19 research outputs found
Stacking-Fault Energy and Anti-Invar Effect in FeMn Alloys
Based on state-of-the-art density-functional-theory methods we calculate the
stacking-fault energy of the paramagnetic random Fe-22.5at.%Mn alloy between
300-800 K. We estimate magnetic thermal excitations by considering longitudinal
spin-fluctuations. Our results demonstrate that the interplay between the
magnetic excitations and the thermal lattice expansion is the main factor
determining the anti-Invar effect, the hcp-fcc transformation temperature, and
the stacking-fault energy, which is in excellent agreement with measurements.Comment: 5 pages, 3 figure
Pareto-optimal alloys
Large databases that can be used in the search for new materials with
specific properties remain an elusive goal in materials science. The search
problem is complicated by the fact that the optimal material for a given
application is usually a compromise between a number of materials properties
and the price. In this letter we present a database consisting of the lattice
parameters, bulk moduli, and heats of formation for over 64,000 ordered
metallic alloys, which has been established by direct first-principles
density-functional-theory calculations. Furthermore, we use a concept from
economic theory, the Pareto-optimal set, to determine optimal alloy solutions
for the compromise between low compressibility, high stability and price.Comment: 5 pages, 2 figures, To be published in Appl. Phys. Let
Alloy Formation and Surface Segregation in Zeolite-Supported PtPd Bimetallic Catalysts
We present extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements on three zeolite-supported bimetallic Pt−Pd catalysts prepared by wet impregnation. Alloy formation and segregation of Pd to the surface of the small bimetallic particles is clearly identified. Ab initio LMTO calculations are used to predict segregation profiles of low-index single-crystal Pt−Pd alloy surfaces. Pd is found to be enriched at the surface. EMT potentials for Pt and Pd are adjusted in order to give the same segregation profiles at low-index single-crystal surfaces as linear muffin tin orbital (LMTO) calculations. Monte Carlo simulations based on the adjusted EMT potentials are then used to calculate the equilibrium structure of small bimetallic Pt−Pd particles. Coordination numbers derived from these equilibrium structures are compared with those derived from EXAFS analysis, and a good agreement is found
Effective interactions and atomic ordering in Ni-rich Ni-Re alloys
Interatomic interactions and ordering in fcc Ni-rich Ni-Re alloys are studied by means of first-principles methods combined with statistical mechanics simulations based on the Ising Hamiltonian. First-principles calculations are employed to obtain effective chemical and strain-induced interactions, as well as ordering energies and enthalpies of formation of random and ordered Ni-Re alloys. Based on the nonmagnetic enthalpies of formation, we speculate that the type of ordering can be different in alloys with Re content less than 10 at.%. We demonstrate that effective chemical interactions in this system are quite sensitive to the alloy composition, atomic volume, and magnetic state. In statistical thermodynamic simulations, we have used renormalized interactions, which correctly reproduce ordering energies obtained in the direct total energy calculations. Monte Carlo simulations for Ni 0.91 Re 0.09  alloy show that there exists a strong ordering tendency of the (112 0) type leading to precipitation of the D1 a  ordered structure at about 940 K. Our results for the atomic short-range order indicate, however, that the presently applied theory overestimates the strength of the ordering tendency compared to that observed in the experiment.QC 20160721</p