L12 Structure Research Articles

Effect of Thermal Processing on the Structural and Magnetic Properties of Epitaxial Co2FeGe Films.

The structure and magnetic properties of epitaxial Heusler alloy films (Co2FeGe) deposited on MgO (100) substrates were investigated. Films of 60 nm thickness were prepared by magnetron co-sputtering at different substrate temperatures (TS), and those deposited at room temperature were later annealed at various temperatures (Ta). X-ray diffraction confirmed (001) [110] Co2FeGe || (001) [100] MgO epitaxial growth. A slight tetragonal distortion of the film cubic structure was found in all samples due to the tensile stress induced by the mismatch of the lattice parameters between Co2FeGe and the substrate. Improved quality of epitaxy and the formation of an atomically ordered L21 structure were observed for films processed at elevated temperatures. The values of magnetization increased with increasing TS and Ta. Ferromagnetic resonance (FMR) studies revealed 45° in-plane rotation of the easy anisotropy axis direction depending on the degree of the tetragonal distortion. The film annealed at Ta = 573 K possesses the minimal FMR linewidth and magnetic damping, while both these parameters increase for another TS and Ta. Overall, this study underscores the crucial role of thermal treatment in optimizing the magnetic properties of Co2FeGe films for potential spintronic and magnonic applications.

Separate magnetic and structural phase transitions in Mn50-xFexRh50 films grown on MgO

Abstract The investigation of the magnetic and structural characteristic of Mn50-xFexRh50 alloy is of physical importance due to the phase transition behavior. In this study, a series of highly ordered epitaxial films with varying Fe concentrations are grown on MgO (001) substrate. At low Fe concentrations (x=0, 2, 6), a separation between the structural phase transition and magnetic phase transition is observed. Different with the structural phase transition, the temperature-dependent magnetization presents a quite large temperature hysteresis behavior. In addition, the structural transition induces further tetragonal distortion and forms intermediate phase between B2 and L10 structure. This separated magnetic and structural phase transition has been further validated through XRD, anisotropic magnetoresistance and spin pumping measurements. Moreover, with the further increase in the Fe concentration, the Mn50-xFexRh50 films shows ferromagnetic behaviors due to the competitive magnetic exchange interaction, while the structural phase transition is suppressed.

Effect of substitutional Sb doping on the structural stability, half-metallicity, elastic properties, electronic properties, and magnetism of the Co₂MnSn full Heusler compound

We conducted Density Functional Theory (DFT) calculations using the full potential linearized augmented plane wave (FP-LAPW) method to modify the Fermi level by introducing Sb doping into Co2MnSn.This was done through the Generalized Gradient Approximation (GGA) and modified Becke-Johnson (mBJ-GGA) formalisms to enhance spin polarization and suggest signs of half-metallicity. For both ferromagnetic and paramagnetic phases, lattice optimization was performed to identify the stable magnetic structure. The results designate that the doped alloys are stable in a ferromagnetic phase with L21 structure. The calculated elastic constants suggest that the doped alloys meet the criteria for mechanical stability. The magnetism in these compounds is primarily due to the localized moments on the Mn and Co atoms. According to mBJ-GGA calculations, the total magnetic moment slightly increases with Sb doping. Analysis of the optical constants revealed the optoelectronic properties, confirming semiconducting behavior. Doping elements to achieve half-metallicity is demonstrated to be an effective method for discovering new materials suitable for spintronics applications.

Influence of direct aging treatment on microstructure and mechanical properties of Al-4Cu-2Mg-0.3Sc-0.7Zr alloy processed by selective laser melting

This work investigates the microstructure and mechanical properties of select laser melting (SLM) fabricated Al-4Cu-2Mg-0.3Sc-0.7Zr alloys after direct aging treatment. In the as-fabricated alloy, typical duplex grain microstructure, primary Al3(Sc, Zr) precipitates and CuMg co-segregation are observed. After aging treatment at 203 °C for 2.5 h, it was found that two types of S (Al2CuMg) precipitates with {100}Al and {120}Al growth direction. The superlattice lattice confirmed the presence of the secondary Al3(Sc, Zr) precipitates with L12 structure after aging at 225 °C to 496 °C. The disappearance of CuMg segregation and the coarser scale Al3(Sc, Zr) particles lead to the poor Zener-pinning effect and abnormal growth of fine grains at 496 °C aged. The fine secondary Al3(Sc, Zr) precipitates enhanced mechanical properties greatly, the yield strength achieved 435.5 MPa (increased about 18 %) with the elongation of 10.5 % after aging at 440 °C.

