B2-type Intermetallic Compound Research Articles

Interplay of hydrogen and point defects in B2-type PdCu: A density functional theory study

To improve the performance of hydrogen-permeable alloys, quantitatively elucidating the metal–hydrogen interactions and identifying the controlling factors that govern the bottleneck step of the hydrogen permeation process are essential. In this study, based on the density functional theory, we characterized the interactions of hydrogen with point defects (such as antisite atoms and vacancies) in B2-type non-stoichiometric PdCu alloys, which are promising candidate materials for low-temperature working membranes for hydrogen purification. We proposed a “double defect” mechanism that produces a new type of defect complex with an atomic configuration similar to the Cu vacancy in B2-type PdCu alloys. We found that both Cu vacancies and double defects can act as strong hydrogen-trapping sites and are thus responsible for the marked decrease in hydrogen diffusivity. In contrast, antisite Cu atoms only repel H atoms and do not significantly hinder hydrogen diffusion. Whereas the binding energy of hydrogen to Cu vacancies and double defects exceeded 0.3 eV when relatively few (one or two) H atoms were trapped, the binding energy of hydrogen to Pd vacancies was substantially smaller (0.02 eV at most). Therefore, in Cu-rich non-stoichiometric PdCu alloys, the double defects may be the major hydrogen trapping sites. Furthermore, the effects of hydrogen on the formation of Pd vacancies, Cu vacancies, and double defects in B2-type PdCu alloys were clarified. Notably, double-defect formation from Pd vacancies in B2-type PdCu was significantly enhanced by the presence of nearby H atoms. This phenomenon, in which vacancies in B2-type intermetallic compounds transform into defect complexes in the presence of hydrogen, is postulated for the first time in this study. The findings of this study contribute to further understanding the driving forces for the formation of vacancy-hydrogen complexes in PdCu alloys at the atomic level and provide theoretical support for identifying the lattice defects that inhibit hydrogen permeability in Pd-based membranes.

Structure and properties of CoCrFeNiMnBe high-entropy alloy films obtained by melt quenching

In this study nanostructured films of the Co16.67Cr16.67Fe16.67Mn16.67Ni16.67Be16.67 high-entropy alloy have been synthesized by the method of melt quenching. The structure and physical properties of melt quenched films and as-cast specimens of identical composition were investigated. The XRD patterns allowed us to establish that the investigated alloy in the as-cast state has a multiphase structure in which there are the FCC phase with lattice parameter a = 0.3598 nm, BCC phase (a = 0.2872 nm), and B2-type intermetallic compounds BeNi(Co) (a = 0.2616 nm). At the same time, the melt quenched sample has a structure consisting of the FCC phase (a = 0.3579 nm), the BeNi(Co) phase (a = 0.2610 nm) and the minor BCC phase. Both as-cast specimens and melt quenched films reveal a typical ferromagnetic behavior and exhibit semi-hard magnetic properties and high mechanical characteristics.

Low temperature deformation mechanisms of polycrystalline CoZr and Co39Ni11Zr50 B2-type intermetallic compounds

The B2-type intermetallic compounds CoZr and Co39Ni11Zr50 were deformed in tension at low temperatures. While CoZr is ductile down to 4 K, Co39Ni11Zr50 becomes brittle below 125 K due to a martensitic phase transformation. Thermal activation analysis shows that CoZr follows the Cottrell-Stokes law indicating forest dislocation cutting as the dominant rate-controlling deformation mechanism, similar to face-centered cubic metals. The moderate ductility of both intermetallic compounds at low temperatures may qualitatively be related to a significant metallic character of bonding giving rise to a low Peierls stress estimated for primary {110}<100> slip and most likely also leads to an easier activation of secondary {110}<110> slip which was proven by transmission electron microscopy. Secondary slip is necessary for the fulfillment of the von Mises criterion for homogeneous plastic deformation of polycrystalline materials. The present results generalize the findings made on the ductile rare earth intermetallics YAg and YCu and, therefore, may help to search for other ductile systems in the broad class of intermetallic compounds.

Low temperature deformation mechanisms of single crystalline intermetallic compound YAg

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that the low yield stress and high work-hardening rate observed cause the moderate ductility at low temperatures.

Low Temperature Deformation Mechanisms of Single Crystalline Intermetallic Compound Yag

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K at which significant ductility was observed. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that a low Peierls stress causes the moderate ductility at low temperatures.

