Binary B2 Research Articles

Towards controllable formation of antiphase boundaries (APBs) in the B2 matrices of the Al-Fe based complex multicomponent alloys

Anti-Phase Boundaries (APBs) formation in B2 (ordered Body Centered Cubic) structures was investigated, focusing on the Al-Fe-Cr/Ni alloys. The current research goal was to propose strategies for controlling APBs formation in multicomponent alloys, following their positive effect on properties of High Entropy Alloys, stated in previous researches. This study is based on our previous investigation of the APBs formation mechanisms in binary B2 Fe50+xAl50-x alloys. There, stable APBs have formed at and above specific Long-Range order parameter. Here, it was established that stabilization of APBs requires disorder at only Al sublattice. Thorough analysis of dislocations revealed that higher complexity accelerates the decrease of order, promoting the formation of APBs. An assessment of disorder in multicomponent systems is challenging and sometimes even impossible, therefore suggested here usage of evaluation of dislocations is an interesting and useful experimental tool to estimate the disorder in these complex systems.

A comparative study of the amorphization of NiTi-B2 structure by anti-site defects

This paper reports a comparative study of the amorphization of B2 structure of near-equiatomic NiTi and three other equiatomic binary B2 alloy systems, including AlFe, CuZr and FeTi. The study was conducted by means of molecular dynamics simulation on the effects of anti-site defects within the B2 structure. It was found that anti-site defects cause severe random distortions to the B2 structures, lowering their crystalline ordering. Among the four B2 systems studied, equiatomic AlFe and FeTi cannot be fully amorphized by anti-site defects but NiTi and CuZr can become fully amorphous at above a threshold concentration of the defects, as compared to the structure of quenched liquid. It is also found that the threshold anti-site concentration for amorphization decreases with increasing temperature. In comparison between the two, NiTi is easier to be amorphized than CuZr by anti-site defects, both in terms of the threshold anti-site defect concentration required and the temperature range within which it is possible. These findings may help to explain the long-observed phenomenon of the easiness of equiatomic NiTi to become amorphous during cold working and the new observation of third element substitutional diffusion caused amorphization.

Structures of Sm–Cu intermetallics with Fe as subphase candidates in SmFe12-based permanent magnets studied by first-principles thermodynamics

Since crystalline grain-boundary subphases are expected in SmFe12-based permanent magnets, stable crystal structures dependent on temperatures and compositions are examined by first-principles thermodynamics. The free-energy landscape of ternary Sm–Cu–Fe is constructed with self-consistent phonon theory to overcome the dynamical instability problem, where a barrier between binary B2 Sm–Cu and bcc Fe is found. Cu-rich B2 Sm–Cu is substantially stabilized by phonons as well as by configurational entropy. Furthermore, Fe atoms in the Cu sublattice of B27 SmCu contribute to the relative phase stabilization. These results indicate the existence of B2–B27 two-phase equilibria expanding the stable composition region of nonmagnetic SmCu-based intermetallics.

Influence of alloying elements and the state of order on the formation of antiphase boundaries in B2 phases

Alloys with single phase B2 structure exhibit interesting physical properties, such as stability at high temperatures and oxidation resistance, but have low fracture toughness and limited ductility. The presence of antiphase boundaries (APBs) has been proposed as one of the factors improving the ductility of the B2 phases. An example of novel B2/A2 dual-phase alloy, which comprises such defects, is the AlCoCrFeNi multicomponent alloy (also known as high-entropy alloy (HEA)). Presence of APBs positively affected the performance of this alloy; therefore, control over the appearance of APBs is important. Current paper presents a first step in this direction, by studying the energy of APBs formation in binary B2 structures forming in the Al–Co–Cr–Fe–Ni system as a function of doping by ternary element from this system. Calculations, based on Density Functional Theory, took into account atom occupancy of the additives (i.e. in Al or in transition metal's sub-lattice) and position (at the APBs or at grains' interior). Following the presented here calculations, casting and characterization of the alloy, where APBs should be promoted, was made, proving the outcomes of this work. AlFe binary system was found to be the most probable (out of all possible binary prototypes) to form APBs. Furthermore, alloying by Cr (which substitutes Al) and Ni (which substitutes Fe) promote even more APBs formation. Taking into account anti-site defects, frequently forming in the AlFe B2 structure, the degree of disorder was related to the temperature – proposing an interesting aspect of promoting the formation of APBs.