Experimental determination of phase stability of the cubic L12 HfAl3-xZnx phase

The present study is devoted to the experimental investigation of homogeneity range and heat of formation (ΔfH° at 300 K) of the cubic L12–HfAl3-xZnx solid solution. A high-temperature direct drop calorimeter has been employed to synthesize and simultaneously determine the ΔfH° of several alloys along the HfZn3–HfAl3 section (25 at. % Hf) whereas X-Ray Powder Diffraction (XRPD) and Scanning Electron Microscopy (SEM) paired with an EDS (Energy Dispersive Spectrometer detector) have been employed to characterize the samples. The performed analysis confirmed that the ternary HfAl3-xZnx alloys were nearly single phase in the range 1 ≤ x ≤ 2.24 having the cubic L12 structure; this in turn helps establish the trend of L12 lattice parameter (at room temperature) with composition. Thanks to the interpolation of our experimental data, the following values of ΔfH° (kJ/mol-atom at 300 K) for the L12–HfAl3-xZnx were determined: -37.1 ± 2.0 (HfAl0.8Zn2.2 corresponding to Hf25Al20Zn55 at. %), -41.7 ± 2.0 (HfAl1.2Zn1.8 corresponding to Hf25Al30.0Zn45.0 at. %), -45.1 ± 2.0 (HfAl1.5Zn1.5 corresponding to Hf25Al37.5Zn37.5 at. %) and -48.5 ± 2.0 (HfAl1.8Zn1.2 corresponding to Hf25Al45.0Zn30.0 at. %). For two pertinent binary intermetallic phases, the following ΔfH° values (in kJ/mol-atom) at 300 K have been obtained: -31.8 ± 3.0 for HfZn3 (unknown structure) and -37.0 ± 2.0 for HfAl3 (tetragonal DO23 – type structure).

Revealing the substantial impact of trace Mg addition on the microstructural configuration of a cast Al-Li-Cu-Zr alloy under various conditions

In this study, the impact of 0.2 wt% Mg addition on the microstructural configuration of a cast Al-2.5Li-2.5Cu-0.15Zr alloy under different heat treatment conditions was investigated using multiscale characterization. Results indicate that in the as-cast state, trace Mg forms the low-melting-point Al2CuMg eutectic phases and promotes grain refinement. In the natural aging state, trace Mg promotes the precipitation of fine Guinier-Preston (GP) zones independent of pre-existing δ′-Al3Li phases. In the artificial aging state, trace Mg mainly leads to the inducing of GP zones, suppression of θ′-Al2Cu phases, and promotion of T1-Al2CuLi phases. δ′ phases with slight diameter reduction and minimal S′-Al2CuMg phases with two variants are also observed. Atomic-level analysis of the two newly formed composite precipitates indicates that L12 structure phases on both sides of the GP zone have an anti-phase relationship. This study is expected to provide theoretical insights into the microstructural origins underlying the beneficial effects of Mg microalloying in cast Al-Li-Cu alloys.

Oxidation behavior of an ultra-high strength and ductile Ni-enriched complex concentrated alloy

Recent research work revealed that Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5 HEA is one of the ultra-high strength and ductile superlattice alloys. In this work, high-temperature oxidation behavior of the as-cast Ni43.9Co22.4Fe8.8Al10.7Ti11.7B2.5 alloy was investigated at 1000 ℃ up to 100 h. The oxidized samples were characterized using X-ray Diffractometer, Energy Dispersive Spectroscopy, and X-ray Photoelectron Spectroscopy. The results revealed that the initial microstructure of the alloy consists of face centered cubic (FCC) and L12 structures. High-temperature exposure resulted in the formation of Al2O3 and TiO2 scales during the initial hours of oxidation, which eventually spall-off after 25 h of exposure allowing further oxidation. The results showed that protective oxide layers such as Al2O3 and TiO2 were not present after 100 h of exposure. The external layer of the 100 h oxidized sample was composed of Fe, Co, and Ni-rich oxides which are known to have mere effective resistance against oxygen ingression. The alloy which has superior strength and ductility may be used for high temperature applications after attaining the thermally stable fine-grain microstructure by suitable thermomechanical processing / providing oxidation resistance coating / by doping with elements having superior oxidation resistance.