Pressure effects on thermo-mechanical properties of intermetallic B2-type FeAl alloy

The pressure effects on thermo-mechanical properties of B2-type FeAl compound have been investigated based on the moment method in statistical mechanics. We derive analytical expressions of equation-of-state, isothermal bulk modulus and specific heats at constant volume and constant pressure of B2-type iron aluminide intermetallic compound. Numerical calculations for lattice parameter, isothermal bulk modulus, Young’s modulus, shear modulus and specific heat at constant pressure of B2-type Fe-40 at.% Al have been performed up to pressure of 10 GPa. Our research shows that the elastic moduli are linear proportional to pressure and the specific heat at constant pressure diminishes strongly at temperature below 300 K. The present statistical moment method results are compared with available experimental data as well as ab initio calculations when possible to verify the developed theory. This research proposes the potential of the moment method in the investigation of thermo-mechanical properties of materials under pressure.

CALCULATIONS OF SELF-DIFFUSION PROCESS AND COEFFICIENT IN ORDERED B2 AlCo

The self-diffusion process in B2 type intermetallic compound AlCo has been investigated by the first-principles calculations within the frame work of density functional theory (DFT). The obtained mono-vacancy formation, migration and activation energies for four self-diffusion mechanisms, the next-nearest-neighbor (NNN) jump, [110] six-jump cycle (6JC), straight [100] 6JC and bent [100] 6JC diffusion show that the NNN jump mechanism of Co vacancy requires the lowest activation energy (Q = 6.835 eV ) in these diffusion mechanisms, which indicates that it is the main way of self-diffusion in AlCo . The electronic structure including the electron density difference on (-1 1 0) plane as well as atomic Mulliken populations were calculated, and the change of bonding behavior during the [110] 6JC process was discussed in detail. Finally, the self-diffusion coefficients of NNN jump and 6JC mechanisms for AlCo were also studied via the first-principles calculations and semi-empirical predictions, which indicates that the self-diffusion coefficients for NNN jump of Co vacancy show the highest value than the others.

Time dependent nature of first order magnetostructural transition in FeRh

B2-type intermetallic compound FeRh shows a typical first order ferro–antiferro magnetostructural transition without any lattice symmetry change but the volume change. In this paper, we found that the first order magnetostructural transition of FeRh exhibits time dependent nature. That is, the transition has a huge incubation time (∼103s) to occur when it is held at temperatures above its transition temperature and proceeds during holding, as in the martensitic transformation with lattice symmetry change.

Lattice instability and martensitic transformation in LaAg predicted from first-principles theory

The electronic structure, elastic constants and lattice dynamics of the B2 type intermetallic compound LaAg are studied by means of density functional theory calculations with the generalized gradient approximation for exchange and correlation. The calculated equilibrium properties and elastic constants agree well with available experimental data. From the ratio between the bulk and shear moduli, LaAg is found to be ductile, which is unusual for B2 type intermetallics. The computed band structure shows a dominant contribution from La 5d states near the Fermi level. The phonon dispersion relations, calculated using density functional perturbation theory, are in good agreement with available inelastic neutron scattering data. Under pressure, the phonon dispersions develop imaginary frequencies, starting at around 2.3 GPa, in good accordance with the martensitic instability observed above 3.4 GPa. By structural optimization the high pressure phase is identified as orthorhombic B19.

Studies of the structure and the atomic diffusion properties of the Σ = 5 [001] twist GB in B2-type intermetallic compound NiAl

The structural properties, the formation energies and the atoms’ diffusion behaviors by vacancy mechanism near the Σ = 5 [001] twist GB of the B2-type intermetallic compound NiAl have been investigated by using the modified analytical embedded-atom method and a molecular dynamics simulation. Both the largest displacement and rippling effect occur at the first layer near the GB. The Ni vacancies at uncoincident sites are most easily formed on the first and second layers of the Ni- and Al-terminations, respectively. Furthermore, the Ni vacancy at an uncoincident site on the second layer of the Al-termination tends to migrate to the coincident Ni site of the first layer of the Ni-termination along a six-step jump path. The Ni vacancies at either the coincident or uncoincident site of the first layer tend to migrate in the first layer and finally return to their original site. Therefore, there is a collective tendency for the Ni vacancies to appear in the GB without local disorder.

