversibility of the transformation in the elastic regimes, and the role of the strain rate on the critical strain necessary for phase transition.

Transformation pathways in the solid-solid phase transitions of iron nanowires

Using molecular dynamics simulations, we study the solid-solid phase transitions induced by strain in Fe nanowires. These show an intricate dependence on the crystallographic orientation of the wire. ⟨001⟩ oriented nanowires exhibit a bcc→fcc transition and preferably follow the Nishiyama–Wassermann path. In ⟨011⟩ and ⟨111⟩ oriented nanowires the transformation is bcc→hcp and proceeds according to the Burgers path. Additionally we show that it is possible to obtain multiple phase transitions accompanied with reorientations.

Collision-spike sputtering of Au nanoparticles

Ion irradiation of nanoparticles leads to enhanced sputter yields if the nanoparticle size is of the order of the ion penetration depth. While this feature is reasonably well understood for collision-cascade sputtering, we explore it in the regime of collision-spike sputtering using molecular-dynamics simulation. For the particular case of 200-keV Xe bombardment of Au particles, we show that collision spikes lead to abundant sputtering with an average yield of 397 ± 121 atoms compared to only 116 ± 48 atoms for a bulk Au target. Only around 31 % of the impact energy remains in the nanoparticles after impact; the remainder is transported away by the transmitted projectile and the ejecta. The sputter yield of supported nanoparticles is estimated to be around 80 % of that of free nanoparticles due to the suppression of forward sputtering.

Thermodynamic interpretation of reactive processes in Ni?Al nanolayers from atomistic simulations

Metals that can form intermetallic compounds by exothermic reactions constitute a class of reactive materials with multiple applications. Ni?Al laminates of thin alternating layers are being considered as model nanometric metallic multilayers for studying various reaction processes. However, the reaction kinetics at short timescales after mixing are not entirely understood. In this work, we calculate the free energies of Ni?Al alloys as a function of composition and temperature for different solid phases using thermodynamic integration based on state-of-the-art interatomic potentials. We use this information to interpret molecular dynamics (MD) simulations of bilayer systems at 800?K and zero pressure, both in isothermal and isenthalpic conditions. We find that a disordered phase always forms upon mixing as a precursor to a more stable nano crystalline B2 phase. We construe the reactions observed in terms of thermodynamic trajectories governed by the state variables computed. Simulated times of up to 30?ns were achieved, which provides a window to phenomena not previously observed in MD simulations. Our results provide insight into the early experimental reaction timescales and suggest that the path (segregated reactants)???(disordered phase)???(B2 structure) is always realized irrespective of the imposed boundary conditions.

The mobility of small vacancy/helium complexes in tungsten and its impact on retention in fusion-relevant conditions

Tungsten is a promising plasma facing material for fusion reactors. Despite many favorable properties, helium ions incoming from the plasma are known to dramatically affect the microstructure of tungsten, leading to bubble growth, blistering, and/or to the formation of fuzz. In order to develop mitigation strategies, it is essential to understand the atomistic processes that lead to bubble formation and subsequent microstructural changes. In this work, we use large-scale Accelerated Molecular Dynamics simulations to investigate small (N = 1,2) VNHeM vacancy/helium complexes, which serve as the nuclei for larger helium bubble growth, over timescales reaching into the milliseconds under conditions typical of the operation of fusion reactors. These complexes can interconvert between different ILVN+LHeM variants via Frenkel pair nucleation (leading to the creation of a additional vacancy/interstitial pair) and annihilation events; sequences of these events can lead to net migration of these embryonic bubbles. The competition between nucleation and annihilation produces a very complex dependence of the diffusivity on the number of heliums. Finally, through cluster dynamics simulations, we show that diffusion of these complexes provides an efficient pathway for helium release at fluxes expected in fusion reactors, and hence that accounting for the mobility of these complexes is crucial.

Mesoscale computational study of the nanocrystallization of amorphous Ge via a self-consistent atomistic phase-field model

Abstract Germanium is the base element in many phase-change materials, i.e. systems that can undergo reversible transformations between their crystalline and amorphous phases. These materials are widely used in current digital electronics and hold great promise for the next generation of non-volatile memory devices. However, the ultra-fast phase transformations required for these applications can be exceedingly complex even for single-component systems, and a full physical understanding of these phenomena is still lacking. In this paper we study the growth of crystalline Ge from amorphous thin films at high temperature using phase-field models informed by atomistic calculations of fundamental material properties. The atomistic calculations capture the full anisotropy of the Ge crystal lattice, which results in orientation dependences for interfacial energies and mobilities. These orientation relations are then exactly recovered by the phase-field model at finite thickness via a novel parametrization strategy based on invariance solutions of the Allen–Cahn equations. By means of this multiscale approach, we study the interplay between nucleation and growth and find that the relation between the mean radius of the crystallized Ge grains and the nucleation rate follows simple Avrami-type scaling laws. We argue that these can be used to cover a wide region of the nucleation rate space, hence facilitating comparison with experiments.

The thermodynamic and kinetic interactions of He interstitial clusters with bubbles in W

Due to its enviable properties, tungsten is a leading candidate plasma facing material in nuclear fusion reactors. However, like many other metals, tungsten is known to be affected by the high doses of helium atoms incoming from the plasma. Indeed, the implanted interstitial helium atoms cluster together and, upon reaching a critical cluster size, convert into substitutional nanoscale He bubbles. These bubbles then grow by absorbing further interstitial clusters from the matrix. This process can lead to deleterious changes in microstructure, degradation of mechanical properties, and contamination of the plasma. In order to better understand the growth process, we use traditional and accelerated molecular dynamics simulations to investigate the interactions between interstitial He clusters and pre-existing bubbles. These interactions are characterized in terms of thermodynamics and kinetics. We show that the proximity of the bubble leads to an enhancement of the trap mutation rate and, consequently, to the...