L10-disorder phase equilibria in the Fe-Pd system investigated by phenomenological calculation

The creep strength of advanced ferritic heat-resistant steels for ultra super critical (USC) power plants is significantly improved by the addition of Pd. It has been found by a detailed transmission electron microscopy (TEM) observation that fine precipitation of an Fe-Pd based L1₀-ordered phase within a lath-martensite grain possessing a certain crystallographic orientation relationship with the matrix is reponsible for the strengthening mechanism. Phenomenological calculation by combining the cluster variation method (CVM) with a Lennard-Jones (L-J) type pair interaction energy is herein attempted for the Fe-Pd binary system to evaluate L1₀-disorder phase equilibria as an initial step to investigate ternary systems. Some of the unknown parameters in the Lennard-Jones pair interaction energies are determined with the help of computations based on a combination of a thermodynamic database and experimental measurement of the latent heat for the L1₀-disorder transition, the latter from differential scanning calorimetry. The experimental L1₀-disorder transition temperature is well reproduced by incorporating a tetragonal distortion of the L1₀-ordered phase into the calculation. In addition, detailed atomistic information, which is indispensable in designing and controlling the morphology of the L1₀-ordered phase, is obtained by the present calculation.     

Deviation of congruent composition in Fe-Pd system

The first-principles calculation of the disordered Ll0 phase boundary for the Fe-Pd system is attempted by combining the FLAPW electronic structure total energy calculations and the cluster variation method via the cluster expansion method. The lattice vibration effects are taken into account based on the Debye-Gruneisen model within quasi-harmonic approximation. The transition temperature is reproduced with very high accuracy. However, the experimental congruent composition of disordered L10 phase that significantly deviates from 1.＂ 1 stoichiometry is not reproduced. Fulther calculations are attempted based on the phenomenological Lennard-Jones type pair potential, which is capable of introducing both tetragonality of the Ll0 ordered phase and additional configurational freedom because of the magnetic spins. The prelimi- nary calculations indicate that the tetragonality enhances the stability of the L10 ordered phase and the magnetic contributions also change the transition temperature. Despite these findings, the shift of the congruent composition still remains as a subject that needs further research. The electronic origin of the shifting of the congruent composition is briefly discussed.     

Thermodynamic properties of the Al–Fe–Ni system acquired via a hybrid approach combining calorimetry,first-principles and CALPHAD

A reaction calorimeter coupled with first-principles calculations was employed to obtain enthalpies of formation for τ₁ (Al₉FeNi) and τ₂ (Al₁₀Fe₃Ni) compounds. The previous thermodynamic model for describing the disorder/order transition (fcc_A1/L1₂) in the Al–Fe–Ni system was modified to extrapolate this model to quaternary and higher-order systems. The first-principles energy calculations for the end-members of sub-lattice models in ternary compounds and L1₂ phase were performed to facilitate subsequent modeling. The existence of the experimentally observed miscibility gap for ternary B2-ordered phase is detected by the present calculation. Such a feature cannot be identified with available thermodynamic software due to the tiny difference between the Gibbs energies associated with different phase assemblages. A set of thermodynamic parameters for the Al–Fe–Ni system was obtained via thermodynamic modeling. Numerous experimental data including phase diagram, thermodynamic properties and site occupation of Fe in B2 phase are well accounted for by the present modeling.     

Morphology of L10-TiAl(Ag) precipitation in Ag-modified L12–Al3Ti

Transmission electron microscopy (TEM) examinations of Ag-modified Al₃Ti with an L1₂-ordered structure have revealed the precipitation of L1₀–TiAl upon aging after quenching from higher temperatures. TEM observations revealed that plate-like L1₀–TiAl precipitates lie on {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. A quantitative X-ray microanalysis for determining the chemical compositions of precipitate and matrix has been done by using an analytical electron microscope. The L1₂ phase field in the Ti–Al–Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.     

Microstructure of carbide precipitates in L12−Ni3Al and L10−TiAl

The crystallographic structures of carbide formed in Ni₃Al- and TiAl-based intermetallics containing carbon are investigated in this study using transmission electron microscopy. In an L1₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2 mol.% to 3.0 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide were formed in the matrix by aging at temperatures around 973 K after solution annealing at 1423 K. TEM examination revealed that the M₂₃C₆ phase and the matrix lattices have a cube-cube orientation relationship and maintain partial atomic matching at the {111} interface. After prolonged aging or by aging at higher temperatures, the M₂₃C₆ precipitates adopt a rod-like morphology elongated parallel to the <100> directions. In L1₀-ordered TiAl containing from 0.1 mol.% to 2.0 mol.% carbon, TEM observations reveal that needle-like precipitates, which lie only in one direction parallel to the [001] axis of the L1₀ matrix appear in the matrix mainly at dislocations. Selected-area electron diffraction (SAED) patterns analyses showed that the needle-shaped precipitate is perovskite-type Ti₃AlC. The orientation relationship between the Ti₃AlC and the L1₀ matrix was found to be (001)_Ti3AlC//(001)_{L10 matrix} and [010]_Ti3AlC//[010]_{L10 matrix}. By aging at higher temperatures or for a longer period at 1073 K, plate-like precipitates of Ti₂AlC with a hexagonal structure form on the {111} planes of the L1₀ matrix. The orientation relationship between the Ti₂AlC and the L1₀ matrix is (0001)_Ti2AlC//(111)_{L10 matrix} and Ti2AlC//L10 matrix.     

