Amdulla O. Mekhrabov

The effect of substitutional impurities on the evolution of FeAl diffusion layer

Abstract The formation and growth characteristics of Fe based aluminide diffusion layers at the FeAl interface have been analysed in terms of interfacial interaction potentials based on the statistico-thermodynamical theory of multicomponent alloys combined with electronic theory in the pseudopotential approximation. The pairwise interatomic interaction potentials and partial ordering energies have been calculated to predict the effect of various alloying additions on the activity coefficient of Al atoms in α-Fe0.95(AlI − nXn)0.05 alloys where X = Si, Ge, Sb, Mg, Cu, Ca, Ag, Cd, Cr, Co, Zn, Mn, Ni, Pb or Bi and considered up to 1 at.%. The results of calculation show that the impurity elements with regard to their effect on the activity coefficient of Al atoms may be classified into two groups; I-group impurity elements, XI = Si, Ti, Ge, Sb, Mg, Cu, Ca, Ag, Cd, and Cr which decrease the activity coefficient of Al atoms, reduce the thickness of the FeAl intermetallic diffusion layer and II-group impurity elements, XII = Co, Zn, Mn, Ni, Pb and Bi, which increase the activity coefficient of Al atoms, tend to increase the thickness of diffusion layer at the FeAl interface. It has been established, in agreement with experimental observation, that the value of activity coefficient of Al atoms in α-Fe0.95(AlI − nXn)0.05 alloys has a strong influence on the formation and growth kinetics of interfacial diffusion layers at the FeAl interface. The analysis of the effects of type and content of XI and XII impurity elements on the activity coefficients of Al atoms, γ Al γ o Al , in α-Fe0.95(Al1 − nXn)0.05 alloys is in good qualitative agreement with the experimental results reported in the literature.

Atomic ordering characteristics of Ni3Al intermetallics with substitutional ternary additions

Abstract The effects of substitutional ternary additions of Me = Zn, Ti, Si, Cr, Mn, Mo, W, Nb, Ta, V, Hf, or Zr on the energetical and structural characteristics of atomic short-range ordering (SRO) of Ni 3 (Al 1 − x Me x ) intermetallics with L1 2 type ordered structure have been analysed by combining the statistical theory of ordering with the electronic theory of alloys in pseudopotential approximation. The partial ordering energies and pairwise SRO parameters were calculated by taking into account the influence of the first three coordination spheres. The results of calculation show that the atoms of Zn, Ti, Si, Mo or V elements substitute mainly for Al sublattice sites, whereas W, Nb, Ta, Hf, or Zr element atoms substitute preferentially for the Ni sublattice sites and Cr or Mn element atoms tend to substitute for both Ni and Al sublattice sites. These theoretical results are in a good qualitative agreement with experimental observations for most of the third component Me elements.

Modelling and Monte Carlo simulation of the atomic ordering processes in Ni3Al intermetallics

The evolution of atomic ordering processes in Ni3Al has been modelled by a Monte Carlo (MC) simulation method combined with the electronic theory of alloys in pseudopotential approximation. The magnitudes of atomic ordering energies of atomic pairs in the Ni3Al system have been calculated by means of electronic theory in pseudopotential approximation up to the 4th coordination spheres and subsequently used as input data for MC simulation for more detailed analysis for the first time. The Bragg–Williams long-range order (LRO) and Cowley–Warren short-range order (SRO) parameters have been calculated from the equilibrium configurations attained at the end of the MC simulation for each predefined temperature and Al concentration levels which reveal the evolution of the system from the L12 → disordered state as the temperature increases. The values of the LRO and SRO parameters at temperatures below 800 °C justify the existence of L12-type ordered Ni3Al alloys for all concentration levels whereas at temperatures above 850 °C, the values of the LRO parameter implies the presence of a disordered solid solution of Ni3Al alloys. However, a higher degree of the SRO order parameter above the predicted order–disorder transition temperature is preserved up to the melting point of Ni3Al intermetallics. The composition dependence of the LRO and SRO parameters agrees well with the experimental results.

A study of impurity effect on ordering characteristics of Fe3Al intermetallics

Abstract A study is carried out into energetical and structural characteristics of atomic ordering processes in Fe 3 Al. The statistico-thermodynamical theory of ordering by a quasichemical method is combined with the electronic theory of alloys in pseudopotential approximation in order to predict impurity effects on DO 3 ↔ B2 phase transition temperature and the characteristics of atomic short-range order in Fe 3 Al-type intermetallics. Impurity elements in Fe 3 (Al, Me) Me = Cr, Si, Ge, Cu, Mn, Zn, V, Nb, Ta, Mo, W, Zr or Hf are considered (up to 1 at.% concentration). The study shows, in agreement with experimental observations in literature, that all impurity elements raise the DO 3 ↔ B2 transition temperature relative to that of the binary alloy. It is further found that impurities of Nb, Ta, Mo, W, Zr or Hf substitute mainly Fe sites in the Fe 3 Al (DO 3 ) superstructure and are more effective in increasing the transition temperature.