Surface structure and energy of B2 type intermetallic compound NiAl

The surface structure and energies for 22 surfaces of NiAl, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (1 1 0) surface belonged to the other kind of the surface consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (1 1 0) texture of NiAl appears mostly in the experiments.

Control of Texture in Intermetallic Compounds by Thermomechanical Processing

Crystallographic relation between NiAl (β ) grains and Ni3Al (γ ' ) precipitate was examined using Ni-38at%Al bicrystals with an oriented grain boundary. Microstructure and texture in B2 type intermetallic compounds such as NiAl and FeAl were controlled by a thermomechanical process focusing on obtaining highly developed &lt;111&gt; texture and improving ductility.

Precipitation of L21-type Ni2AlTi Phase in B2-type Intermetallic Compounds NiTi

Precipitation behavior has been studied in NiTi-based ordered alloy using transmission electron microscopy. The hardness after solution treatment is high in NiTi alloy suggesting the large contribution of solid solution strengthening in this alloy system. However, the amount of age hardening is not large as compared to the large microstructural variations during aging. At the beginning of aging, the <TEX>$L2_1-type$</TEX> <TEX>$Ni_2AlTi$</TEX> precipitates keep a lattice coherency with the NiTi matrix. By longer periods of aging <TEX>$Ni_2AlTi$</TEX> precipitates lose their coherency and change their morphology to the globular ones surrounded by misfit dislocations. Misfit dislocations, which are observed on {100} planes of H-precipitates have the Burgers vector of a <100> with a pure edge type. The lattice misfits of <TEX>$NiTi-Ni_2AlTi$</TEX> system is estimated from the spacings of misfit dislocations to be 1.3% at 1273 K. The lattice misfits decrease with increasing aging temperature in this system.

Identification of lattice vacancies in the B2-phase region of Ni–Al system by positron annihilation

The point defects of B2-type intermetallic compound Ni–Al have been investigated as a function of different Ni compositions ranging from 47 to 55 at.% by means of positron lifetime and coincidence Doppler-broadening (CDB) measurements. The position lifetime of the annealed samples, expressed by single-component lifetime of τ = 170–180 ps, indicates positron saturation trapping at single vacancies in Ni–Al samples. The CDB measurements have been applied to identify these vacancy-type defects formed on the Ni sublattice in the B2-phase Ni–Al alloys.

Texture evolution during hot-rolling and recrystallization in B2-type FeAl, NiAl and CoTi intermetallic compounds

The texture evolution during the hot-rolling and the recrystallization of B2-type Fe–48Al, Ni–50Al and Co–50Ti (expressed by at.%) intermetallic compounds were investigated. By hot-rolling at 973 K, Fe–48Al showed a microstructure with coarse grains elongated along rolling direction, while Ni–50Al and Co–50Ti showed a deformed microstructure featured by the heavily distorted (elongated) grains and/or the deformation bands. The hot-rolling texture of Fe–48Al was composed of {111} , while those of Ni–50Al and Co–50Ti were composed of {111} and {111} , respectively. After annealing, the recrystallized grains were preferentially nucleated at the grain boundaries for Fe–48Al, and in the heavily distorted regions or the deformation bands for Ni–50Al and Co–50Ti. The orientations of the recrystallized grains were similar with those of the deformed matrix, especially for Ni–50Al and Co–50Ti. The recrystallization textures were generally more dispersive than the hot-rolling texture. Based on these results, the texture evolution during the hot rolling and the recrystallization of the B2-type intermetallic compounds were discussed.

Hot rolling workability, texture and grain boundary character distribution of B2-type FeAl, NiAl and CoTi intermetallic compounds

Hot-rolling workability, texture and grain boundary character distribution of B2-type FeAl, NiAl and CoTi intermetallic compounds were investigated as a function of alloy stoichiometry. All the FeAl (i.e., Fe-38Al, -43Al and -48Al, denoted by at.%), and stoichiometric NiAl and CoTi were successfully hot-rolled at 1273 K but off-stoichiometric NiAl (Ni-48Al and -52Al) and CoTi (Co-48Ti and -49Ti) failed. After hot-rolling, all the FeAl showed microstructures with recrystallized coarse grains while (stoichiometric) CoTi retained a deformed microstructure. Hot-rolled (stoichiometric) NiAl showed an intermediate microstructure between FeAl and CoTi. The hot-rolling and annealing textures of FeAl essentially consisted of {111}〈u v w〉. For NiAl and CoTi, {111}〈110〉 and {111}〈112〉 were prominent in the hot-rolling texture, respectively. Both the hot-rolled NiAl and CoTi showed fully-recrystallized microstructures by subsequent annealing, but the resultant recrystallization textures were similar to their hot-rolling textures. On the other hand, grain boundary character distributions of FeAl, NiAl and CoTi with fully-recrystallized microstructures were similar to one another and characterized by a high frequency of low angle boundaries (i.e. Σ 1 boundaries). Based on these results, recrystallization and grain boundary structure of the B2-type ordered intermetallic compounds were briefly discussed.

