D03 Structure Research Articles

What is the true ground state of intermetallic compound Fe3Al?

We discuss recent doubts about the true ground-state (GS) structure of the intermetallic compound Fe3Al. It seems that it should be the D03 structure (observed experimentally), but there are some considerations that, perhaps, D03 might be a high-temperature (>400 K) structure and the GS at 0 K might be the L12 structure because there might be a high energy barrier between both structures and, when the temperature is lowered, the system is not able to transform into the (perhaps) lower-energy L12 structure. To elucidate this problem, we re-interpret our recent extended ab initio electronic structure calculations for Fe3Al performed with the help of the VASP code and using various exchange-correlation energies within the generalized gradient approximation (GGA). Regrettably, some calculations provide the L12 and some of them D03 as the GS structure.To resolve this question, we performed further calculations testing 9 frequently applied metaGGAs, such as TPSS, revTPSS, M06-L, SCAN(-L), rSCAN(-L) and r2SCAN(-L) representing a higher rung of the Jacob's ladder. It turns out that also here some meta-GGAs lead to L12 and some others to D03 GS structure and, again, we cannot decide.In this way, the present results represent the very first step on the way to understand the energetics of the Fe3Al compound and its ground state. We hope they may motivate future theoretical and experimental work in this direction.

Microscopic mechanism of the L12–D019 phase transformation in a Co-base single crystal superalloy

In γ′-strengthened superalloys based on the Co-Al-W system, the stability of the γ′ phase is often limited, as it is found to transform into other phases such as B2 and D019 after long-term annealing. To explore the details behind the annealing-induced transformation from the metastable L12-γ′ phase to the thermodynamically stable D019-χ phase in the single-crystalline Co-base superalloy ERBOCo-VF60 (Co79.8Al8.9W9.0Ta2.3 in at.%), scanning transmission electron microscopy and atom probe tomography are employed. Due to the structural similarity between the L12 and D019 structures, coherent plate-shaped χ precipitates are formed in the γ/γ′ microstructure parallel to {111} planes through a shear-based transformation mechanism. While the χ phase precipitates slowly during isothermal aging, its formation can be significantly accelerated locally by coating the superalloy with a Cr-rich layer, which indirectly stabilizes the χ phase as revealed by thermodynamic calculations. This procedure allowed us to obtain an intermediate (incomplete) state of χ-phase formation in terms of both crystal structure and composition. By characterizing the partial dislocations at the tip of growing χ precipitates, the microscopic details of the shear-based cubic-to-hexagonal transformation from L12 to D019 are uncovered. The compositional aspect of the transformation involves a significant diffusion-mediated enrichment of W in the χ phase, accompanied by a simultaneous W depletion in the γ′ phase, leading to its transformation to the γ phase.

Magnetization and exchange-stiffness constants of Fe–Al–Si alloys at finite temperatures: A first-principles study

We investigated the magnetic properties of Sendust (Fe-Al-Si) alloys not only at 0 K but also at finite temperatures by means of the first-principles calculations assuming A2, B2, and D03 structures. We confirmed that the itinerant characteristics of 3d electrons of Fe are not negligible and a significantly small exchange stiffness constant exists at zero temperature in a B2 structure. However, the calculated Curie temperatures are in the same order for all structures; this indicates that the Curie temperature cannot be determined only by the exchange interactions at zero temperature in itinerant electron systems. Temperature dependence of the exchange interaction, namely, spin configuration dependence, also might be important for determining it. In addition, this property might also be related to the unique behavior of the temperature dependence of the exchange stiffness constant for the B2 structure, which does not decrease monotonically as temperatures increase, contrary to the behavior expected from the Heisenberg model. In addition, we investigated composition dependence on the exchange stiffness constant at zero temperature and confirmed that the substitution of Si with Al could improve the amplitude of the exchange stiffness constant at zero temperature for all structures.

