D03 Structure Research Articles

Ab initio study of magnetic and structural properties of Fe-Ga alloys

The structural and magnetic properties for a series of Fe100-xGax alloys (x = 18 – 30 at.%) are studied in the framework of first-principles calculations and Monte Carlo simulations. The both, general gradient approximation and local density approximation are considered for the exchange-correlation functional. The ground state ab initio calculations are performed for both D03 and L12 crystal structures. It is shown that for general gradient approximation, the optimized lattice parameters and total magnetic moments are found in the better agreement with experimental ones. Using the calculated exchange coupling constants for studied compositions, Curie temperatures are estimated by means of Monte Carlo simulations of Heisenberg Hamiltonian.

Natural and artificial aging in Mg-Gd binary alloys

Early precipitation stages are now frequently observed in Mg alloys with rare earth due to a development of new microscopy techniques. Information about a very early precipitation at room temperature (i.e. natural aging, NA), however, is rare even in Mg alloys with a very well described precipitation sequence. Moreover, there are only few works dealing with an influence of NA on subsequent thermal processes in Mg alloys. The aim of this work was to study kinetics of the NA performed up to 5 years in an Mg-RE system and its influence on a subsequent artificial aging. The solution treated Mg-Gd was chosen as a model binary system where the NA as well as precipitation development during isochronal and isothermal heat treatment was observed. Electrical resistivity measurements at low and well-defined temperature (77 K) and Vickers microhardness were used for monitoring of a solute clustering during the NA. The kinetics of this process is much slower than observed in aluminum alloys and both methods give identical values of kinetics parameters in simple models. The validity of concentration dependences of microhardness and yield stress values supports these models. The NA intensifies precipitation strengthening in solution treated Mg-Gd alloy with 15 wt % Gd isochronally annealed up to 200 °C. Formation of short range ordered super hexagons consisting of 6 columns of Gd atoms along the [0001] direction resembling the D019 structure was revealed by electron diffraction in specimen isochronally annealed up to 200 °C. The maximum hardening during isochronal annealing up to 300 °C shifts to higher annealing temperatures in the naturally aged specimen. Precipitates of the orthorhombic β′ phase (aβ’ = 2aMg, bβ’ = 4√3aMg, cβ’ = cMg) were identified after isothermal artificial aging for 165 h at 200 °C in the Mg-Gd with 15 wt% in a cell-like arrangement despite of the NA. Since the mesh size is much smaller in the naturally aged alloy, however, it causes the better hardening effect.

First-principles screening of structural properties of intermetallic compounds on martensitic transformation

Martensitic transformation with good structural compatibility between parent and martensitic phases are required for shape memory alloys (SMAs) in terms of functional stability. In this study, first-principles-based materials screening is systematically performed to investigate the intermetallic compounds with the martensitic phases by focusing on energetic and dynamical stabilities as well as structural compatibility with the parent phase. The B2, D03, and L21 crystal structures are considered as the parent phases, and the 2H and 6M structures are considered as the martensitic phases. In total, 3384 binary and 3243 ternary alloys with stoichiometric composition ratios are investigated. It is found that 187 alloys survive after the screening. Some of the surviving alloys are constituted by the chemical elements already widely used in SMAs, but other various metallic elements are also found in the surviving alloys. The energetic stability of the surviving alloys is further analyzed by comparison with the data in Materials Project Database (MPD) to examine the alloys whose martensitic structures may cause further phase separation or transition to the other structures.