Novel Multicomponent B2-Ordered Aluminides: Compositional Design, Synthesis, Characterization, and Thermal Stability

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi followed to produce Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi), and Al(CoCuFeMnNi) multicomponent alloys. CALculation of PHAse Diagrams (CALPHAD) approach was used to predict the phases in these alloys. X-ray diffraction and transmission electron microscopy were used to confirm the B2 ordering in the alloys. Thermal stability of the B2 phase in these alloys was demonstrated by prolonged heat treatments at 1373 K and 1073 K up to 200 h.

Lattice Superdeformation as the Synergy of Two Martensitic Transformations in Titanium Nickelide Alloys

The uniqueness of titanium nickelide alloys consists in the binary B2 → R, B2 → B19) martensitic instability of the isotropically soft B2 lattice and in the synergetic effects induced by the superposition of these two transformations, which form the synergetic B19' lattice with martensitic superdeformation formed in the transformations B2 → R → B19', B2 → B19', and B2 → B19 → B19'. The effects of synergy of the two martensitic transformations are observed during the relay B2 → B19 → B19', and the superdeformation effects are most pronounced at the stage B19 → B19'.

Multiscale analysis of an ODS FeAl40 intermetallic after plasma-assisted nitriding

The binary B2 (ordered bcc-type structure) FeAl40 Grade 3 aluminide containing 40 at.% Al was nitrided at 600 °C for 15 min with a pulsed DC plasma in a 95% N2/5% H2 gas mixture. The nitrided layer was described at the micro-, nano- and atomic scales. The nitrided layer consists of a thin outer sublayer (∼1 μm in thickness) containing γ′-Fe4N and having a moderate hardness (870 Hv) below which a thicker (∼36 μm in thickness) and harder (1400 Hv) inner sublayer is formed. While the outer sublayer is completely depleted in Al, the inner sublayer is made of a mixture of hex. AlN and α-Fe phases regularly structured at different levels: i) At the micrometre scale, observations showed a network of α-Fe “wave-like” veins (∼2 μm in length and ∼100 nm in width) located at the grain boundaries and preferentially aligned parallel to the nitrided surface. ii) At the nanometre scale, observations evidenced that such veins are embedded in a “lamellar-like” morphology matrix alternately composed of α-Fe and α-Fe/hex. AlN lamellae (∼300 nm in length and ∼10 nm in width). iii) At the atomic scale, analysis pointed out that each α-Fe/hex. AlN lamella presents a finer structural arrangement in which hex. AlN-rich clusters (∼4 nm in diameter) are trapped between thin α-Fe walls (∼2 nm in thickness). The diversity of the morphologies evidenced in the present study highlights the complexity of the nitriding mechanisms of iron-based aluminides. The observations support however a mechanism of discontinuous precipitation in the inner nitrided sublayer which seems to be strongly promoted by the high amount of Al in the FeAl40 grade 3 alloy (40 at.%).