Microstructural Evolution of High-Entropy Intermetallic Compounds during Detonation Spraying

This study aims at investigating the feasibility of depositing quality coatings from various high-entropy intermetallic compounds (HEICs) using detonation spraying (DS). Four different HEIC coatings, namely (NbTaVCrTi)Al3, (NbTaVNiFe)Al3, (NbTaVZrHf)Al3, and (FeNiCoCrMn)(MoCr), were prepared by DS on low alloy steel substrates. The HEIC powders were first prepared by arc melting followed by ball milling and then used as reinforcement particles to deposit HEIC coatings. Elemental segregation was observed for all the as-cast samples. Powders with average particle sizes of about ~25 µm for (NbTaVCrTi)Al3, ~22 µm for (NbTaVNiFe)Al3, ~34 µm for (NbTaVZrHf)Al3, and ~18 µm for (FeNiCoCrMn)(MoCr) were obtained. (NbTaVCrTi)Al3, (NbTaVNiFe)Al3, and (NbTaVZrHf)Al3 HEICs exhibited a nearly single D022 (TaAl3 type) structure, while (FeNiCoCrMn)(MoCr) exhibited a single D8b (FeCr type) structure. Dense coatings consisted of a lamellar microstructure and sound bonding with the substrate, and low porosity was obtained for all the samples. Crystal structures of the HEIC samples were highly retained during DS, whereas all the samples underwent some degree of oxidation. Microhardness values of 745 HV for (NbTaVCrTi)Al3, 753 HV for (NbTaVNiFe)Al3, and 862 HV for (NbTaVZrHf)Al3 were obtained, which are significantly higher than the microhardness of the substrate (~140 HV). Among all the samples, (FeNiCoCrMn)(MoCr) exhibited the highest microhardness values of about 1047 HV.

Lightweight Al3Ti-based medium-entropy alloys with well-balanced strength and ductility

The lightweight Al3Ti intermetallic compound shows superior properties at elevated temperatures but its room-temperature brittleness largely limits its usefulness. We propose Al3Ti-based lightweight medium-entropy alloys (MEAs) with selected alloying elements, which change Al3Ti from a tetragonal D022 structure to a ductile cubic L12 one with embedded hard B2 and D8a phases. A combination of improved ductility and strength can be achieved. Four lightweight AlTiCrMnFeCu MEAs (ρ=3.65∼4.27 g/cm3) with tunable balance between hardness and ductility were fabricated. Ti and Fe promoted the formation of hard B2 phase, while Cr and Mn favored the formation of D8a phase which strengthened the alloy at the minor expense of ductility. The compressive yield and peak strengths of the MEAs e.g., Al55Ti20Cr5Mn10Fe5Cu5, can reach as high as 650 MPa and 1366 MPa, respectively, with its fracture strain around 17 %. This study provides useful information for designing Al3Ti-based lightweight MEAs with desirable properties.

A critical comparative review of generalized gradient approximation: the ground state of Fe3Al as a test case

Quantum-mechanical calculations have become an indispensable tool for computational materials science due to their unprecedented versatility and reliability. Focusing specifically on the Density Functional Theory (DFT), the reliability of its numerous implementations was tested and verified mostly for pure elements. An extensive testing of binaries, ternaries and more-component phases is still rather rare due to a vast configurational space that is nearly infinite already for binaries. Importantly, there are well known cases of theoretical predictions contradicting experiments. In this paper, we analyze the failure of theory to reproduce correctly the ground state of the Fe3Al intermetallic compound. Namely, most exchange-correlation (xc) energies within the generalized gradient approximation (GGA) predict this material in the L12 structure instead of the experimentally found D03 structure. We test the performance of 36 combinations of 6 different GGA parametrizations and 6 different Fe and Al potentials. These combinations are evaluated employing a multi-dimensional multi-criteria descriptor {, a, {}, {C ij }} consisting of fundamental thermodynamic properties (energy difference between the D03 and L12 structures), a structural aspect (lattice parameter a), electronic-structure related magnetic properties (local magnetic moments of Fe atoms {}) and elastic properties (a complete set of second-order elastic constants {C ij }). Considering the thermodynamic stability as the most critical aspect, we identify the Perdew–Wang (1991) GGA xc-functional parametrization as the optimum for describing the electronic structure of the Fe3Al compound.

