Co-based Alloys Research Articles

First-principles calculations on novel Co-based Equiatomic Quaternary Heusler Alloys for Spintronics

The structural, mechanical, electronic, and magnetic properties of the new series of Co-based Equiatomic Quaternary Heusler Alloys (EQHAs) XX'YZ (X = Co; X' = Zr, Ru; Y = Ti, V; Z = Sn, Al, Ga, Si, Ge) have been calculated using first-principles calculations. The Full Potential-Linearized Augmented Plane Wave (FP-LAPW) approach was used in association with Density Functional Theory (DFT). We treated the exchange-correlation (XC) energy functional using a Generalized Gradient Approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) scheme. We included the TB-mBJ potential (GGA-mBJ) and the Hubbard correction (GGA + U) to improve the precision of the band gap values. Spin-orbit coupling (GGA-SOC) calculations are required due to the existence of heavier elements such as Zr and Ru. We have included the GGA-SOC calculations to estimate the total and partial magnetic moment and their impact on the electronic structure of Co-based EQHAs. The ground state parameters of proposed Co-based EQHAs were determined, including the lattice parameter, total energy, bulk modulus, and its derivative. The electronic structure of these materials revealed the presence of half-metallic ferromagnetism and spin-gapless semiconductor behavior. In addition, the origin of the band gap in spin channels was also investigated. In addition, we employed the mean-field approximation (MFA) method based on the Heisenberg model to determine the Curie temperature (TC). This theoretical analysis adds a new level of sophistication to the application of Co-based EQHAs in spintronic applications.

Prediction of half-metallic gap formation and Fermi level position in Co-based Heusler alloy epitaxial thin films through anisotropic magnetoresistance effect

We have investigated the temperature $(T)$ dependence of the anisotropic magnetoresistance (AMR) effect of ${\mathrm{Co}}_{2}{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) epitaxial thin films having different compositions and atomic orders to examine the relation between AMR and the half-metallic electronic structure based on a developed theoretical model. The $T$ dependence of the resistance change (\ensuremath{\Delta}\ensuremath{\rho}) of the AMR normalized at 10 K is minimal in the CFGG films having a standard composition and high atomic order. In contrast, the films having a large atomic disorder due to the Co-rich composition exhibit a large reduction of \ensuremath{\Delta}\ensuremath{\rho} with $T$. Our theoretical model of AMR well explains this behavior; namely, the half-metallic ferromagnets having the Fermi level $({E}_{\mathrm{F}})$ around the center of the half-metallic gap are predicted to show a small $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} because of no/few localized $d\ensuremath{\downarrow}$ states at ${E}_{\mathrm{F}}$. On the contrary, a large change of \ensuremath{\Delta}\ensuremath{\rho} with $T$ is expected for the half-metallic materials having ${E}_{\mathrm{F}}$ close to gap edges or large in-gap states because of the contribution of thermally excited s-d scattering involving the $d\ensuremath{\downarrow}$ states. Moreover, we have also investigated the variation of \ensuremath{\Delta}\ensuremath{\rho} with $T$ for the thin films of various half-metallic Co-based Heusler alloys and found the behavior that agrees with our theoretical prediction. The present study proves that the formation of a half-metallic gap and the position of ${E}_{\mathrm{F}}$ in a half-metallic material are readily predictable from the $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} of the AMR effect, which can be a facile way for efficient screening of half-metallic materials.

Design of Co-based amorphous alloys with magnetocaloric effect near room temperature

In this work, a Co-based amorphous alloy with magnetocaloric effect near room temperature was designed successfully by modulating the chemical compositions. Firstly, the temperature dependence of the magnetization for Co75-xMo10Si10B5+x (x=5, 10, 15) amorphous ribbons was investigated. The Curie temperature increases linearly from 104 to 343K as the Co content increases from 60 to 70 at. %. Thus, the Co content of 68.5 at. % was chosen, which has a magnetocaloric effect near room temperature. The magnetic entropy change under a maximum applied field of 5 T of Co68.5Mo10Si10B11.5 amorphous ribbons can reach 0.86 J kg−1K−1 near 310 K. This work provides a new strategy to regulate the magnetic transition temperature of Co-based amorphous alloys.