First principles study on structural, electronic and elastic properties of AgX and AuX (X = Mg, Sc, Zn and Cd) intermetallic compounds

The electronic, elastic and mechanical properties of eight binary B2 – type intermetallic compounds of Ag and Au (AgX and AuX, X=Mg, Sc, Zn and Cd) have been studied systematically using first principles density functional theory within generalized gradient approximation. Ground state properties such as lattice constant (a0), bulk modulus (B), its pressure derivative (B′) and elastic constants are calculated. The present results are in good agreement with the experimental and other available theoretical results. Poisson’s ratio (σ), Young’s moduli (E), shear moduli (GH) and the ratio of elastic anisotropy factor (A) are also estimated. Ductility for these compounds is further analysed by calculating the ratio of B/GH and Cauchy pressure (C12–C44). Amongst all the intermetallics AuZn is found to be most ductile due to the presence of strong metallic bonding, and AgZn is least ductile in all the Au and Ag intermetallic compounds.

Reversible stress-induced martensitic phase transformations in a bi-atomic crystal

In an earlier work, Elliott et al. [2006a, Stability of crystalline solids—II: application to temperature-induced martensitic phase transformations in bi-atomic crystals. Journal of the Mechanics and Physics of Solids 54(1), 193–232], the authors used temperature-dependent atomic potentials and path-following bifurcation techniques to solve the nonlinear equilibrium equations and find the temperature-induced martensitic phase transformations in stress-free, perfect, equi-atomic binary B2 crystals. Using the same theoretical framework, the current work adds the influence of stress to study the model's stress-induced martensitic phase transformations. The imposition of a uniaxial Biot stress on the austenite (B2) crystal, lowers the symmetry of the problem, compared to the stress-free case, and leads to a large number of stable equilibrium paths. To determine which ones are possible reversible martensitic transformations, we use the (kinematic) concept of the maximal Ericksen–Pitteri neighborhood (max EPN) to select those equilibrium paths with lattice deformations that are closest, with respect to lattice-invariant shear, to the austenite phase and thus capable of a reversible transformation. It turns out that for our chosen parameters only one stable structure (distorted α IrV ) is found within the max EPN of the austenite in an appropriate stress window. The energy density of the corresponding configurations shows features of a stress-induced phase transformation between the higher symmetry austenite and lower symmetry martensite paths and suggests the existence of hysteretic stress–strain loops under isothermal load–unload conditions. Although the perfect crystal model developed in this work over-predicts many key material properties, such as the transformation stress and the Clausious–Clapeyron slope, when compared to real experimental values (based on actual polycrystalline specimens with defects), it is—to the authors' knowledge—the first atomistic model that has been demonstrated to capture all essential trends and behavior observed in shape memory alloys.

Effect of vanadium on atomic ordering characteristics and anti-phase boundary energies of B2–FeCo alloys

Effect of V on the energetical and structural characteristics of atomic ordering processes of B2–FeCo intermetallics have been investigated via electronic theory of multi-component alloys combined with Monte Carlo (MC) simulations. Emphasis has been placed on the variation of anti-phase boundary (APB) energies with V content and temperature to establish a relationship between ductility and atomic ordering characteristics of (FeCo) 100− x V x ( x = 0–5 at%) alloys. It was shown that APB energies are highly affected by actual degree of long range order (LRO) and also dissolution mode of V alloying element atoms. Results of present simulation reveals that partially ordered (FeCo) 100− x V x alloy having x = 2 at% V facilitates the highest loss in {110} APB energy in temperature range of 300–750 K and yields drastic increase in <111> {112} APB energies with increasing concentration of V due to pinning effect. Owing to the lowest APB energy of {110} planes, (FeCo) 98V 2 alloy would be predicted to display higher ductility, in agreement with experimental observations, relative to binary B2–FeCo intermetallic due to more independent movement of a/2 <111> partials at low temperatures.