Multi-phase-field simulation of D019-χ transformation in Co-Al-W superalloy with L12-γ′ + fcc-γ phases

The L12-γ′-Co3(Al, W) phase in Co-based superalloys substantially improves the creep properties and oxidation resistance. However, the cubic γ′ phase in Co-Al-W superalloy becomes unstable at high temperature and transforms into the strip-shaped D019-χ-Co3W phase with prolonged aging. The transformation mechanism and evolution kinetics of the χ phase in Co-8.25Al-10 W (at.%) superalloy with γ + γ' phases are studied by establishing the multi-phase-filed model with the sublattice free energies of fcc-L12 and hcp-D019 phases. Aged at 1173 K, the γ' phase transforms into χ phase through in-situ transformation by superlattice intrinsic stacking faults (SISF), the strip-shaped χ phase grows in direction of [0001]χ[111]γ and forms an angle of 60° with the alignment orientations [100]γ′ and [010]γ′ of γ′ phase. The strip-shaped χ phase causes the stress concentration and the large difference of diffusion potential, promoting the continuous transformation of the γ' phase to χ phase, the Ostwald ripening of γ′ phase happens simultaneously at the regions with low diffusion potential. The chemical potential shows a dominated effect on driving the transformation of γ′ phase to χ phase. The study reveals the mechanisms and driving force of multi-phase transformation from fcc to D019 structure with the stress change in these phases.

Synergistic enhancement of high temperature strength and ductility with a novel γ/ε dual-phase hetero-nanostructure in NiCoCr-based alloys

In most γ"-strengthened superalloys, the γ"→δ stabilization is generally considered as an unfavorable transformation process due to the incapability of δ phase on conferring high-temperature strengthening. While recent studies in multi-component alloys indicate that another stabilized γ" phase (ε phase with D019 structure) has been found and proven to be beneficial to mechanical properties. This work shows that the precipitation of ε phase could be tailored in such a way, that it is accompanied by the formation of nano-twins in low stacking fault energy NiCoCr-based multi-component alloys, establishing a unique matrix/(nano-twin/ε/nano-twin)/matrix double-stacked-sandwich heterogeneous nanostructure. This novel ε-plate/nano-twin structure has been found to provide satisfying high temperature strength-ductility synergy and structural stability after aging, thereby leading to a yield-strength enhancement of ∼120% at 25 °C and ∼98% at 700 °C, respectively, while still maintaining sound ductilities. This unique self-organized, heterogeneous, lamellar nanostructure opens up new avenues for further development of hierarchically-nanostructured, high-temperature structural materials.

One-Dimensional Mn5Si3 Nanorods: Fabrication, Microstructure, and Magnetic Properties via a Novel Casting-Extraction Route.

This study presents a simple and innovative approach for producing one-dimensional Mn5Si3 nanorods through a casting-extraction process. In this technique, the Mn5Si3 nanorods were synthesized by reacting Mn and Si during brass solidification and extracted by electrochemical etching of the brass matrix. The effect of the cooling rate during casting on the nanorods' dimension, morphology, and magnetic properties was investigated. The results demonstrate that the prepared high-purity Mn5Si3 nanorods had a single-crystal D88 structure and exhibited ferromagnetism at room temperature. The morphology of the nanorods was an elongated hexagonal prism, and their preferred growth was along the [0001] crystal direction. Increasing the cooling rate from 5 K/s to 50 K/s lead to a decrease in the dimension of the nanorods but an increase in their ferromagnetism. At the optimal cooling rate of 50 K/s, the nanorods had a diameter and length range of approximately 560 nm and 2~11 μm, respectively, with a highest saturation magnetization of 7.5 emu/g, and a maximum coercivity of 120 Oe. These properties make the fabricated Mn5Si3 nanorods potentially useful for magnetic storage applications, and this study also provides a new perspective on the preparation of one-dimensional nanomaterials.