Observations of crack tip process zones in cubic titanium trialuminide intermetallics

Despite the fact that the single-phase L1₂-ordered titanium trialuminides, derived from D0₂₂-ordered Al₃Ti by alloying with fourth-period transition elements such as Cr, Mn, Fe, Co, Ni, Cu, and Zn have a cubic lattice structure, their room temperature fracture toughness remains quite low (4–5 MPa m^1/2). In general, process zones developed at the crack tips determine the fracture toughness of a material. In this work the results of the crack tip fracture studies of cubic (L1₂) Al₃Ti alloys stabilized with Mn are presented. The process zones at the crack tip in nearly stoichiometric single-phase L1₂ 9Mn–25Ti (at.%) titanium trialuminides were not observed in most of the specimens studied. Occasionally, two types of process zones were observed: either small, heavily localized process-plastic zones accompanied by a crack tip “collapse”, or “pseudo-bifurcated” ones, reminiscent of those in brittle ceramics. Observations of the crack tip process zones in multiphase, high Ti (up to ∼33 at.%), B-doped trialuminides, exhibiting increased fracture toughness (∼7 MPa m^1/2), show the presence of secondary microcracks in the zone ahead of the crack tip and adjacent to the propagating crack, and more plasticity at the crack tip.     

Microstructure Control of L10-Ordered FePt Granular Film for Heat-Assisted Magnetic Recording (HAMR) Media

L1₀-ordered FePt nanogranular film is one of the promising candidates for the next-generation high areal density media for heat-assisted magnetic recording (HAMR). To realize a suitable microstructure for the HAMR media, the granular structure composed of uniformly dispersed L1₀-FePt nanoparticles with various segregants has been optimized using various combinations of segregants and seed layer materials. Although the microstructure of FePt-C came so close to the ideal one for HAMR, it would not be the final answer as the growth of the columnar grain structure is not possible due to too strong phase separation between FePt and C. In this article, we review recent investigations aiming at achieving the ideal microstructure for HAMR media.     

Density functional study of the L10-αIrV transition in IrV and RhV

Both IrV and RhV crystallize in the αIrV structure, with a transition to the higher symmetry L1₀ structure at high temperature, or with the addition of excess Ir or Rh. Here we present evidence that this transition is driven by the lowering of the electronic density of states at the Fermi level of the αIrV structure. The transition has long been thought to be second order, with a simple doubling of the L1₀ unit cell due to an unstable phonon at the R point (0 1/2 1/2). We use first-principles calculations to show that all phonons at the R point are, in fact, stable, but do find a region of reciprocal space where the L1₀ structure has unstable (imaginary frequency) phonons. We use the frozen phonon method to examine two of these modes, relaxing the structures associated with the unstable phonon modes to obtain new structures which are lower in energy than L1₀ but still above αIrV. We examine the phonon spectra of these structures as well, looking for instabilities, and find further instabilities, and more relaxed structures, all of which have energies above the αIrV phase. In addition, we find that all of the relaxed structures, stable and unstable, have a density comparable to the L1₀ phase (and less than the αIrV phase), so that any transition from one of these structures to the ground state will have a volume change as well as an energy discontinuity. We conclude that the transition from L1₀ to αIrV is probably weakly first order. We also examine the behavior of similar compounds, and show that the αIrV structures of both IrTi and RhTi are lower in energy than the experimentally observed high-temperature L1₀ structure.     

Crystallography and morphology of D023-Al11Ti5 precipitation in Ag-modified L12-Al3Ti

A detailed transmission electron microscopy (TEM) study has been made of the crystallographic characteristics of precipitation of D0₂₃-Al₁₁Ti₅ in Ag-modified Al₃Ti with an L1₂-ordered structure. TEM observations revealed that plate-like D0₂₃-Al₁₁Ti₅ precipitates form multi-domain structures which can be considered as a twin related structures with the tetragonal axis of twin I parallel to [100] direction and that of twin II parallel to [010] direction of the matrix. The particular habit plane, orientation relationship and twin plane variants were carefully determined. A quantitative X-ray microanalysis for determining the chemical compositions of the precipitate and matrix has been done by using an analytical electron microscope. It is found that these crystallographic and morphological features can be explained excellently by adopting the linear elastic theory developed primarily by Khachaturyan. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.     