Tb-dependent phase transitions in Fe-Ga functional alloys

Effect of microalloying by Tb on the microstructure, phase transitions, and functional properties of the Fe-19Ga-xTb and Fe-27.4Ga-xTb (where x = 0, 0.15, 0.3, 0.4, or 0.5 at.%) alloys was investigated by several techniques including in-situ neutron diffraction, X-ray diffraction (XRD), SEM-EBSD, magnetic measurements, and mechanical spectroscopy (internal friction). We could establish a microstructural understanding to figure out why doping with the trace amounts of Tb leads to an enhanced magnetostriction in the Fe-27.4Ga alloy up to 242 ppm. Our results demonstrate that the microalloying of Fe-27.4Ga with Tb significantly suppresses the formation of the phases with close-packed L12 and D019 structures and stabilizes the dominant phase with D03 structure. The L12 has negative magnetostriction compared to positive magnetostriction of the D03 matrix; therefore, the lower the volume fraction of the L12 is the higher the saturation magnetostriction at room temperature is. The influence of Tb on decelerating the kinetics of the phase transitions in the Fe-27Ga alloys can be reasonably explained by considering a possible competition between precipitation of the Tb-enriched phase and the nucleation of the L12 at the grain boundaries. Both Tb-enriched and the L12 prefer D03 grain boundaries for their nucleation as demonstrated by EBSD analysis. Nevertheless, the Tb-enriched phase already precipitates during casting, and therefore, at subsequent annealing, these precipitations prevent the nucleation of the L12 on the grain boundaries of the as-cast (D03) structure.

Magnetic properties of Fe100−xGax: Ab initio and Monte Carlo study

This work presents a theoretical study of magnetic properties for Fe100-xGax (x = 25, 28.125%) alloys in the framework of first-principles calculations and Monte Carlo simulations. In particular, the zero-temperature ground state properties, magnetocrystalline energy, and magnetic exchange coupling constants for both D03 and L12 structures are calculated. It is shown that for both structures, the magnetocrystalline anisotropy energy change the sign from positive to negative during tetragonal distortion from c/a<1 to c/a>1, respectively. Using the exchange interaction constants within Heisenberg model, the thermomagnetization curves and Curie temperatures for compositions studied are obtained. Calculated data are in a good agreement with experimental ones.

Short-range order and correlations of S atoms in thin-layer PbS structures

The correlations of sulfur atoms and vacancies in the planar square and hexagonal nonmetal sublattices formed by the 4(b) and 8(c) sites in thin-layer PbS films with a cubic D03 structure have been studied.

Thermal cycling of Fe3Al based iron aluminide during the wire-arc additive manufacturing process: An in-situ neutron diffraction study

Fe3Al based iron aluminide has continuously been attractive because of its excellent oxidation resistance, corrosion resistance, light weight and low material cost. It has been considered as a promising replacement of regular stainless steel in fossil energy industry. However, the industrial application of iron aluminide is limited by its low room temperature ductility and high fabrication cost. In recent years, additive manufacturing processes have been proved capable of producing iron aluminide with relatively lower cost as compared to traditional powder metallurgy processing. In the present research, the influence of thermal cycling during the additive manufacturing of Fe3Al based iron aluminide on the phase fraction inside the deposited material has been simulated and investigated using in-situ neutron diffraction. Upon heating, the Fe3Al based iron aluminide has experienced Fe3Al↔FeAl phase transformations, FeAl phase ordering-disordering, and Fe3Al phase transformation from imperfectly ordered B2 structured to perfectly ordered D03 structure. Also, the existence of the forbidden Fe3Al 110 reflection has been determined by neutron diffraction and further evaluated. In addition, the variation of phase fractions throughout the heat treatment has been quantitatively analyzed by Rietveld refinement.

Heteroepitaxial growth of tetragonal Mn2.7−xFexGa1.3 (0 ≤ x ≤ 1.2) Heusler films with perpendicular magnetic anisotropy

This work reports on the structural and magnetic properties of Mn2.7−xFexGa1.3 Heusler films with different Fe content x (0 ≤ x ≤ 1.2). The films were deposited heteroepitaxially on MgO single crystal substrates, by magnetron sputtering. Mn2.7−xFexGa1.3 films with the thickness of 35 nm were crystallized in a tetragonal D022 structure with (001) preferred orientation. Tunable magnetic properties were achieved by changing the Fe content x. Mn2.7−xFexGa1.3 thin films exhibit high uniaxial anisotropy Ku ≥ 1.4 MJ/m3, coercivity from 0.95 to 0.31 T, and saturation magnetization from 290 to 570 kA/m. The film with Mn1.6Fe1.1Ga1.3 composition shows high Ku of 1.47 MJ/m3 and energy product (BH)max of 37 kJ/m3 at room temperature. These findings demonstrate that Mn2.7−xFexGa1.3 films have promising properties for mid-range permanent magnet and spintronic applications.