Derivation of phase field model for the second nearest interatomic potential of D022 structure and sedimentation process in N0.75AlxV0.25−x alloy

This paper obtains a new phase field method that can simultaneously calculate the interaction potential between the first and second neighbor atoms of the D022 structure Ni3V phase in the Ni0.75AlxV0.25−x alloy, and analyzes the interaction potential between the two neighbor atoms of the D022 structure with the change trend of temperature and concentration. The feasibility of using the phase field method to invert the interaction potential between two neighboring atoms of the D022 structure in the Ni0.75AlxV0.25−x alloy laws and the precipitation of D022 and the types of interfaces formed by D022 and L12 during the aging process to be used in the microscopic phase field equation is studied. The results show that with the increase/decrease of the temperature or the concentration of solute atoms, the first neighbor interaction potential between the first neighbor atoms of the D022 structure increases/decreases and the neighbor interaction potential between the second neighbor atoms of the D022 structure decreases/increases; Ni0.75AlxV0.25−x alloys form two ordered phase interfaces between L12 and D022 phases during high temperature precipitation; at the same temperature and the concentration, the calculated interatomic potential is similar to the interatomic potential obtained by other methods. The calculated interatomic potential is substituted into the phase field model, and the simulated alloy precipitation results are consistent with the experimental results.

The Effect of Doping with Al on the Stability of D03 and L12 Phases in Fe73.44(Ga,Al)26.56 Alloys: Ab Initio Calculation and Monte Carlo Modeling

The effect of doping with Al on the stability of D03 and L12 phases was studied in the magnetostric-tion Fe–Ga and Fe–Ga–Al alloys with a high content of nonmagnetic atoms of ≈27 at %. For the studied D03 and L12 structures, the tetragonal shear modules C' = (C11 – C12)/2 and the Debye temperatures ΘD were found by the methods of density functional theory. The replacement of 4.58 at % of Ga by Al atoms was shown to lead to an increase in ΘD and a decrease in C′. Using the combined approach ab initio and Monte Carlo modeling, the calculations of free energies were performed, and the D03–L12 phase transition temperatures were determined. The Al addition to the Fe–Ga system was shown to decrease the difference between the energies of the D03 and L12 structures.

Термическая стабильность структурно-фазового состояния алюминия и сплавов системы Al – Si, легированных атомами циркония при воздействии компрессионных плазменных потоков

The results of structural-phase state investigations of Аl, Al – 12,6 wt. % Si – 0,1 wt. % Fe and Al – 12,6 wt. % Si – 0,5 wt. % Cu – 2,7 wt. % Mg – 0,3 wt. % Ni – 0,3 wt. % Fe – 0,2 wt. % Mn alloys surface layers alloyed by zirconium atoms under action of compression plasma flows are presented. The formation of metastable compounds Al3Zr with L12 structure and (Al,Si)3Zr with D022 structure in the alloyed layers of aluminum and silumin samples, respectively, was found. Alloying with zirconium atoms makes it possible to increase the microhardness of surface layer in 1.4 – 3 times. Thermal stability of the structure and phase composition of alloyed layers at 550 °C was investigated. The transformation of Al3Zr – L12 into Al3Zr – D023 phase in aluminium and (Al,Si)3Zr disintegration with ZrSi2 in Al – Si alloys was observed during annealing. The formed surface layers had elevated temperature strength.