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.

Effect of Ti doping on phase stability and half-metallicity of the Co2FeGe compound: GGA and mBJ-GGA approaches

Density Functional Theory (DFT) calculations were performed using the full-potential linearized augmented plane wave (FP-LAPW) method to study the effect of Ti doping on Co2FeGe systems. The both GGA and mBJ-GGA formalisms were employed in this study. For both the L21 and XA structures, ferromagnetic, ferrimagnetic and paramagnetic phases were considered, and lattice optimization was conducted to determine the most stable magnetic phase. The Co2FeGe and Ti2FeGe Heusler alloys were found to be stable in the ferromagnetic state with the L21 structure. In contrast, the XA-type ordering was observed for the CoTiFeGe alloy. The density of states and band structure calculations for the CoTiFeGe (XA structure) alloy revealed half-metallic behavior, which is notable for its potential in achieving high spin polarization. Additionally, half-metallic character was observed for the Co2FeGe alloy with L21-type ordering when using the mBJ-GGA formalism. The calculated elastic constants confirmed that these systems satisfy the mechanical stability criteria. Furthermore, the systems with x = 0.00 (Co2FeGe) and x = 1.00 (Ti2FeGe) comply with the Slater-Pauling rule for half-metallicity in alloys, given by Mtot = NV-24. According to the mBJ-GGA method, the total magnetic moments of Co2FeGe and CoTiFeGe are 6 μB and 1 μB, respectively. Meanwhile, the Ti2FeGe compound follows the condition Mtot = NV-18 and has a total magnetic moment of approximately 2 μB. The optoelectronic properties were examined through analysis of the optical constants, confirming semiconducting behavior for both x = 0.00 and x = 1.00.

Atomically L12 ordered (PtHf)x(NiTi)100-x/CNT as bifunctional catalysts for hydrogen evolution reaction and methanol oxidation reaction with high activity and durability

Instead of a sluggish oxygen evolution reaction (OER), a methanol oxidation reaction (MOR) is combined with a hydrogen evolution reaction (HER) for efficient hydrogen production. Here, by data-driven and density functional theory (DFT) calculation, we establish a combined theoretical–and experimental strategy for designing L12 ordered structures. Based on L12-PtNiTi, Hf element is added. Hf atoms occupy the α-site (face-centered site of FCC) with Pt. Ni atoms occupy the β-site (corner site of FCC) with Ti. The (PtHf)x(NiTi)100-x nano-alloy maintains L12 structure, enhancing intrinsic anti-poisonous performance and reducing Pt content. L12 (PtHf)50(NiTi)50/CNT exhibits an overpotential of 19 mV for HER and a mass activity of 4.725 A/mgPt for MOR in acidic electrolyte. For coupling HER with MOR, the working voltage is merely 0.55 V at 10 mA cm−2. It is demonstrated by DFT calculations that ordered (PtHf)50(NiTi)50/CNT has superior electrocatalytic performance attributed to smaller H adsorption energy, stronger OH binding energy and weaker CO adsorption energy.

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.

Elemental distribution and site occupancy in a Co-Ni-Al-Ti-Ta-W superalloy

γ'-precipitate strengthened cobalt-based superalloys show promise for higher temperature-capability over nickel-based superalloys due to higher melting temperatures. This study investigates a multicomponent CoNi-based superalloy with composition of 54Co-30Ni-9Al-1Ta-4Ti-2-W (at.%) in terms of elemental partitioning and site occupancy. Atom probe tomography (APT) reveals preferential partitioning of Ni, Al, Ti, Ta and W to the γ'-precipitate and Co to the γ-matrix. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectrometer (EDS) along the [001] zone axis of γ'-precipitate with L12 structure indicate that Co, Ni substitute at A-sites, whereas Al, Ti, Ta, W prefer the B-sites. Density functional theory (DFT) calculations further confirm the results of partitioning behaviors and site preferences. Clarifying elemental partitioning and site occupancy provides valuable insights into alloy design principles for advanced CoNi-based superalloys.

