Carlos Amador

Site Occupancy of Ternary Additions to B2 Alloys

Abstract In this broad-based survey study, the substitutional site preference of ternary alloying additions to B2 compounds (stable at room temperature and 50/50 composition) is determined using the Bozzolo–Ferrante–Smith (BFS) method for alloys. The method is applied to Ni, Al, Ti, Cr, Cu, Co, Fe, Ta, Hf, Mo, Nb, W, V and Ru additions to NiAl, FeAl, CoAl, CoFe, CoHf, CoTi, FeTi, RuAl, RuSi, RuHf, RuTi, and RuZr. The results are compared, when available, to experimental data and other theoretical results.

An Introduction to the BFS Method and Its Use to Model Binary NiAl Alloys

We introduce the Bozzolo–Ferrante–Smith (BFS) method for alloys as a computationally efficient tool for aiding in the process of alloy design. An intuitive description of the BFS method is provided, followed by a formal discussion of its implementation. The method is applied to the study of the defect structure of NiAl binary alloys. The groundwork is laid for a detailed progression to higher order NiAl-based alloys linking theoretical calculations and computer simulations based on the BFS method and experimental work validating each step of the alloy design process.

Density Functional Theory

The constrained search approach, mappings to external potentials, and virial-like theorems for electron-density and one-matrix energy-functional theories.- Density matrices, reduced density matrices, a geometric investigation of their properties, and applications to density functional theory.- Properties of one-matrix energy functionals.- Self-interaction correction.- Some recent developments in density functional theory.- Electron gas models and density functional theory.- Electron structure calculations for heavy atoms: A local density approach.- Density functionals obtained from models of the electron first and second order density matrices.- Free energy density functionals for non-uniform classical fluids.

BFS simulation and experimental analysis of the effect of Ti additions on the structure of NiAl

The Bozzolo–Ferrante–Smith (BFS) method for alloy energetics is applied to the study of ternary additions to NiAl. A description of the method and its application to alloy design is given. Two different approaches are used in the analysis of the effect of Ti additions to NiAl. First, a thorough analytical study is performed, where the energy of formation, lattice parameter and bulk modulus are calculated for a large number of possible atomic distributions of Ni, Al and Ti. Substitutional site preference schemes and formation of precipitates are thus predicted and analyzed. The second approach used consists of the determination of temperature effects on the final results, as obtained by performing a number of large-scale numerical simulations using the Monte Carlo–Metropolis procedure and BFS for the calculation of the energy at every step in the simulation. The results indicate a sharp preference of Ti for Al sites in Ni-rich NiAl alloys and the formation of ternary Heusler precipitates beyond the predicted solubility limit of 5 at. % Ti. Experimental analysis of three Ni–Al–Ti alloys confirms the theoretical predictions.

Atomistic Simulations of Ti Additions to NiAl

The development of more efficient engines and power plants for future supersonic transports depend on the advancement of new high-temperature materials with temperature capabilities exceeding those of Ni-based superalloys. Having theoretical modelling techniques to aid in the design of these alloys would greatly facilitate this development. The present paper discusses a successful attempt to correlate theoretical predictions of alloy properties with experimental confirmation for ternary NiAI-Ti alloys. The B.F.S. (Bozzolo- Ferrante-Smith) method for alloys is used to predict the solubility limit and site preference energies for Ti additions of I to 25 at. % to NiAI. The results show the solubility limit to be around 5 % Ti, above which the formation of Heusler precipitates is favored. These results were confirmed by transmission electron microscopy performed on a series of NiAI-Ti alloys.

Local Approximations in the Applications of Density Functional Theory

We present two examples of the use of local approximations in density functionals (DF) theory: 1) the estimation of the ionization potentials and spectroscopic terms for atoms and 2) the calculation of spin susceptibility for atoms in metals, and use them to review some current trends in the applications of DF. In the first case we stress that symmetry and statistics should be built into the basic formulation of the computational schemes in order to obtain realistic results. In the second example we show that DF allows the treatment of small regions of a material as a system in itself, provided the proper boundary conditions are considered for the density function and for the auxiliary Kohn-Sham wave functions. Some numerical results are given for several materials.

Chemical bonding and structural relaxation of amorphous ZrxCu1−x

Abstract Experimental evidence for the thermally induced structural relaxation of amorphous Zr 70 Cu 30 with changes in transport and superconducting properties has been found by De la Cruz. We show that chemical bonding between Zr and Cu, favoring an optimum ZrCu distance can be responsible for this behavior. Our discussion is based on electronic structure calculations for discrete clusters embedded in the metallic medium showing this bonding and the related energy changes. The found electronic density of states is correlated with the transport properties as a function of structural relaxation.

El mundo finito: Desarrollo sustentable en el siglo de oro de la humanidad

Abstract Natural resources are limited. At the ever-growing need for energy, the human kind have devised new and improved ways to provide energy from the natural materials from the world. But these products are limited and one day, they will end. How much time before we run out or reserves? Nobody knows although many studies, debates and research has been developed. When the natural resources end, we will have to devise as a society the technological success we have accomplished thanks to the abundance of them. Education, government normativity and social consciousness besides real world collaboration will be key factors when that day approaches.

Phase Diagrams from First-Principles Calculations: the One Magnetic Component Ni-Pt Alloy

The cluster expansion technique is used in combination with first-principles calculations of the total energy of ordered compounds to study the energetics of ordered and disordered phases of the NiPt alloy. The first principles calculations (at 0 K) are performed allowing for magnetization. Stable magnetic phases for (fcc) Ni, (L12) Ni3Pt, and (L10) NiPt are found. The Hamiltonian proposed by Dahmani et al. 1 is used to describe the energetics of these phases at finite temperature. However, in the present case, the parameters are derived from the first-principles calculations with no resort to empirically adjusted parameters. Very good agreement with the observed Curie temperatures for the disordered phase of compositions x Ni = 0 and x Ni = 0.25 is found. The effect of the elastic energy arising from the big size mismatch of the constituting compounds is also discussed.

The Formulation of Density Functional Theory

We have generalized the formulation of density functional theory in a form suitable for multiconfiguration calculations, and for the calculation of electric and magnetic response functions. The method allows the direct calculation of mixed valence states, spectra (and not just the energy eigenvalues and their differences) and in general, of the elementary excitations of the system.