Disordering in Fe3Ga alloy of D03 structure: Effect on stability and magnetostriction

The effect of the structural disorder and Ga content variation on the mechanical stability and magnetostrictive properties of the Fe3Ga alloy of structure D03 was studied by density functional theory methods. There are contradictions between experimental data and simulation results for the compound under investigation. Calculations show that, in contrast to the experimental results, the magnetostriction coefficient should be negative. In addition, the energy-strain simulation predicts the mechanical instability of the perfect D03 lattice. Here it is demonstrated that the permutation of Fe and Ga atoms stabilizes the Fe–25Ga structure. However, the sign of the magnetostriction coefficient remains negative. The further structure modification by partially replacing Fe atoms with Ga leads to a change in the sign of the magnetostriction coefficient in agreement with the experiment.

Grain Boundary Precipitation Control of GCP Phase Using TCP or A2 Phase in Ni-Based Alloys

To cover the grain boundary (GB) of the Ni phase with precipitates, the GB precipitation behavior of both topologically close-packed (TCP) or A2 and geometrically close-packed (GCP) phases was investigated in two Ni–Nb–(Co, Cr) ternary systems. The Ni/TCP or A2/GCP three-phase region existed in both systems. In the Ni-Nb-Co ternary system, Nb was approximately equally partitioned into both Co7Nb2 (mC18 structure, TCP) and (Ni, Co)3Nb (D019 structure, GCP) phases. In the Ni–Nb-Cr ternary system, Nb and Cr were mainly partitioned into the Ni3Nb (D0a structure, GCP) and Cr (A2 structure) phases, respectively. In the Ni–Nb–Co ternary system, the Co7Nb2 phase grew along the GB, whereas the (Ni, Co)3Nb phase grew toward the grain interior (GI). However, the growth of the Ni3Nb phase toward the GI was suppressed in the Ni–Nb–Cr ternary system. The suppression of growth of the GCP phase and covering the GB using both the TCP or A2 and GCP phases might be possible in a system where the precipitation of the GCP phase nucleating on the GB prior to the TCP or A2 phase increases supersaturation for precipitation of the TCP or A2 phase.

Alloying and magnetic disordering effects on the thermodynamic properties and phase stability of Co2Cr(Ga, Si) and Co2Cr(Al, Si) Heusler alloys

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the alloying and magnetic disordering effects on the thermodynamic properties and phase stability of the ferromagnetic L21- and D022-CoxCr78−xZ11Si11 (Z=Ga and Al, 46≤x≤62) alloys are systematically investigated. It is shown that at 0 K, with increasing x the D022 structure gets relatively more and more stable in the energy and stabilized instead in the two groups of alloys with x≥56 and 60, respectively. There, upon the martensitic transformation (MT) the bulk modulus gets a little larger than that of the austenite, and both significant magnetocaloric and magnetovolume effects could be also expected. The tetragonal shear elastic constant (C′=(C11−C12)/2) of the L21 phase decreases whereas that (Cs=C11+C12+2C33−4C13) of the D022 phase increases with the Co addition, mechanically favoring the relative stability of the more ductile D022 martensite as well. The magnetic excitations decrease the electronic total energy but increase the phonon vibrational free energy of the D022 phase relative to the L21 one. The two effects nevertheless almost offset each other at their Curie temperature. The reentrant MT (RMT) is then predicted without the magnetic excitations effect in the Z=Ga and Al alloys with 50≤x≤54 and 54≤x≤58, respectively, and their critical temperatures (TM) for the RMT decrease with increasing x, in line with the measured data of the Z=Ga alloys. With the Co addition, the increase of the relative stability of their D022 phase is finally confirmed maybe originating from the increase of the Co 3d minority DOS around the Fermi level in their L21 phase.

