Abstract: The crystallisation behaviour of Vitreloy 105 during different thermo-mechanical exposure conditions has been studied in a Gleeble 3500 thermo-mechanical simulator (TMS). Strains up to 0.1 applied at rates of 0.001 – 0.01 s-1 in the supercooled liquid region (SLR) from 420 – 435 °C resulted in no change in the crystallisation kinetics compared with purely thermal exposure for the same times and temperatures. Separate deformation studies on the amorphous bulk metallic glass confirmed that the deformation conditions used corresponded to Newtonian flow conditions. It remains to be confirmed whether the lack of influence of permanent deformation persists for deformation at higher strain rates in the non-Newtonian regime.
Abstract: We apply density functional perturbation theory together with experimental studies in order to investigate the structure and physical properties of possible stable and metastable copper(I) compounds with oxygen and hydrogen. Copper(I) hydride, CuH, is found to be a metastable phase which decomposes at ambient conditions and exhibiting a semiconducting gap in the electronic spectrum. The calculated structure and phonon spectra are found to be in good agreement with experimental data. The phonon spectra of a novel metastable phase, copper(I) hydroxide, are also determined.
Abstract: Vacancy-solute interactions play a crucial role in diffusion-controlled phase transformations, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. The knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) under irradiation, as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on density functional theory and employing the locally self-consistent Green’s function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Ti – Cu), and 4d (Nb – Ag) elements. Interactions within the first two coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a very strong effect on the vacancy-solute interactions.
Abstract: We present a study of the electronic properties of Tl5Te3, BiTl9Te6 and SbTl9Te6 compounds by means of density functional theory based calculations. The optimized lattice constants of the compounds are in good agreement with the experimental data. The band gap of BiTl9Te6 and SbTl9Te6 compounds are found to be equal to 0.589 eV and 0.538 eV, respectively and are in agreement with the available experimental data. To compare the thermoelectric properties of the different compounds we calculate their thermopower using Mott’s law and show, as expected experimentally, that the substituted tellurides have much better thermoelectric properties compared to the pure compound.
Abstract: Self-diffusion of the metal and carbon atoms in TiC and ZrC carbides is studied by first principles methods. Our calculations yield point defects energies, vacancy jump barriers and diffusion pre-factors in TiC and ZrC. The results are in reasonable agreement with the available experimental data and suggest that the self-diffusion mechanism for metal atoms in these carbides may involve nearest-neighbor vacancy pairs (one metal and one carbon vacancy).
Abstract: Molecular dynamics and molecular statics have been used to explore the transition between the partially Zener ordered state of carbon in octahedral sites. In this communication we have specifically used isothermal molecular dynamics with a recent Fe-C EAM potential to examine the observed high temperature transition from the Zener ordered state where carbon resides on only 1/3 of all octahedral sites to a state where all octahedral sites are available for occupation. It is shown that the Zener ordered state begins to disorder at temperatures well below the transition temperature and that this disordering occurs without any spatial correlation.
Abstract: Magnetic Cluster Expansion method is applied to the investigation of magnetic properties of Fe-Cr alloys treated as a function of Cr content, the spatial distribution of Cr atoms, and temperature. Random Fe-Cr alloys and Cr clusters formed in concentrated alloys are analyzed. We find significant differences between the types of magnetic order characterizing those systems, which are reflected in the characteristic variation of the temperature-dependent magnetic specific heat. Simulations show that in random Fe-Cr alloys and in alloys containing Cr clusters, the interplay between antiferromagnetic interactions characterizing Fe-Cr and Cr-Cr atom pairs gives rise to unusual patterns of finite temperature magnetic ordering.
Abstract: Surface segregation in transition metals can be analysed within a generalised Ising model,derived from Tight-Binding electronic structure calculations, which identiﬁes three driving forces:the difference in surface energy and atomic volume between the two components and their tendencyto order or phase separate in the bulk. Using this ”three effects” rule, we present here general mapswhich predict the tendency of the solute metal element to segregate (or not) at the surface of a metalmatrix, for the 702 solute/matrix systems that can be formed with transition metal elements. Ourpredictions compare fairly well to the existing ab initio calculations and experimental data availableon these systems. The few exceptions, which mainly concern given matrix elements are discussed indetails.
Abstract: The decomposition of Fe-Cr solid solutions during thermal aging is modeled by Atomistic Kinetic Monte Carlo (AKMC) simulations, using a rigid lattice approximation with composition dependant pair interactions that can reproduce the change of sign of the mixing energy with the alloy composition. The interactions are fitted on ab initio mixing energies and on the experimental phase diagram, as well as on the migration barriers in iron and chromium rich phases. Simulated kinetics is compared with 3D atom probe and neutron scattering experiments.
Abstract: Kinetic Monte Carlo (KMC) simulation is a valuable tool to investigate conﬁgu-ration changes in intermetallic compounds. The elementary process is the jump of an atomfrom a lattice site to a neighboring vacancy. In classical transition state theory the jump ratecontains the energy diﬀerence between the original equilibrium state and the saddle point (=transition) state. In traditional KMC the saddle point has mostly received rather careless treat-ment, setting it constant or relating it to the type of jumping atom. In the present work, saddlepoint heights were considered explicitly. Taking L12 ordered Ni3Al as an example, jump energyproﬁles for various atom environments were calculated ab initio in relaxed conﬁgurations ofa 3x3x3 supercell, employing the Nudged Elastic Band method where necessary. From theseresults, eﬀective ’pure’ saddle point heights were extracted. To show the eﬀect on kinetics,simulations of order-order transitions were done with jump probabilities based on these results.When compared to the old assumption of constant saddle point heights, both overall kineticsand detailed jump statistics result considerably changed.