Effects of alloying elements on the structural, elastic and thermodynamic properties of Co3Ta compounds from first-principles calculations

In this work, we investigated the effects of alloying elements X (X = Al, Ti, V, Nb, Mo and W) on the structural stability, elastic, electronic and thermodynamic properties of Co3Ta compounds using the density functional theory (DFT) calculations. The investigations of atomic structure reveal that all of the alloying elements prefer to occupy Co sites rather than Ta sites in both the L12 and D019 structures, by which Mo is found to play the most effective role in the structural stabilization for γ′ phase. Both bulk moduli and shear moduli of Co3Ta compounds are reduced by the additions of alloying elements, except that the shear moduli are slightly increased by alloying Mo and W. By using the quasi-harmonic Debye model, the thermodynamic properties, including thermal expansion coefficients, heat capacity and bulk modulus, are predicted as a function of temperature for these compounds. The results from this work provide fundamental information for the further development of Co-Ta-based superalloys.

Structure of the high-entropy alloy AlxCrFeCoNi: fcc versus bcc

The effect of Al on the crystal structures of the high-entropy alloy AlxCrFeCoNi is discussed using first-principles electronic structure calculations. When the atomic configuration is totally random, AlxCrFeCoNi has the fcc structure. However, the total energy difference between the fcc and bcc structures decreases as the Al concentration increases. In the calculations Cr and Fe stabilize the bcc structure and Ni and Co work as fcc stabilizer in the alloys, as is observed in experiments. Moreover, the interactions between Al and transition metal elements are strongly attractive. As a result, partially disordered structures such as L12, D03 and B2, where the Al atoms are ordered and the transition metal atoms are still random, are more stable than the totally disordered phases. Especially, the energy gain by the D03 structure is large and this leads to the transition from fcc to bcc for strongly increased Al concentration.

Texture evolution, phase transformation mechanism and nanohardness of selective laser melted Ti-45Al-2Cr-5Nb alloy during multi-step heat treatment process

This paper for the first time systematic study the effect of heat treatment on the grains characteristics, texture evolution, phases transformation mechanism and nanohardness of Ti-45Al-2Cr-5Nb (at.%) alloy processed by selective laser melting (SLM). With increasing the annealing temperature, the grain size generally increases while the crystallographic orientations basically remain unchanged. The grains of all the samples are dominated by high-angle grain boundaries, with its volume fraction increases with increasing the annealing temperature. The content of B2 phase decreases, while the contents of α2 and γ phases increase with the increase of annealing temperature. The phases evolution mechanism during the heat treatment is described as: (200)β→(112¯0)α2+(111)γ. The structure change from β(B2) phase to α(α2) phase is most likely achieved by the dispelling of Nb and Cr atoms, namely through transforming from the “… Ti-Nb/Cr-Al-Ti-Nb/Cr-Al …” stacking sequence in (110) plane of the bcc structure to the “… Ti-Al-Ti-Ti-Ti-Al-Ti-Ti …” stacking sequence in (0001) plane of the D019 (hcp) structure. The structure change from α(α2) to γ is most likely achieved by the rearrangement of Ti and Al atoms through transforming from the “… Ti-Al-Ti-Ti-Ti-Al-Ti-Ti …” stacking sequence in (0001) plane of the D019 (hcp) structure to the “… Ti-Al-Ti-Al …” stacking sequence in (110) plane of the L10 (bcc) structure. The nanohardness ranging from 8.36 ± 0.42 to 9.75 ± 0.49 GPa with increasing the annealing temperature, which is much higher than the roll bonding fabricated TiB2 reinforced TiAl alloy.

On the strengthening precipitate phases and phase transformation of β″/β′ in a Mg-Sm-Zr alloy

Strengthening precipitate phases in a Mg-4Sm-0.4Zr (wt%) alloy, aged isothermally at 473K, were characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. The β″ (D019) precipitates appear in continuous and discontinuous D019 structures, which are designated as β″ (I) and β″ (II) respectively and the β″ (II) is the dominant phase in the peak-aged state. A small number of β' (cbco) precipitates were observed in Mg-Sm-Zr alloy, and these phases have two types of lattice parameters where one is a=0.64nm, b=1.14nm and c=0.52nm, and the other is a=0.64nm, b=2.28nm and c=0.52nm, designated as βL′ and βS′ respectively. The precipitation sequence in Mg-Sm-Zr alloys can be presented as super-saturated solid solutions (S.S.S.S)→β″→β'→β1 (fcc)→β (bct), indicating almost Mg-RE (RE=Nd, Sm, Gd, Y, Dy) alloys have the same ageing precipitation sequence. A phase transformation model between β″/β′ was proposed based on the experimental results.