A generalized defect correlation model for B2 compounds

A generalized statistical-thermodynamic model for non-stoichiometric binary B2 phases was developed on the basis of a Bethe–Bragg–Williams approximation, with pair interaction energies as input parameters. The model considers both vacancies and anti-structure atoms as possible point defects on the two sublattices; it allows therefore a description of all types of B2 compounds, independently of their particular defect mechanism. At the same time, the model describes both long-range ordering and short-range ordering in the crystal lattice, i.e. it includes the possible correlation of nearest neighbour point defect combinations. It was tested with ab initio values of pair interaction energies for NiAl from the literature, and good agreement was obtained between calculated values and experimental data for both, aluminum activities and vacancy concentrations. Full sets of pair interaction energies were suggested for three other B2 phases, i.e. FeAl, CoAl, and PdAl, based on experimental data for thermodynamic activities as well as, in the case of FeAl, also for vacancy and Fe anti-structure atom concentrations. The (hypothetical) critical temperatures of the order–disorder transformations were derived from the model calculations.

Ternary Diffusion in a RuAl-NiAl Couple

A ternary diffusion couple assembled with NiAl and RuAl disks and annealed at 1100 °C was examined by scanning electron microscopy and analyzed for concentration profiles by electron microprobe analysis. Complete mutual solid solubility with continuous variations in compositions was observed between the binary B2 aluminides. Ternary interdiffusion coefficients were determined with the aid of a program called MultiDiFlux over two composition ranges, one Ru-rich and the other Ni-rich, within the diffusion zone. The interdiffusion coefficient, \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{RuRu}}}} \) varies little with variation in composition, but the interdiffusion coefficient, \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{NiNi}}}} \) decreases by an order of magnitude from the Ni-rich region to the Ru-rich region. \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{NiNi}}}} \) is larger than \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{RuRu}}}} \) in the Ni-rich region by an order of magnitude. The cross coefficients, \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{NiRu}}}} \) and \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{RuNi}}}} \), are both positive. \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{NiRu}}}} \) is comparable in magnitude to the main coefficient \( \ifmmode\expandafter\bar\else\expandafter\=\fi{ \ifmmode\expandafter\tilde\else\expandafter\sim \fi{D}}^{{{\text{Al}}}}_{{{\text{NiNi}}}} \) in the Ni-rich region; hence, Ni interdiffusion flux is enhanced down a Ru concentration gradient but decreased against it. Similarly, Ni interdiffusion is reduced down Al gradients. Characteristic depth parameters calculated for Ni and Ru are larger on the NiAl side than on the RuAl side. Approximate calculations of cumulative intrinsic diffusion fluxes past a Kirkendall plane suggest that the atomic mobility of Ni is larger than that of Ru.

Phase stability in Cu–Zn-based ternary alloys with elements of column 14

By adding small amounts of ternary elements Ge, Si or Sn, it is possible to raise the martensitic transformation temperature M S of binary B2 ordered Cu–Zn towards room temperature. Several poly and single crystal ternary alloys have been studied using resistivity and calorimetric measurements, tensile experiments and optical observations. The influence of the composition and thermomechanical pretreatment on the martensitic transformation has been analyzed. It is shown that the increase in the transformation temperature at a constant electron concentration favors competing decomposition mechanisms.

B2 Phases and their Defect Structures: Part I.I. Ab initio Vibrational and Electronic Free Energy in the Al-Ni-Pt-Ru System

ABSTRACTIn this work, we calculate the finite temperature thermodynamic properties of the binary B2 phases in the Al-Ni-Pt-Ru system, particularly the B2 RuAl phase in the Pt-Al-Ru ternary, through the incorporation of the vibrational and electronic contributions to the total free energy. The harmonic approximation is used to consider the atomic vibrations, with the quasi-harmonic correction to account for volume expansion effects on the vibrational entropy as the temperature increases. The vibrational entropy calculations are incorporated through the supercell approach. The calculated phonon dispersion curves show that the B2 PtRu structure is mechanically unstable at low temperatures, while B2 PtAl is marginally stable. The thermal electronic contribution is added to the total free energy. Finally, the formation enthalpies and entropies of B2 RuAl are calculated as a function of temperature.