Precipitate evolution of as-cast Mg-9Gd-2Y-0.5Zn-0.5Zr alloy containing LPSO, β′′, β′ and γ'' phases during isothermal aging at 200 ℃

In this work, the evolution process, crystallinity, microstructure and morphology of Mg-9Gd-2Y-0.5Zn-0.5Zr (wt%) alloy precipitates were analyzed during isothermal aging at 200 ℃. During the early stages of aging, the precipitate phase distributed in the matrix is mainly the β′′ phase, which exhibits a hexagonal D019 structure. In the peak-aged stage, the γ′′ phase, LPSO phase, and β′ of the precipitates coexist in the matrix. The synergism between the three types of phases is significant with respect to the improved age-hardening. We propose that the optimized mechanical properties of the alloy originate from the co-precipitation of the LPSO, β′, and γ'' phases. Furthermore, it is an important way to improve the performance of the alloy at high temperatures.

Precipitation enhancing strain hardening capability of ductile AlTiVZrNb high-entropy alloys upon dynamic loading

Strain softening upon dynamic loading is a roadblock of most body-centered cubic (BCC) structured high entropy alloys (HEAs), which can easily lead to dynamic plastic instability. Here, the precipitation was applied in Al–Ti–V–Zr–Nb system BCC HEAs for enhancing strain hardening capability upon dynamic loading. The α2 precipitates (D019 structure, P63/mmc space group) with contents of ∼25% are developed with burgers orientation relationship (BOR) interface and homogenously dispersed in the BCC matrix. The α2 precipitates can effectively reduce the slip length, activate more slip systems, and impose high density geometrically necessary dislocations near interface for hardening. Owing to the BOR, dislocations can slip transfer across phase boundaries offering the co-deformation for plasticity. Compared with single BCC counterpart, the strain hardening rate is changed from negative to positive and the fracture feature transforms from cleavage to dimples. Consequently, a ∼60% increasement in the product of strength and ductility is achieved upon dynamic loading.

An Experimental Evaluation of Quenched Fe-Ga Alloys: Structural Magnetic and Magnetostrictive Properties

This study examines the effect of quenching on Fe100-x-Gax (x – 20 & 25) alloys. The long range of D03 ordering causes a minor variation in unit cell, which reduces peak intensity. This existence of D03, coupled with the A2 phase, leads to a decrease in magnetostriction in the quenched 25 at.% Ga alloy, which promotes D03 ordering. An Fe-Ga alloy having 20 at.% Ga that has been quenched possesses the A2 phase, the production of D03 is a first-order transition. Continuous ordering attempts to suppress D03 in 25 at. % Ga alloys were ineffective. Quenched Fe80-Ga20 alloy's saturation magnetization is larger than Fe75-Ga25 alloy. This suggests that lowering the nonmagnetic element Ga promotes saturation magnetization. The rise in material flaws and dislocations is due to the increased Ga content and higher quenching temperature. In a single-phase region, Fe80-Ga20 has the greatest magnetostriction at 85 ppm. Magnetostriction diminishes as Ga content rises to 25%, the D03 structure is responsible for this drop.

Structural and magnetic properties of Fe–Al alloys: Ab initio studies

The structural and magnetic properties of Fe100−xAlx (5≤x≤25 at.%) alloys with different structural order are investigated within the framework of density functional theory. Using the Korringa–Kohn–Rostoker Green’s function method with a coherent potential approximation, the equilibrium lattice parameters, ground state energy and shear moduli for D03, B2 and A2 structures are calculated. For all structures, the optimized lattice constant increases and the shear modulus shows a decrease with increasing Al content. The tetragonal magnetostriction constants are estimated by the torque method. The A2 and B2 structures provide a positive contribution to the tetragonal magnetostriction. Using Monte Carlo simulations of the three-dimensional Heisenberg model, Curie temperatures are obtained over a wide concentration range.

Atomic defects and diffusion in intermetallic compounds with D03 structure: an ab-initio study

Article Atomic defects and diffusion in intermetallic compounds with D03 structure: an ab-initio study was published on October 1, 2004 in the journal International Journal of Materials Research (volume 95, issue 10).