Study of cracking and microstructure in Co-free valve seat hardfacing

• Through-thickness cracks were found on Co-free hardfacing with NOREM 02. • Brittle cracking propagate along the interdendritic network of carbides. • Solidification defects and lacking of ferrite phase was observed. For reducing the radiological hazard, Co-free hardfacing alternatives have been used to replace the traditional Co-based hardfacing alloys in nuclear power plant applications. However, manufacturing a defect-free Co-free hardfacing is challenging. Reduced galling resistance and mechanical properties exhibited by common replacement alloys in some conditions limit the application. This issue is important for the operating nuclear power plants with lifetime extension and also for the new builds. In this paper, a failed industrial hardfacing was studied. Through-the-thickness cracks of the Co-free hardfacing deposit with NOREM 02 on a nuclear swing check plug were detected. The potential cause of the brittle interdendritic cracking was studied and the detailed microstructure of the Co-free hardfacing analysed. The solidification defects during the manufacturing and the lacking of ferrite phase in the microstructure together with the potentially non-optimized handling procedures are concluded to be the causes of the observed cracking.

Formation of laser-induced periodic surface structures on amorphous Fe- and Co-based alloys and its impact on magnetic properties

Laser-induced periodic surface structures, so-called LIPSS, were obtained using linearly polarized femtosecond laser pulses on metal glasses with compositions Fe73.5Nb3Cu1Si15.5B7 and Co73Fe1Mn3Mo1Si13B9. The effect of laser parameters on spatial period and regularity of LIPSS was observed. Highly regular and low spatial frequency LIPSS with a period in the range of 850–950 nm was revealed using FESEM studies. The backscattered X-ray diffraction method revealed no devitrification of the amorphous structure upon LIPSS forming. Magnetic properties including saturation magnetization, coercive force, thermomagnetic curves were measured for both materials before and after LIPSS generation. It was observed that LIPSS impacts the coercive force of the amorphous Fe- and Co-based alloys.

Influence of a Partial Substitution of Co by Fe on the Phase Stability and Fatigue Behavior of a CoCrWC Hard Alloy at Room Temperature

The deformation-induced phase transition from fcc to hcp causes local embrittlement of the metal matrix in Cobalt-base alloys, facilitating subcritical crack growth under cyclic loading and reducing fatigue resistance. Our approach to increasing the fatigue life of Co-based hard alloys is to suppress the phase transition from fcc to hcp by an alloy modification that increases the stacking fault energy (SFE) of the metal matrix. Therefore, we substitute various contents (15, 25, and 35 mass pct) of Co by Fe and analyze the effect on the fatigue life and resistance against subcritical crack growth. Subcritical crack growth in the specimens takes place in a cyclic load test. The proceeding crack growth and the occurrence of phase transformations are monitored by scanning electron microscope (SEM) investigations and electron backscatter diffraction (EBSD). We determined an SFE of 35 mJ/m2 at an iron content of 35 mass pct, which leads to a change of the main deformation mechanism from deformation-induced martensitic transformation to deformation twinning. Analysis of cyclically loaded specimens revealed that the resistance against subcritical crack growth in the metal matrix is facilitated with increasing Fe content, leading to a significant increase in fatigue life.

Microstructural Transformation and High-Temperature Aluminum Corrosion Properties of Co-Based Alloy Coating Prepared by Laser Cladding

A Co-based alloy coating was deposited on H13 steel substrate via pulsed Nd:YAG laser and the corrosion resistance to and mechanism of corrosion in molten aluminum were explored. The results showed that the coating was mainly composed of γ-Co dendrite and M23C6 precipitation. The average hardness in the cladding layer was 732.6 HV0.5, which was 3.55 times greater than that of the H13 substrate. During the molten aluminum corrosion test, the surface of the Co-based alloy coating was immersed for 4, 8, 16 and 24 h at 700 °C. The corrosion rate decreased with increases in aluminum erosion time. It was observed that there were two intermediate layers between the coating and the liquid Al, with (Co, Fe, Cr)2Al9 intermetallic compounds (IMCs) layer near the coating side and the (Fe, Cr)4Al13 and (Co, Fe, Cr)2Al5 intermetallic compounds (IMCs) layer near the Al solidification side. After 24 h of static corrosion, the Co-based alloy coating could still maintain its integrity to protect the substrate.