Structural, energetic and dynamical properties of ordered and disordered bcc Fe25at.%Ni alloys: A first-principles study

Using first-principles phonon calculations and the Quasi-Harmonic approach, the structural, energetic and dynamical properties of ordered (D03) and disordered Fe25at.%Ni bcc alloys have been investigated. All calculations are based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structure. It is found that 25at.% Nickel substitution increases the lattice parameter and consequently decreases the C11 and C12 elastic constants and bulk modulus of bcc-iron. Simultaneously, the C44 elastic constant increases with Ni substitution. The calculated formation energies of the disordered structures at 0K are lower than those of the ordered one. The predicted phonon spectra reveal the dynamical stability of both SQS and D03 structures. Finally, the Gibbs free energy functions indicate that the relative stability of the disordered structure vs the ordered one increases with increasing temperature.

Electronic structures and thermodynamic properties of HfAl3 in L12, D022 and D023 structures

To better understand the relative stability and bonding characteristics of the L12, D022 and D023 structures for HfAl3, the formation enthalpies, electronic structures and thermodynamics properties were investigated by first-principles calculations. The agreement of calculated equilibrium lattice parameters and formation enthalpies with experimental results indicates the reliability of this work. The order of structural stability is D023>D022>L12. The results of densities of states, atomic Mulliken charge and bond population support the best structural stability for D023 structure. Variations of thermodynamic properties with temperature were predicted via phonon frequencies calculation. The enthalpy, entropy, free energy of D023 structure change more quickly than those of the other two structures. The Debye temperatures of L12, D022 and D023 structures are 399, 407 and 416 K, respectively. The volume thermal expansions for HfAl3 increase exponentially at the low temperature, whereas the thermal expansion coefficients increase linearly at the high temperature.

Martensitic transformation and shape memory effect at high temperatures in off-stoichiometric Co2VSi Heusler alloys

Martensitic transformation in off-stoichiometric Co2VSi Heusler alloy was firstly confirmed in this work. The martensite phase presents an ordered 2M non-modulate D022 structure transformed from L21 Heusler structure. In the Co63.5V17.0Si19.5 alloy, the forward and reverse martensitic transformation temperatures are extremely high which reach up to 683°C and 693°C, respectively, higher than most known Heusler-type high-temperature shape memory alloys. And results also indicate that the single martensite phase alloy exhibits a good ductility and strength with the compressive strain of over 15% and strength of about 2300MPa, respectively. What's more, the recoverable strain being maximal to around 3.1% can be obtained at high temperature (750–800°C) due to its reverse martensitic transformation.

Isotropic, high coercive field in melt-spun tetragonal Heusler Mn3Ge

Nanostructured Mn3Ge ribbons with a composition ranging from 77 to 74 at.% Mn were prepared using induction melting, melt-spinning, and subsequent heat treatment. The hard magnetic properties of the ribbons originate from the highly anisotropic tetragonal D022 structure of Mn3Ge. Depending on the composition and the amount of ferrimagnetic Mn5Ge2 as a secondary phase, a coercivity of up to μ0HC = 2.62 T was obtained for the Mn3Ge ribbons. Microstructure investigations by transmission electron microscopy confirmed the formation of the secondary phase. All samples show isotropic coercive fields, i.e., independent of the direction of the applied magnetic field in contrast to already known epitaxial thin films. The Curie temperature was found to be higher than 800 K, which is the temperature of the phase transition from the tetragonal D022 structure to the hexagonal D019 structure.

