The elastic stiffness coefficients of iron-aluminum alloys—I experimental results and thermodynamic analysis

Influence of long- and short-range chemical order on spontaneous magnetization in single-crystalline Fe0.6Al0.4compound thin films

We systematically investigate the long- and short-range chemical order, lattice volume, and spontaneous magnetization in single-crystalline Fe0.6Al0.4compound thin films. The vapor-quenching method based on a molecular beam epitaxy technique is utilized to fabricate the single-crystalline Fe0.6Al0.4compound with the differentB2 long-range order parameterS. Swas varied by the deposition temperatureTd,and it increases with increasingTd. The lattice volumeVdecreased with increasingTd, while the tetragonal distortion, ∼4%, due to epitaxial strain were observed. The changes inSandVwere accompanied with the change in the magnetic moment per Fe,μFe.μFeshowed the monotonic decrease as a function ofSwhereasμFemonotonically increases withV. With considering tetragonal distortion,μFe-Vrelationship has a good agreement with the previous reports. TheμFe-Srelationship showed the steep decrease ofμFearoundS∼ 0.6. In contrast toμFe-Vrelationship,μFe-Srelationship does not match only from ours to previous studies but also among other reports. It implies the statistical number of the nearest-neighbor Fe-Fe bonds, i.e.S, cannot be an enough explanatory parameter. To clarify the structural origin of change inμFe, the short-range order (SRO) parameter inferred from the analysis of superlattice diffractions were introduced. They showed the clear difference for the films with high and lowμFe. The results suggest that the transition from the long- to the SRO state plays the significant role onμFe.

Elasticity of Phases in Fe-Al-Ti Superalloys: Impact of Atomic Order and Anti-Phase Boundaries

We combine theoretical and experimental tools to study elastic properties of Fe-Al-Ti superalloys. Focusing on samples with chemical composition Fe71Al22Ti7, we use transmission electron microscopy (TEM) to detect their two-phase superalloy nano-structure (consisting of cuboids embedded into a matrix). The chemical composition of both phases, Fe66.2Al23.3Ti10.5 for cuboids and Fe81Al19 (with about 1% or less of Ti) for the matrix, was determined from an Energy-Dispersive X-ray Spectroscopy (EDS) analysis. The phase of cuboids is found to be a rather strongly off-stoichiometric (Fe-rich and Ti-poor) variant of Heusler Fe2TiAl intermetallic compound with the L21 structure. The phase of the matrix is a solid solution of Al atoms in a ferromagnetic body-centered cubic (bcc) Fe. Quantum-mechanical calculations were employed to obtain an insight into elastic properties of the two phases. Three distributions of chemical species were simulated for the phase of cuboids (A2, B2 and L21) in order to determine a sublattice preference of the excess Fe atoms. The lowest formation energy was obtained when the excess Fe atoms form a solid solution with the Ti atoms at the Ti-sublattice within the Heusler L21 phase (L21 variant). Similarly, three configurations of Al atoms in the phase of the matrix with different level of order (A2, B2 and D03) were simulated. The computed formation energy is the lowest when all the 1st and 2nd nearest-neighbor Al-Al pairs are eliminated (the D03 variant). Next, the elastic tensors of all phases were calculated. The maximum Young’s modulus is found to increase with increasing chemical order. Further we simulated an anti-phase boundary (APB) in the L21 phase of cuboids and observed an elastic softening (as another effect of the APB, we also predict a significant increase of the total magnetic moment by 140% when compared with the APB-free material). Finally, to validate these predicted trends, a nano-scale dynamical mechanical analysis (nanoDMA) was used to probe elasticity of phases. Consistent with the prediction, the cuboids were found stiffer.

Strength and Brittleness of Interfaces in Fe-Al Superalloy Nanocomposites under Multiaxial Loading: An ab initio and Atomistic Study

We present an ab initio and atomistic study of the stress-strain response and elastic stability of the ordered Fe3Al compound with the D03 structure and a disordered Fe-Al solid solution with 18.75 at.% Al as well as of a nanocomposite consisting of an equal molar amount of both phases under uniaxial loading along the [001] direction. The tensile tests were performed under complex conditions including the effect of the lateral stress on the tensile strength and temperature effect. By comparing the behavior of individual phases with that of the nanocomposite we find that the disordered Fe-Al phase represents the weakest point of the studied nanocomposite in terms of tensile loading. The cleavage plane of the whole nanocomposite is identical to that identified when loading is applied solely to the disordered Fe-Al phase. It also turns out that the mechanical stability is strongly affected by softening of elastic constants C′ and/or C66 and by corresponding elastic instabilities. Interestingly, we found that uniaxial straining of the ordered Fe3Al with the D03 structure leads almost to hydrostatic loading. Furthermore, increasing lateral stress linearly increases the tensile strength. This was also confirmed by molecular dynamics simulations employing Embedded Atom Method (EAM) potential. The molecular dynamics simulations also revealed that the thermal vibrations significantly decrease the tensile strength.

Theoretical elastic moduli of ferromagnetic bcc Fe alloys

The polycrystalline elastic parameters of ferromagnetic Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 ≤ x ≤ 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poisson's ratio (ν) of bcc Fe. The complex composition dependence of the C(44) single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Young's modulus (E), and Debye temperature (Θ). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects.

Modified embedded-atom method interatomic potential for the Fe-Al system

An interatomic potential for the Fe-Al binary system has been developed based on the modified embedded-atom method (MEAM) potential formalism. The potential can describe various fundamental physical properties of Fe-Al binary alloys--structural, elastic and thermodynamic properties, defect formation behavior and interactions between defects--in reasonable agreement with experimental data or higher-level calculations. The applicability of the potential to atomistic investigations of various defect formation behaviors and their effects on the mechanical properties of high aluminum steels as well as Fe-Al binary alloys is demonstrated.

Ideal Tensile Strength of Ni3Al and Fe3Al with D03 Structure

The ideal tensile strength along the [111] direction in the Fe3Al and Ni3Al intermetallic compounds with the D03 structure has been calculated from the first principles using the fullpotential linearized augmented plane-wave method (FP LAPW) within the density functional theory (DFT). The strains corresponding to the maximum sustainable stresses in both materials were determined and compared. The behavior of atomic magnetic moments as a function of strain was analyzed. The tensile test simulations have been theoretically simulated employing both the local density approximation (LDA) and generalized gradient approximation (GGA) for the exchangecorrelation potential.

Electron microscopy of antiphase domain boundary tubes in deformed ordered alloys

Antiphase domain boundary (APB) tubes have been observed in a slightly deformed Fe–30.5 at. % Al alloy in dark field electron microscope images taken in superlattice reflexions. The image contrast theory has been developed and accounts satisfactorily for the nature of the APB tube contrast observed. The contrast theory is used to estimate the heights of the tubes. The widths of the tubes range from 20 to 50 ņ, and the measured heights are in the range of one to six times the elementary height. Evidence is presented that tubes are generated by the Vidoz & Brown (1962) mechanism and by a mechanism of cross-slip and annihilation of screw superdislocations. The possible effects of tubes on flow stress are discussed.