Precipitation behavior of Al-Ti-Ag alloy system

Transmission electron microscopy (TEM) examinations of Al₃Ti with an L1₂-ordered structure have revealed the precipitation of D0₂₃-Al₁₁Ti₅ and L1₀-TiAl upon aging after quenching from higher temperatures. TEM observations revealed that fine uniform precipitation of Al₂Ti occurs when the supersaturation is sufficiently high, and, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L1₂-Al₃Ti is supersaturated with DO₂₂-Al₃Ti, DO₂₃-Al₁₁Ti₅ with a multidomain structure is formed during aging. On the other hand, plate-like L1₀-TiAl precipitates lie on the {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. The Ll₂ phase field in the Al-Ti-Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.     

Evolution of microstructure and defect structure in L10-ordered manganese aluminide permanent magnet alloys

Defects produced in massively transformed L1₀-ordered τ-MnAl have been characterized by detailed TEM studies. The defect population in massive τ-MnAl comprises arrays of overlapping octahedral stacking faults, {111}-conjugated microtwins, thermal antiphase boundaries and dislocations. The genesis of these defects has been attributed to atomic attachment faulting on {111}- and {020}-type facets of the essentially incoherent growth interface between the parent and product phases. The features of the defect genesis in τ-MnAl are discussed with respect to the role of atomic level processes at solid-state transformation interfaces in general and growth interfaces in massively transforming materials systems in particular.     

Effect of heat treatment on pseudoelasticity in Fe–25.0 at.% Al single crystals

The pseudoelastic behaviour of Fe–25.0 at.% Al single crystals annealed in the D0₃, (α + D0₃), (α + B2) or B2 phase region was examined focusing on the dislocation and ordered domain structures. When large pseudoelasticity appeared in the crystals, 1/4[111] superpartial dislocations moved individually dragging the nearest-neighbour antiphase boundaries (NNAPB). During unloading, the superpartials were pulled back by the surface tension of the NNAPB resulting in pseudoelasticity. Large pseudoelasticity took place only in the D0₃-ordered crystals, while the crystals annealed in the (α + D0₃), (α + B2) or B2 phase region exhibited small strain recovery. The surface tension of the NNAPB in the D0₃ phase was higher than that in the B2 one resulting in the larger recoverable strain in the D0₃-ordered crystals. Since a specific type of ordered domain boundaries in the D0₃ phase played an important role in the pseudoelasticity, a coarsening of the ordered domains caused a decrease in strain recovery.     

Thermodynamic modeling of the Pd-X (X=Ag, Co, Fe, Ni) systems

A set of self-consistent thermodynamic model parameters is presented to describe the phase equilibria of the Ag-Pd, Co-Pd, Fe-Pd, and Ni-Pd systems. In most cases, the calculated values using the optimized model parameters agree very well with the experimental data. The FePd and FePd₃ phases with large homogeneity ranges are described by the compound energy formalism. At present, insufficient thermodynamic data are available for these two phases. Therefore, experimental data on the heat of formation and/or the first-principle calculation of cohesive energies will be very useful for further refinement of thermodynamic parameters of FePd and FePd₃ phases.     

Cold-working and annealing of the chemically ordered L10-phase Fe50–Pd50

We performed an experimental investigation of the annealing behavior of equiatomic γ₁-FePd at temperatures below the critical disordering temperature after cold deformation by rolling in the fully L1₀-ordered state. We used scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to monitor microstructural changes. We used vibrating sample magnetometer (VSM) measurements to determine the evolution of the coercivity of the material in response to cold deformation and annealing. We discuss our findings in the context of previous reports related to studies of recovery and recrystallization during annealing after cold deformation in intermetallic systems that undergo ordering transformations.     

RETRACTED ARTICLE: A study on the microstructure of precipitation strengthened B2-ordered NiAl

Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)₂Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)_p//(111)B2 and [[`1]\bar 12[`1]\bar 10]_p//[[`1]\bar 110]_B2, where the suffixes p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles.     

Thermodynamic modeling of the Pd-X (X=Ag, Co, Fe, Ni) systems

A set of self-consistent thermodynamic model parameters is presented to describe the phase equilibria of the Ag-Pd, Co-Pd, Fe-Pd, and Ni-Pd systems. In most cases, the calculated values using the optimized model parameters agree very well with the experimental data. The FePd and FePd₃ phases with large homogeneity ranges are described by the compound energy formalism. At present, insufficient thermodynamic data are available for these two phases. Therefore, experimental data on the heat of formation and/or the first-principle calculation of cohesive energies will be very useful for further refinement of thermodynamic parameters of FePd and FePd₃ phases.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Microstructures and morphologies of B2-Ordered NiAl(Co) and FeAl(Co)

Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)₂Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix was (0001)_p//(111)_B2 and $[\bar 12\bar 10]_p //[\bar 110]_{B2}$ , where the suffix p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.