Room-temperature tensile property and fracture behavior of recrystallized B2-type CoZr intermetallic compound

Abstract Thermo-mechanically fabricated equi-atomic B2-type CoZr intermetallic compound exhibited a recrystallized microstructure containing C15-type Co2Zr Laves phase dispersions. CoZr tensile-deformed at room temperature showed tensile elongations of ∼7.5% accompanied with a transgranular ductile fracture mode mixed with an intergranular fracture mode.

Self-diffusion of cobalt in B2 type intermetallic compound CoAl

The self-diffusion coefficient of 57Co in B2 type intermetallic compound CoAl at the concentrations of 42, 46, 50 and 52 at.% Al has been determined in the temperature range between 1250 and 1630 K by use of the serial radio-frequency sputter-microsectioning technique. The diffusion coefficient of cobalt, D Co * , decreases with increase in Al concentration from the Co-rich composition to the stoichiometry, while D Co * in Al-rich Co-52 at.% Al alloy is almost equal to that at the stoichiometry. At higher temperature region above 0.8 T m ( T m: melting temperature of alloys), D Co * deviates upward largely from the Arrhenius line, suggesting that the diffusion mechanism changes with increasing temperature from the monovacancy to the divacancy mechanism.

Thermodynamic analysis of the Co–Al–C and Ni–Al–C systems by incorporating ab initio energetic calculations into the CALPHAD approach

The Co–Al–C ternary phase diagram has been constructed by combining ab initio energetic calculations with the CALPHAD approach, and the results have been compared with the Ni–Al–C ternary phase diagram obtained using the same procedure. In the thermodynamic analysis, special care was taken evaluating the expression of the free energy for the L1 2 and E2 1 structures. To treat these two structures as a continuous solution, the free energy was expressed using the (M,Al) 3(Al,M) 1(C,V a) 1-type sublattice model. Because of the lack of experimental data, the thermodynamic properties of the E2 1 structure were evaluated using the Full Potential Linearized Augmented Plane Wave method. The calculated results show that the E2 1 phase in the Co–Al–C system is in equilibrium with fcc Co, the B2-type intermetallic compound, and the graphite phase. This finding is in good agreement with previous experimental results. On the other hand, the E2 1 phase does not exist in the Ni–Al–C ternary system. The phase equilibrium of the Co–Ni–Al–C quaternary system is discussed, taking into account the two-phase separation between the E2 1 phase and the L1 2 phase.

Thermodynamic Analysis of the Fe-Al-C Ternary System by Incorporating ab initio Energetic Calculations into the CALPHAD Approach

A thermodynamic analysis of the Fe-C-Al ternary system has been carried out covering a wide range of temperatures and composition. Special care was taken to the expression of the free energy for the ternary Perovskite carbide phase, Fe 3 AlC (κ), by considering the crystallographic similarity between the K phase and the L12 structure. The free energy was calculated using the (Fe,Al) 3 (Fe,Al) 1 (C,Va) 1 sublattice model, and the K and L12 structures were treated as a continuous solution. Because of the lack of experimental information available, the thermodynamic properties of the E2 1 structure were evaluated using the Full Potential Linearized Augmented Plane Wave (FLAPW) method. The ab initio energetic calculations show that the stable E2 1 structure of the K phase is highly preferred compared with the metastable Fe 3 Al-L1 2 structure. Contour plots of the charge density for the K phase indicate that the bonding between the Fe and C atoms forms in the Fe 3 AlC-E2 1 structure, and that this interaction between the atoms enhances the energetic stability of the K phase. According to our phase diagram calculations, the K phase is in equilibrium with the fcc Fe, B2-type intermetallic compound, as well as the graphite phase. This finding is in good agreement with previous experimental results.