Structure induced anelasticity in Fe3Me (Me = Al, Ga, Ge) alloys

Neutron diffraction and mechanical spectroscopy techniques were combined in one study in order to interpret structure induced anelasticity of Fe-(25–27)Me alloys where Me is Al, Ga or Ge. The following sequences of phase transitions at continuous heating in the 20–850 °C temperature range were recorded: (i) in Fe-27Al the B2 (only in water quenched state) → D03 → B2 → A2, (ii) in Fe-27Ga: D03 → L12 → D019 → A2(B2), and (iii) in Fe-25Ge: D019 (+A2 very limited amount) → A3 (+A2 very limited amount). At continuous cooling from 850 °C, we recorded the following transitions: (i) in Fe-27Al A2 → B2 → D03, (ii) in Fe-27Ga: A2 → B2 → L12 (+ very limited amount of D019 and remaining A2 phases which underwent to D03 ordering), and (iii) in Fe-25Ge A3 (+A2 very limited amount) → D019 (+A2 very limited amount). These transition sequences determine different temperature dependencies of magnetic and anelastic properties in the studied alloys. In Fe-27Al and Fe-27Ga water quenched samples the bcc-born phases (A2, B2, D03) dominate at room temperature and a Snoek-type anelastic relaxation is recorded in both alloys. The Snoek relaxation is not recorded in Fe-25Ge alloy which has a close-packed D019 structure. Zener relaxation was recorded in all three systems Fe-Al, Fe-Ga and Fe-Ge both at heating and cooling. The first order irreversible D03 → L12 transition (in Fe-27Ga) at continuous heating generates internal stresses due to a huge jump-like increase of atomic volume with temperature and leads to a well-pronounced transient internal friction effect. The reversible D03 ↔ B2 transition (in Fe-27Al) also leads to a transient but much smaller anelastic effect. Much slower phase transition in Fe-25Ge alloy does not influence temperature dependent anelasticity in this alloy.

Observation of hyperfine structure of D022-Mn3−xFexGa by Mössbauer effect

In this work, to obtain the design guideline for a magnet made of a Mn-based alloy, Mn3−xFexGa alloys were prepared by arc melting and the magnetic state of Fe in the alloys and hyperfine structure were investigated on the basis of the Mössbauer effect. As a result, D022-Mn2.2Fe0.5Ga alloys were obtained by annealing at 350 °C for 2 days. From the Mössbauer spectrum of Mn2.2Fe0.5Ga, it was clear that Fe replaced Mn in the Mn II site of the D022 structure. In addition, it was also found that the hyperfine field of Fe is extremely lower in the Mn II site than in the Mn I site.

Study of the enhancement of the magnetic properties of Fe70Al30−xSix alloys in the order-disorder transition

The aim of this work is to study the influence of the coexistence of different ordered structures on the evolution of the magnetism in the order-disorder transition produced by crushing and mechanical milling in the intermetallic Fe70Al30−xSix alloys. First, XRD measurements show that the three ordered samples have both D03 and B2 structures simultaneously and that the disordering process takes place in two stages. In the first one the D03 structure starts to disorder and in the second one, once the D03 structure has almost disappeared, the B2 starts to disorder. Second, Mössbauer spectroscopy and magnetic measurements show that there is an enhancement of the magnetic properties related to the structure change and to the increase of the lattice parameter in the order-disorder transition. The change of the magnetic behaviour is linked to the disordering of B2 structure in these alloys.

HRTEM studies of aging precipitate phases in the Mg-10Gd-3Y-0.4Zr alloy

Rare-earth (RE) element addition can remarkably improve the mechanical properties of magnesium alloys through precipitation hardening. The morphology, distribution and crystal structure of precipitates are regarded as major strengthening mechanisms in the Mg-RE alloys. In order to understand the formation of precipitates during aging at 225 °C in a Mg-10Gd-3Y-0.4Zr alloy (GW103K) with high strength and heat resistance, a high-resolution transmission electron microscopy (HRTEM) was employed to characterize the microstructural evolution. It was found that three types of precipitates were observed in the alloy at the early stage, named as: single layer D019 structure, one single layer D019 structure and one layer of Mg, two parallel single layers (containing RE) and Mg layer in between, which was regarded as ordered segregation of RE, precursors to form β″ and β′ phase, respectively. Both of β″ and β′ phase were transformed from the precursors. It was also found that large size of β′ phase and the small size of β″ phase were constantly existent in the whole aging process. β′ phase played a major role in the strengthening of the GW103K alloys and the decrease of the hardness was caused by the coarsening of β′ phase.