Guillermo Bozzolo

Multilayer Relaxation and Surface Energies of FCC and BCC Metals Using Equivalent Crystal Theory

Abstract The multilayer relaxation of fcc and bcc metal surfaces is calculated using equivalent crystal theory. The results for changes in interplanar spacings of planes close to the surface and the ensuing surface energies are discussed in reference to other theoretical results and compared to available experimental data. The calculation includes high-index surfaces for which no other theoretical results are known.

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.

Modeling of ternary element site substitution in NiAl

It is well recognized that ternary alloying additions can have a dramatic impact on the behavior of ordered intermetallic alloys such as nickel aluminides. Properties as diverse as yield strength, fracture strength, fracture mode, cyclic oxidation resistance, creep strength, and thermal and electrical diffusivity can change by orders of magnitude when a few percent or less of a ternary element is added. Yet our understanding of the resulting point defect structures and the simple site preferences of ternary alloying additions is poor because these are extremely difficult characteristics to determine. This disconnection between the understanding of the structure and properties in ordered alloys is at least in part responsible for the limited development and commercialization of these materials. Theoretical methods have provided useful but limited insight in this area, since most techniques suffer from constraints in the type of elements and the crystallographic structures that can be modeled. In an effort to overcome these limitations, the Bozzolo–Ferrante–Smith (BFS) method for alloys was designed. After a brief description of this approximate quantum mechanical approach, we use BFS to investigate the energetics of Si, Ti, V, Cr, Fe, Co, Cu, Zr, Nb, Mo, Ru, Hf, Ta and W additions to B2-ordered, stoichiometric NiAl. In addition to determining the site preference for these alloying additions over a range of compositions, we include results for the concentration dependence of the lattice parameter. In this introductory paper, we performed our analyses in the absence of constitutional and thermal vacancies for alloys of the form Ni50(Al,X)50. Where data exist, a comparison between experimental, theoretical, and BFS results is also included.

Surface segregation in multicomponent systems: Modeling of surface alloys and alloy surfaces

Abstract The study of surface segregation, though of great technological importance, has been largely restricted to experimental work due to limitations associated with theoretical methods. However, recent improvements in both first-principles and semiempirical methods are opening the doors to an array of new possibilities for surface scientists. We apply one of these techniques, the BFS method for alloys, which is particularly suitable for complex systems, to several aspects of the computational modeling of surfaces and segregation, including alloy surface segregation, structure and composition of alloy surfaces, and the formation of surface alloys. We conclude with the study of complex NiAl-based binary, ternary and quaternary thin films (with Ti, Cr and Cu additions to NiAl). Differences and similarities between bulk and surface compositions are discussed, illustrated by the results of Monte Carlo simulations. For some binary and ternary cases, the theoretical predictions are compared to experimental results, highlighting the accuracy and value of this developing theoretical tool.

Modeling of the Site Preference in Ternary B2-Ordered Ni-Al-Fe Alloys

The underlying equilibrium structure, site substitution behavior, and lattice parameter of ternary Ni-Fe-Al alloys are determined via Monte Carlo-Metropolis computer simulations and analytical calculations using the BFS method for alloys for the energetics. As a result of the theoretical calculations presented, a simple approach based on the energetics of small atomic clusters is introduced to explain the observed site preference schemes.

Atomistic Modeling of Surface and Bulk Properties of Cu, Pd and the Cu-Pd System

Abstract The BFS method for alloys is applied to the study of the Cu–Pd system. A variety of issues are analyzed and discussed, including the properties of pure Cu or Pd crystals (surface energies, surface relaxations), Pd/Cu and Cu/Pd surface alloys, segregation of Pd (or Cu) in Cu (or Pd), concentration dependence of the lattice parameter of the high temperature fcc CuPd solid solution, the formation and properties of low temperature ordered phases, and order–disorder transition temperatures. Emphasis is made on the ability of the method to describe these properties on the basis of a minimum set of BFS universal parameters that uniquely characterize the Cu–Pd system.

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.

Atomistic modeling of RuAl and (RuNi)Al alloys

Abstract Atomistic modeling of RuAl and RuAlNi alloys is performed. The BFS method for alloys and its first-principles-based parameters are tested by comparing to the lattice parameter and energy of formation of B2 RuAl and (Ru 50− x Ni x )Al 50 alloys as a function of Ni concentration. Additional tests include Monte Carlo simulations for compositions close to Ru 25 Ni 25 Al 50 showing no obvious evidence of a miscibility gap and separation of the individual B2 phases.

Heat of segregation of single substitutional impurities

The method of Bozzolo, Ferrante and Smith (BFS) is applied for the calculation of the heat of segregation of single substitutional impurities in fcc metals. A simple equation for predicting the heat of segregation is derived for the rigid case (no atomic relaxations). The results of including atomic relaxation using a Monte Carlo method are also presented and the results compared with a number of experimental and theoretical results.

Multilayer relaxation and surface structure of ordered alloys

Using BFS, a new semiempirical method for alloys, we study the surface structure of fcc ordered binary alloys in the L12 structure (Ni3Al and Cu3Au). We show that the surface energy is lowest for the mixed-composition truncation of the low-index faces of such systems. Also, we present results for the interlayer relaxations for planes close to the surface, revealing different relaxations for atoms of different species producing a rippled surface layer.