Structure Optimization and Electronic Properties of Co2tix(X=Si, Ge) Heusler Alloys: A DFT Study

A theoretical study of structural, electronic, magnetic, mechanical and vibrational properties of Co-based heusler alloys, namely Co2TiX(X=Si,Ge); have been studied by the first principle calculations with the Generalized Gradient approximation(GGA) based on Density functional theory(DFT). The total spin magnetic moments per unit cell is 2µB, obeying the Slater-Pauling rule. The electronic band structure along the high symmetry points reveal that majority spin states have metallic character and minority spin states have a band gap at the Fermi level. There are no imaginary phonon mode which illustrate the dynamical stability. The Bulk modulus, young’s modulus, shear modulus, Pugh’s ratio, sound velocities, Debye temperature in accordance with the Voigt-Reuss-Hill approximation. The elastic constant results show that these alloys are mechanically stable.

Stress-induced giant magneto-impedance effect of amorphous CoFeNiSiPB ribbon with magnetic field annealing

The development of high-performance magnetic field-sensitive materials is in urgent need for magnetic sensors. In this paper, the effect of magnetic field annealing on the soft magnetic properties, thermal stability and stress-induced giant magneto-impedance (GMI) effect of Co69Fe5.5Ni1Si14.5-xPxB10 (x = 0, 0.5, 1, 1.5, 2) amorphous alloys were investigated in detail. Thermal analysis shows that replacing Si with a small amount of P can increase the thermal stability of the amorphous alloys. The permeability of the samples with magnetic field annealing is about 150% higher than that of the samples with stress-relieve annealing. The stress-induced GMI (GSI) results show that when the applied tensile stress is 0 MPa, 100 MPa, and 300 MPa, the maximum GMI ratio of the 1 at. % P-doping alloy with magnetic field annealing first increases from 76.6% to 102.8% and then decreases to 82%. Under the optimized measurement conditions, the magnetic field response sensitivity ξ of the Co69Fe5.5Ni1Si13.5P1B10 ribbon reached to 35.3%/Oe, the GSI ratio reached 34.3%, which has significant improvement compared to that of the stress-relieve annealing samples. Therefore, the magnetic field annealing technique greatly improved the GSI effect of the Co-based amorphous alloys, and the results obtained show a new way to develop high-performance GSI sensor materials and can be used in the construction of stress sensors.

High-entropy approach starting from a corner of the phase diagram in designing high strength Fe–Mn–Co-based alloys with good tensile ductility

High-entropy approach starting from a corner of the phase diagram in designing high strength Fe–Mn–Co-based alloys with good tensile ductility

Aging strengthening treatment of laser cladding Co-based alloy coating

Co-based alloy coatings were prepared using laser cladding technology and subjected to aging treatments at 850 °C for 4, 8, 16 hours. The influences in aging time on coating structure and performance are investigated. Results show that aging treatment could cause changes in the crystal structure strength and phases of coating. The precipitates in the coatings were mainly M23C6 carbides, and with the extension of the aging time, they changed from small and dispersed distribution to aggregate nucleation and growth. The Rockwell hardness and wear resistance of the coating was improved by 9.2% and 29.7%, respectively, after aging 8 h. Aging 16 h, the Rockwell hardness and wear resistance enhanced 7.1% and 18.9%. And the wear mechanism of the two coatings was mainly abrasive wear.

Fabrication of Co2FeSi Heusler-alloy epitaxial film on NbN epilayer with improved surface morphology

We demonstrate the process to improve surface morphology of an epitaxial NbN layer and the epitaxial growth of a Co-based Heusler alloy, Co2FeSi (CFS), film on the surface-flattened NbN epilayer. The surface roughness of the epitaxial NbN layers was suppressed by employing Cr buffer layers and post-deposition annealing at 800 ˚C, resulting in a very small average surface roughness of ∼1.7 Å. The CFS layers were epitaxially formed on the surface-flattened NbN layers, and we confirm that the CFS/NbN and NbN/Cr interfaces were atomically flat using high-angle annular dark-field scanning transmission electron microscopy. Although energy-dispersive X-ray spectroscopy (EDS) elemental maps show almost no significant interdiffusion of constituent atoms in the CFS/NbN/Cr on the MgO substrate, EDS concentration profiles indicate minor diffusion of N atoms of the NbN layer into the Cr buffer layer. The diffusion of N atoms could be an origin of the degradation of crystallinity of the NbN layer.