The Effect of Alloying Additions on the Interfacial Interactions at the Fe-Al Interface During Coating

New phases or compounds, mainly intermetallics, appear if two different materials are in contact at sufficiently high temperatures, i.e. during joining operation, sintering, coating and other surface treatments. The formation and development of new intermetallic phases in those treatments depend on the range of interfacial interactions of the constituent materials which are governed by the crystal structure, solid solubility limit, the magnitude and sign of interatomic interaction potentials between the atoms of two different materials and also their interdiffusion coefficient.

High-temperature site preference and atomic short-range ordering characteristics of ternary alloying elements in γ’-Ni3Al intermetallics

Abstract Remarkable high-temperature mechanical properties of nickel-based superalloys are correlated with the arrangement of ternary alloying elements in L12-type-ordered γ′-Ni3Al intermetallics. In the current study, therefore, high-temperature site occupancy preference and energetic-structural characteristics of atomic short-range ordering (SRO) of ternary alloying X elements (X = Mo, W, Ta, Hf, Re, Ru, Pt or Co) in Ni75Al21.875X3.125 alloy systems have been studied by combining the statistico-thermodynamical theory of ordering and electronic theory of alloys in the pseudopotential approximation. Temperature dependence of site occupancy tendencies of alloying X element atoms has been predicted by calculating partial ordering energies and SRO parameters of Ni-Al, Ni-X and Al-X atomic pairs. It is shown that, all ternary alloying element atoms (except Pt) tend to occupy Al, whereas Pt atoms prefer to substitute for Ni sub-lattice sites of Ni3Al intermetallics. However, in contrast to other X elements, sub-lattice site occupancy characteristics of Re atoms appear to be both temperature- and composition-dependent. Theoretical calculations reveal that site occupancy preference of Re atoms switches from Al to both Ni and Al sites at critical temperatures, Tc, for Re > 2.35 at%. Distribution of Re atoms at both Ni and Al sub-lattice sites above Tc may lead to localised supersaturation of the parent Ni3Al phase and makes possible the formation of topologically close-packed (TCP) phases. The results of the current theoretical and simulation study are consistent with other theoretical and experimental investigations published in the literature.

A generalized polytetrahedral cluster approach to partial coordination numbers in binary metallic glasses

Partial coordination numbers (CNs) play a substantial role in description of hetero-coordinated local structure and related short-range order in metallic glasses. By defining a polytetrahedral aggregation for solute–solvent type atomic clusters, which retains cluster sphericity, high solute–solvent and solvent–solvent CNs, a preliminary model is developed to estimate the CN of a solute atom within a non-isolated cluster embedded in the glassy environment. Employing the result, a generalized quasi-hard sphere model is constructed by defining intra- and inter-cluster correlations, which can yield significantly close values to experimentally derived partial CNs in a great many metallic glass systems, such as Fe–B, Ni–B, Ni–P, Co–P, Pd–Si, Al–Y, Co–Zr, Co–Ti, Ni–Ti, Zr–Ni, Zr–Pd, Zr–Pt and Cu–Zr. The approach allows evaluation of a complete set of partial CNs in a given binary system as a function of atomic radii and composition.

Modeling the kinetics of atomic ordering in high temperature intermetallics

Canonical ensemble Monte Carlo study was performed for the analysis of ordering characteristics of the high temperature X 3 Al (X=Fe and Ni)-based ordered intermetallics. Calculated partial ordering energies by means of the electronic theory of alloys in pseudopotential approximation were utilized as input data to determine the Hamiltonian of the system. Bragg-Williams long-range order (LRO) and Cowley-Warren short-range order (SRO) parameters were calculated from the equilibrium configurations attained at the end of Monte Carlo simulation for each predefined temperature and Al concentration levels of X 3 Al intermetallics. It was shown that the current predictions agree well with the experimental values of LRO and SRO parameters, indicating that the systems have transformed into perfect ordered superlattices by forming a single domain in the lattice structures. It seems that this present approach on the combination of Monte Carlo simulation methods with the electronic theory of alloys in pseudopotential approximation can be successfully applied to elucidate the ordering characteristics of high temperature Fe 3 Al and Ni 3 Al intermetallics.