A combination strategy of strengthening for enhancing impact-resistance property in high entropy alloys

Developing new high-entropy alloy system has been a subject of significant research interest in recent years. However, it is a challenge to enhance the impact-resistance property in existing high-entropy alloys. Here we proposed a combination strategy of strengthening via second phase and grain refining to build a network microstructure in Co2Ni2CrVTa0.3 high entropy alloys that could significantly enhance the ballistic limit velocity from 407.8 m/s to 435.4 m/s. Especially, under a high impact velocity of 750 m/s, the energy absorption of the alloy was evidently larger than that of the Ta-free alloy. The microstructure of Co2Ni2CrVTax alloys could be sensitively tuned via Ta addition because of the Ta-induced multi-scale microstructure involving the micro-scale Laves phase and nano-scale L12 structure precipitates. This should be the structural mechanism of the enhanced impact-resistance property, and will shed light on the development of alloys with excellent impact-resistance property.

Crystal Structure and Properties of Heusler Alloys: A Comprehensive Review

Heusler alloys, which were unintentionally discovered at the start of the 20th century, have become intriguing materials for many extraordinary functional applications in the 21st century, including smart devices, spintronics, magnetic refrigeration and the shape memory effect. With this review article, we would like to provide a comprehensive review on the recent progress in the development of Heusler alloys, especially Ni-Mn based ones, focusing on their structural crystallinity, order-disorder atoms, phase changes and magnetic ordering atoms. The characterization of the different structures of these types of materials is needed, where a detailed exploration of the crystal structure is presented, encompassing the influence of temperature and compositional variations on the exhibited phases. Hence, this class of materials, present at high temperatures, consist of an ordered austenite with a face-centered cubic (FCC) superlattice as an L21 structure, or body-centered cubic (BCC) unit cell as a B2 structure. However, a low-temperature martensite structure can be produced as an L10, 10M or 14M martensite structures. The crystal lattice structure is highly dependent on the specific elements comprising the alloy. Additionally, special emphasis is placed on phase transitions within Heusler alloys, including martensitic transformations ranging above, near or below room temperature and magnetic transitions. Therefore, divers’ crystallographic defects can be presented in such types of materials affecting their structural and magnetic properties. Moreover, an important property of Heusler compounds, which is the ability to regulate the valence electron concentration through element substitution, is discussed. The possible challenges and remaining issues are briefly discussed.

Effects of addition of Er and Zr on microstructure and mechanical properties of Al–Cu–Mn–Si–Mg alloy

Abstract The effects of addition of Er and Zr on the aging precipitation phase and mechanical properties in cast Al–Cu–Mn–Si–Mg alloy were studied. The addition of Er and Zr can refine the as-cast grains. The average grain size of as-cast alloy containing Er and Zr is 140.60 μm, while the average grain size of as-cast alloy without Er and Zr is 168.54 μm. After T6 heat treatment, the yield strength of the alloy containing Er and Zr reached 334.3 MPa, while the yield strength of the alloy without Er and Zr was only 284.3 MPa. After aging the strengthening precipitates of both alloys contain θ′ phase, Q′ phase and T phase. In the alloys containing Er and Zr the Al3(Er, Zr) phase with L12 structure precipitated during the solution process, which reduced the diameter of the θ′ phase during subsequent aging. After T6 heat treatment, the precipitation strengthening contributed 67.09 % and 61.31 % to the total strength of the alloy with Er, Zr and without Er, Zr, respectively.

Enhancing Surface Strain of Intermetallic Fuel Cell Catalysts by Composition-Induced Phase Transition.

The lattice parameter of platinum-based intermetallic compounds (IMCs), which correlates with the intrinsic activity of the oxygen reduction reaction (ORR), can be modulated by crystal phase engineering. However, the controlled preparation of IMCs with unconventional crystal structures remains highly challenging. Here, we demonstrate the synthesis of carbon-supported PtCu-based IMC catalysts with an unconventional L10 structure by a composition-regulated strategy. Experiment and machine learning reveal that the thermodynamically favorable structure changes from L11 to L10 when slight Cu atoms are substituted with Co. Benefiting from crystal-phase-induced strain enhancement, the prepared L10-type PtCu0.8Co0.2 catalyst exhibits much-enhanced mass and specific activities of 1.82 A mgPt-1 and 3.27 mA cmPt-2, which are 1.91 and 1.73 times higher than those of the L11-type PtCu catalyst, respectively. Our work highlights the important role of crystal phase in determining the surface strain of IMCs, and opens a promising avenue for the rational preparation of IMCs with different crystal phases by doping.