Cobalt based new quaternary Heusler alloys for Spintronic and thermoelectric applications: an Ab-initio study

ABSTRACT In this paper, we employed Density Functional Theory (DFT) to study structural and mechanical stability, electric, magnetic and electronic properties of cubic Co-based new quaternary half-Heusler alloys CoZrCrZ (Z = Al, Ga, In) using WIEN2k. Volume optimisation suggests that these alloys are stable in the Y1 structure and show ferromagnetic behavioTAur. Generalised Gradient approximation calculations confirm the half-metallic nature of the reported alloys, which show metallic nature, and semiconducting band gaps exist in spin-up and spin-down channels, respectively. From the calculated cubic elastic constants, the reported Heusler alloys shows ductile nature. The calculated spin-magnetic moments of CoZrCrZ (Z = Al, Ga, In) are consistent with the Slater-Pauling rule. The very fine narrow band gap in the spin-down channel enhances the thermoelectric properties. The reported ferromagnetic half-metals with good thermoelectric parameters suggests that these alloys have possible applications in spin-based electronics and green energy technology.

Synthesis and characterization of Co-based alloy

This study aims to effectively produce samples of Co-20Cr-10Al-1.5Y2O3 (wt%) with proper powder ratios through the powder metallurgy route followed by mechanical alloying in a planetary ball mill. After mechanical alloying, the samples are sintered at 1200 °C and 1400 °C. Both the optical & FESEM microstructures are investigated. Mechanical properties (micro-hardness), microstructural morphology, and phase analysis using X-ray diffraction & EDX are studied. Results reveal that the new phases and intermetallics such as AlCo, Co0.8Cr0.2, Al5Co2, and AlCo2Cr are formed during fabrication through sintering. Considerable grain growth in the microstructures has also been observed with an increase in sintering temperature. An increase in density, densification parameter & shrinkage is also noted. The mechanism underlying the observed performance variations has also been investigated.

Определение температурных режимов отжига ферромагнитных аморфных проводов, полученных методом Улитовского – Тейлора, при нагреве прямым пропусканием тока

The direct current (dc) heating effect of ferromagnetic amorphous wires (AW) on the electromagnetic properties of the serial Co-based alloy, Co – Fe – Cr – Si – B, is investigated. The Ulitovsky – Taylor method was used to produce AW with diameters D = 40, 70, 82, 98, 123, and 145 μm in a glass shell, that was removed mechanically by elastic bending on a laboratory bench. The developed in situ method of stepwise cyclic dc heating of AW of various diameters enables to choose the optimal values of the current (Ia) for carrying out the selected modes that maximize the magnetic properties. AW electromagnetic properties were monitored by registering the amplitude of the intrinsic electromagnetic signal of electromotive force (EMF) of the AW sample under study. Since reliable direct measurements of AW temperature (Ta) when passing the current (Ia) quite an undertakings, we used indirect method of AW heating temperature estimation by establishing the relationship Ta = f (Ia). For the AW specified diameters, the direct current values were experimentally determined, which ensure optimal heat treatment in the range of temperature stability of the amorphous phase, up to the crystallization temperature, Tx. A calibration curve was constructed using reference temperature points, i.e. Tx and the Curie temperature (Tc). The proposed technique for calibration graph construction I = f(D) may be used for different AWs that have different diameters and same composition; it allows one to determine the modes of current annealing required for the best EMF amplitude values of amorphous wires of the given diameter, up to 100 μm.

Tribological Properties of Laser Cladding Co-Based Alloy Coating Under Different Loads

Tribological Properties of Laser Cladding Co-Based Alloy Coating Under Different Loads

Analysis of current-in-plane giant magnetoresistance using Co2FeAl0.5Si0.5 half-metallic Heusler alloy

There has been renewed interest in current-in-plane giant magnetoresistance (CIP-GMR) devices for high-sensitivity magnetic sensors. However, further improvement in magnetoresistance (MR) ratio is necessary to achieve sufficient magnetic field sensitivity. Use of a half-metallic Co-based Heusler alloy ferromagnetic (FM) layer has been demonstrated to be effective in enhancing GMR in the configuration with current perpendicular to the plane; however, only small MR ratios are obtained in the CIP configuration. To understand the origin of the disappointingly low MR in the CIP configuration when using Heusler alloy FM layers, we investigated the magnetotransport properties of CIP-GMR devices using half-metallic Co2FeAl0.5Si0.5 (CFAS) Heusler alloy and conventional CoFe alloy as the FM layers in combination with Ag or Cu as a nonmagnetic (NM) spacer layer. Regardless of the high lattice and electronic band matching at the CFAS/Ag interface, CFAS/Ag CIP spin valves (SVs) show a MR ratio of only 1.2% at room temperature, which is much smaller than those of reference CoFe/Cu and CoFe/Ag SVs (21.6% and 8.4%, respectively). Current density distribution simulations suggest that large current shunting occurs in the Ag layer due to the significant resistivity gap between CFAS and Ag, which limits the generation of highly spin-polarized current from the CFAS layer, resulting in the very small MR ratios. To enhance the MR ratio in CIP-GMR using half-metallic materials, resistivity matching between FM layers and the NM layer is required, in addition to the high electronic band match that has been considered, as a key factor to obtain a high MR ratio in CIP-GMR devices.

