Raymundo Arroyave

Thermodynamic properties of binary hcp solution phases from special quasirandom structures

Three different special quasirandom structures (SQSs) of the substitutional hcp A1–xBx binary random solutions (x=0.25, 0.5, and 0.75) are presented. These structures are able to mimic the most important pair and multi-site correlation functions corresponding to perfectly random hcp solutions at those compositions. Due to the relatively small size of the generated structures, they can be used to calculate the properties of random hcp alloys via first-principles methods. The structures are relaxed in order to find their lowest energy configurations at each composition. In some cases, it was found that full relaxation resulted in complete loss of their parental symmetry as hcp so geometry optimizations in which no local relaxations are allowed were also performed. In general, the first-principles results for the seven binary systems (Cd-Mg, Mg-Zr, Al-Mg, Mo-Ru, Hf-Ti, Hf-Zr, and Ti-Zr) show good agreement with both formation enthalpy and lattice parameters measurements from experiments. It is concluded that the SQSs presented in this work can be widely used to study the behavior of random hcp solutions.

Thermodynamic modeling of the Hf–Si–O system

Abstract The Hf–O system has been modeled by combining existing experimental data and first-principles calculation results through the CALPHAD approach. Special quasirandom structures of α and β hafnium were generated to calculate the mixing behavior of oxygen and vacancies. For the total energy of oxygen, vibrational, rotational and translational degrees of freedom were considered. The Hf–O system was combined with previously modeled Hf–Si and Si–O systems, and the ternary compound in the Hf–Si–O system, HfSiO4 has been introduced to calculate the stability diagrams pertinent to the thin film processing.

Ab-initio aprroach to the electronic, structural, elastic, and finite-temperature thermodynamic properties of Ti2AX (A = Al or Ga and X = C or N)

In this work, the electronic, structural, elastic, and thermodynamic properties of Ti2AX MAX phases (A = Al or Ga, X = C or N) were investigated using density functional theory (DFT). It is shown that the calculations of the electronic, structural, and elastic properties of these structures, using local density approximation (LDA) and generalized gradient approximation (GGA) coupled with projected augmented-wave (PAW) pseudopotentials, agree well with experiments. A thermodynamic model, which considers the vibrational and electronic contributions to the total free energy of the system, was used to investigate the finite-temperature thermodynamic properties of Ti2AX. The vibrational contribution was calculated using the supercell method, whereas the electronic contribution resulted from one-dimensional integration of electronic density of states (DOSs). To verify the model, the specific heats of pure elements were calculated and compared to experimental data. The DFT-D2 technique was used to calculate the ...

Structural and transport properties of epitaxial NaxCoO2 thin films

We have studied structural and transport properties of epitaxial NaxCoO2 thin films on (0001) sapphire substrate prepared by topotaxially converting an epitaxial Co3O4 film to NaxCoO2 with annealing in Na vapor. The films are c-axis oriented and in-plane aligned with [101¯0]NaxCoO2 rotated by 30° from [101¯0] sapphire. Different Na vapor pressures during the annealing resulted in films with different Na concentrations, which showed distinct transport properties.

Stabilization of bcc Mg in Thin Films at Ambient Pressure: Experimental Evidence and ab initio Calculations

Recent experiments suggest that bcc Mg can be stabilized when grown together with bcc Nb in Mg/Nb multilayer films at ambient conditions. This finding is remarkable as (pure) bcc Mg has only been observed under very high pressures and is in fact (mechanically) unstable under room conditions. Density functional theory calculations were performed to gain insight into the stability of Mg in the bcc structure. Calculations of the thermodynamic, electronic and structural stability of bcc Mg show that this structure is in fact metastable under thin film conditions, when Mg grows epitaxially on bcc Nb, in agreement with experiments.

Magnetocaloric effects in Ni-Mn-Ga-Fe alloys using Monte Carlo simulations

Heusler alloys based on the Ni2MnGa system have been shown to exhibit strong magneto-thermo-structural couplings that make them very attractive multi-functional materials. In this work, first principles calculations combined with Monte Carlo simulations have been used to study the magnetocaloric effect (MCE) in Fe-doped Ni-Mn-Ga alloys. The first principles calculations have been used to determine the magnetic properties of the alloys—specifically, magnetic exchange couplings—and to construct a lattice-based Hamiltonian (q-state Potts model) for the description of the magnetic transformations. The magnetic Hamiltonian is then coupled to a lattice description of the structural (martensitic) transformation, leading to the development of phenomenological models for the magneto-thermo-structural phase transformation. This model Hamiltonian is then investigated through a Monte Carlo framework to describe the coupled phase transformations as well as the magnetocaloric effect. The field-induced entropy change d...

Lattice Vibrations Boost Demagnetization Entropy in Shape Memory Alloy

Magnetocaloric (MC) materials present an avenue for chemical-free, solid state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC material Ni45Co5Mn36.6In13.4. Upon heating across TC, the material exhibits an anomalous increase in phonon entropy of 0.17 0.04 k_B/atom, which is nine times larger than expected from conventional thermal expansion. We find that the phonon softening is focused in a transverse optic phonon, and we present the results of first-principle calculations which predict a strong coupling between lattice distortions and magnetic excitations.

Spatial Control of Functional Response in 4D-Printed Active Metallic Structures

We demonstrate a method to achieve local control of 3-dimensional thermal history in a metallic alloy, which resulted in designed spatial variations in its functional response. A nickel-titanium shape memory alloy part was created with multiple shape-recovery stages activated at different temperatures using the selective laser melting technique. The multi-stage transformation originates from differences in thermal history, and thus the precipitate structure, at various locations created from controlled variations in the hatch distance within the same part. This is a first example of precision location-dependent control of thermal history in alloys beyond the surface, and utilizes additive manufacturing techniques as a tool to create materials with novel functional response that is difficult to achieve through conventional methods.

A first-principles approach to transition states of diffusion

We propose a first-principles approach for treating the unstable vibrational mode of transition states in solid-state diffusion. It allows one to determine a number of fundamental quantities associated with the transition state, in particular the enthalpy and entropy of migration and the characteristic vibrational frequency, along with their temperature dependences. Application to pure face centered cubic Al shows good agreement with available experimental measurements and previous theoretical calculations. The procedure is further applied in calculations of the migration properties of Mg, Si and Cu impurities in Al, and the differences among Mg, Si and Cu are discussed.

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.