Effect of Al and Nb addition on the precipitation hardening and deformation mechanism of CoCrNi medium entropy alloy

Single-phase medium-entropy alloys (MEAs) with face-centered cubic (fcc) structure exhibit high elongation and toughness but suffer from insufficient strength. In this work, two novel (CrCoNi)Al4 and (CrCoNi)Al4Nb2 MEAs were designed with the aim of introducing precipitation hardening effect and thus improve strength of the alloys. It was revealed that addition of Al alone, only slightly increased the strength, but the elongation decreased severely. Conversely, the combination of Al and Nb can significantly increase strength and maintain acceptable elongation. The Al addition promotes the formation of a disperse distribution of BCC/B2 composite precipitates, however, the large size and incoherent nature of these composite precipitates produce minor hardening response. Except for the BCC/B2 composite precipitates, a high density of the nanoscale γ′ phase with L12 structure and stoichiometry of (Ni, Co, Cr)3(Al, Nb) is produced in the Al4Nb2 MEA, which significantly increases the strength of the alloy. After the addition of Al and Nb, the deformation mechanism of the alloys is changed from nanotwins to stacking fault (SF) dominated plasticity. Such phenomenon mainly relates to the increasing critical twinning stress affected by the channel width of the matrix. This study provides new insight for the composition design of precipitation-hardening MEAs.

Hysteresis-free reversible magnetocaloric effect and critical behavior of Ni39.56Fe7.27Co2.97Mn40.65Sn9.55 full heusler alloy

Utilizing the magnetocaloric effect (MCE), magnetic refrigeration technology offers an advantage over traditional gas-based refrigerators, which produce harmful gases. Herein, we study the MCE and critical magnetic behavior of Fe and Co substituted Ni39.56Fe7.27Co2.97Mn40.65Sn9.55 Heusler alloy near the paramagnetic (PM) to ferromagnetic (FM) transition temperature. With Fm3̅m space group, the present sample crystallizes in cubic L21 structure. Arrott plots and hysteresis-free continuous phase transition confirm a second-order transition from FM to PM. Again, this second-order transition is verified by collapsing scaled entropy curves for various fields into a single universal curve. The critical exponent values obtained using several analytical techniques, like the Kouvel-Fisher method, modified Arrott plot, and critical isotherm, correspond to the mean-field theory universality class having long-range ordering. The above findings suggest our sample to be a suitable magnetic refrigerant by overcoming the difficulty of obtaining a reversible and high value of magnetic entropy change along with substantial relative cooling power across the second-order phase transition.

Influence of structural modifications on mechanical and magnetic properties of Co2FeGa full Heusler alloys: A comparative study on L21 and XA structures

In this study we are investigating the effect of pressure and tetragonal distortion of Co2FeGa full Heusler compound in L21 and XA structures on the elastic, magnetic and thermomagnetic properties. Our approach for this is by using density functional theory (DFT) and Monte Carlo (MC) simulation. Our result reveals a significant effect of pressure on the magnetization and Curie temperature on both prototypes. The elastic constants calculated here determine the stable configuration on both structures. The exchange interaction is also influenced by the pressure. The same sort of behavior of the effect of tetragonal distortion is seen on the elastic constants, magnetization, and exchange interaction. Therefore, a detailed significant effect is found in this work and compared to previous studied.

Exploring the thermomagnetic behavior of Co2TiZ (Z=Al, Si, Ga, Ge, and Sn) alloys: a computational study

A comprehensive computational exploration of the structural and thermomagnetic properties of Co2TiZ (Z = Al, Si, Ga, Ge, and Sn) Heusler alloys are conducted utilizing both density functional theory (DFT) and Monte Carlo simulations (MC). Our calculations revealed that the XA prototype consistently exhibited larger lattice parameters than the L21 structure. Furthermore, the investigation of exchange parameters uncovered distinct differences between the L21 and XA prototypes. The L21 structures consistently exhibited stronger Co-Co interactions, while the XA prototypes showcased more pronounced Co-Ti interactions. The calculated Curie temperatures (Tc) varied between the L21 and XA prototypes, highlighting the significance of atomic arrangement. The calculated critical temperature Tc of Co2TiAl exhibited variation depending on the structural prototype, and it is determined to be equal to 131K for the L21 prototype, while in the XA structure, it increases significantly to 248K. The higher Tc indicates improved thermal stability, expanding the material’s operational range and making it suitable for applications that require magnetic functionality at higher temperatures.