Comparative study of the electronic structure and optical properties of the Heusler alloys Co2MGa and Co2MAl (M = Fe and Ni)

The electronic structure and optical properties of the Heusler alloys Co2NiGa, Co2NiAl, Co2FeGa, and Co2FeAl are reported and compared in this work. In the Fe-based alloys, Co2FeGa and Co2FeAl, the electronic structure is found to have 100% spin polarization with the indirect energy gap in the [Formula: see text]-[Formula: see text] direction, whereas in Co2NiGa and Co2NiAl, the density of states is metallic in both spin projections with spin polarization of 55% (Co2NiGa) and 37% (Co2NiAl). Total and Co partial magnetic moments of all Heusler alloys for the optimized lattice parameters were found in a good agreement with previous calculations and experimental data. The frequency dependence of the real and imaginary parts of the complex dielectric constant for the Heusler alloys is studied in the spectrum region of 0.08–5 eV. The research results are discussed based on the performed calculations of the electronic structure. It was found that the character of variations of the spectral parameters of the alloys is typical for media with the metallic conductivity. In the IR region, the mechanism of the intraband acceleration of electrons by the light wave field dominates. The significant changes in the optical spectrum, magnetic moment, spin polarization and electronic structure were revealed in Co2MGa and Co2MAl for different M atoms which motivate further investigations of the Co-based Heusler alloys as promising materials for spintronics.

A combined experimental and ab initio molecular dynamics study on a novel B-based B50Sm10Co40 amorphous alloy

A novel B-based B50Sm10Co40 amorphous alloy was fabricated by melt-spinning technique. The crystallization temperature, Vickers hardness, and Young’s modulus of the alloy are 1068 K, 15.2 GPa, and 230 GPa, respectively, much higher than those of a Co-based Co65Sm10B25 amorphous alloy in the same system. Ab initio molecular dynamics simulation based on density-functional theory indicates that the short-range order of the B50Sm10Co40 amorphous alloy is dominated by B-centered tri-capped trigonal prisms (TTPs) and bi-capped square Archimedean antiprisms, whereas the Co65Sm10B25 alloy mainly contains B-centered TTPs and Co-centered distorted icosahedral-like polyhedra, and the fraction of the TTPs is lower than that in the B50Sm10Co40. The dense-packed B-centered prism-type clusters with a high proportion of stable B–B covalent bonds reduce the atomic diffusivity and mobility, and increase the resistance to crystallization and shear transformation, which contribute to the high thermal stability and hardness/modulus of the B-based B50Sm10Co40 amorphous alloy.

Additive manufacturing of cobalt-based alloy on tool steel by directed energy deposition

Cladding hard-surfacing alloys on tool steel is an effective approach to enhance the surface properties of tool steel. In this study, a Co-based alloy was deposited on tool steel by Directed Energy Deposition (DED) following a three-factor three-level design of experiment matrix with varied laser power, scan speed, and powder flow rate. The microstructure of the deposits was characterized using scanning electron microscopy (SEM). The residual stress on the surface of the samples was measured by the X-ray diffraction (XRD) sin2ψ technique. The parameters that produced promising deposits were used to fabricate samples for tensile test, four-point bending test, Charpy impact test, and hardness measurement. The result reveals that the processing parameters have a significant role in the residual stress of the coatings. Residual stress reduces with the increase of laser energy density. Cracks were found at samples with energy density below a threshold. Tensile testing of the coating/substrate combined structure reveals fracture at the coatings with an ultimate tensile strength of 633.9 ± 54.7 MPa. The bi-material interface survived the tensile test, indicating a strong interfacial bond. The four-point bending test of coating/substrate laminates shows an ultimate flexure strength of 860.6 ± 36.9 MPa. Cracks initiated from the coatings ignored the interface and penetrated the substrate, suggesting a solid bi-material bond. Charpy impact test shows the absorbed energy of coating/substrate laminates is more than doubled that of the substrate.