Correlations in the arrangement of antistructure point defects in intermetallic phases with B2 (CsCl) structure

Abstract Based on the Bragg-Williams approximation, a statistical-thermodynamic approach is presented for non-stoichiometric binary B2 phases with a pure anti-structure defect mechanism. Whereas in the original Bragg-Williams model only the state of long-range order is considered and no correlations are allowed in the arrangement of the point defects, short-range order is introduced as an equilibrium property of the system in the present work. This new approach is applicable not only both to highly ordered intermetallic compounds and to completely disordered alloys but also in the region of the order-disorder transition. The obtained phase boundary which describes the order-disorder transition is in good agreement with the results predicted by Monte Carlo calculations.

Correlations in the arrangement of antistructure point defects in intermetallic phases with B2 (CsCl) structure

Abstract Based on the Bragg-Williams approximation, a statistical-thermodynamic approach is presented for non-stoichiometric binary B2 phases with a pure anti-structure defect mechanism. Whereas in the original Bragg-Williams model only the state of long-range order is considered and no correlations are allowed in the arrangement of the point defects, short-range order is introduced as an equilibrium property of the system in the present work. This new approach is applicable not only both to highly ordered intermetallic compounds and to completely disordered alloys but also in the region of the order-disorder transition. The obtained phase boundary which describes the order-disorder transition is in good agreement with the results predicted by Monte Carlo calculations.

Point defect concentrations and hardening in binary B2 intermetallics

Point defect hardening in binary B2 intermetallic compounds with the anti-structure defect structure (FeCo and AuZn) and the triple defect structure (NiAl, FeAl, and CoAl) was investigated. Thermodynamic modeling combined with experimental measurements of lattice parameters and bulk densities were used to establish point defect concentrations as functions of composition and temperature. Microhardness measurements were made on samples of varying compositions and quenching temperatures. Solution hardening rates of vacancies were found to be significantly larger than those of anti-site defects. It was possible to relate the hardening rates of anti-site defects to the magnitude of the lattice dilation. This suggests that the elastic size effect was the primary hardening mechanism. No such correlation was found for vacancies.

Dislocation core structure in the B2 phase of Nb-40%Ti-15%Al

Dislocation cores in the B2 phase of Nb rich alloys in the Nb-Ti-Al system were simulated using the embedded atom method. The alloy composition examined is Nb-15Al-40Ti. The site occupancies, elastic constants and average relaxed APB energies in these ternary B2 intermetallics were determined previously and compare favorably to the experimental values. We simulated 1/2〈111〉 and 〈100〉 dislocations in various orientations of the dislocation line. The results were compared with the dislocation core structures that are typical of other B2 compounds. Although the values of the APB's that characterize this ternary alloy are relatively low, the 〈100〉 dislocations were not found to split spontaneously during the simulation into two 1/2〈111〉 partials. The structures observed for these dislocations are basically similar to those found in other B2 binary compounds. The main difference is that the core structures in the ternary compound tend to loose the well defined preference for certain crystallographic pines that is typical of binary B2 compounds.

The B2 Aluminides As Alternative Materials

ABSTRACTAs part of NASA's Conservation of Strategic Aerospace Materials (COSAM) Program, a research effort is underway to explore the potential of the B2 aluminides as structural material alternatives for the strategic element containing superalloys currently used in gas turbine engines. Emphasis is being place on the equiatomic Fe and Ni aluminides. Although Co is a strategic material, the equiatomic Co aluminide is also being studied to gain a more complete understanding of these fourth period intermetallics. The research effort is a cooperative program involving in-house research at NASA Lewis plus several university grant programs. Research focuses on initial processing techniques such as ingot melting, powder metallurgy, and rapid solidification with and without additional thermomechanical processing; high temperature deformation - primarily compressive creep; compositional effects within the binary B2 aluminides; third-element alloying addition effects on high temperature strength and oxidation resistance; and near room temperature ductility as influenced by processing, alloying, and grain size. This paper will review the various programs now underway and present some of the highlights of research results.