Bulk Alloy Research Articles

Screening of the FeCrAl LBE corrosion-resistant coatings: The effect of Cr and Al contents

Sixteen kinds of FeCrAl coatings with different Cr and Al contents were prepared by the magnetron sputtering method, and the thermal shock resistance and static lead‑bismuth eutectic (LBE) corrosion behavior (500– 650 °C for 1000 h) were investigated. The results show that the coating with poor thermal shock resistance could also peel off during the corrosion test, and the corrosion resistance of the coating is strongly related to the Cr and Al composition. A relatively high Al content could be required for the FeCrAl coatings to maintain outstanding performance, which is different from the traditional design of the FeCrAl bulk alloys.

M-plane AlGaN digital alloy for microwire UV-B LEDs

The growth of non-polar AlGaN digital alloy (DA) is achieved by metal-organic vapor phase epitaxy using GaN microwire m-facets as the template. This AlGaN DA consisting of five periods of two monolayer-thick layers of GaN and AlGaN (approximately 50% Al-content) is integrated into the middle of an n-p GaN/AlGaN junction to design core-shell wire-μLED. The optical emission of the active zone investigated by 5 K cathodoluminescence is consistent with the AlGaN bulk alloy behavior. Several contributions from 295 to 310 nm are attributed to the lesser thickness and/or composition fluctuations of AlGaN DA. Single-wire μLED is fabricated using a lithography process, and I–V measurements confirm a diode rectifying behavior. Room temperature UV electroluminescence originating from m-plane AlGaN DA is accomplished at 310 nm.

Magnetic glassy state in combination with ferromagnetism in Fe0.05(SnTe)0.97Sb0.03 bulk alloy

In this study on Fe0.05(SnTe)0.97Sb0.03 bulk alloy, we found signatures of presence of both magnetic glassy state and ferromagnetism. The bulk alloy is prepared by modified solid state technique and the sample is characterized for its structural, electrical and magnetic properties. Electrical resistivity plot shows semiconducting nature of the sample, however below 25 K, a sudden increase in the electrical resistivity value is observed. The transport mechanism is explained on the basis of small polaron hopping (SPH) model and variable range hopping (VRH) model. A large bifurcation observed between zero-field cooled and field-cooled magnetization at low temperature hints towards existence of a magnetic glassy state. M-H curve exhibits hysteresis behaviour for the measurements carried out at 10, 100 and 300 K. However, absence of saturation of the curves at 10 and 100 K suggests co-existence of ferromagnetic and glassy state. Presence of magnetic glassy state can also be confirmed from the Arrott plot and AC Susceptibility measurement. The susceptibility curves are found to undergo relatively small shift of peaks with frequency and theoretical fitting of the data supports presence of a cluster-glass state.

Abnormal grain growth of 68Cu–16Al–16Zn alloys for elastocaloric cooling via cyclical heat treatments

Cu-based superelastic shape memory alloys are promising for low-stress elastocaloric cooling. We have synthesized bulk alloys of 68Cu–16Al–16Zn under different conditions in order to promote its grain growth and enhance its elastocaloric properties. High-temperature x-ray diffraction of untreated 68Cu–16Al–16Zn alloy showed that the phase boundary between the α + β mixed phases and the high temperature phase (β phase) was between 973 K and 1023 K. Based on this result, the 68Cu–16Al–16Zn alloy was heated and cooled in a furnace repeatedly between 773 K and 1173 K. The maximum grain size after heat treatment of the ingot rolled to 67% reached 11.1 mm. The latent heat of the martensitic transformation after grain growth was 6.3 J g−1, which is higher than the previously reported value for the compound. The stress–strain curve of 68Cu–16Al–16Zn rolled to 67% rolling with cyclical heat treatments showed a maximum stress of 106 MPa at 4.5% strain, with adiabatic temperature change of 5.9 K in heating during stress loading and 5.6 K in cooling in stress removal. Furthermore, no fatigue in the stress–strain behavior was observed up to at least 60 000 mechanical cycles at 2% strain.

Improved mechanical properties of a Ti-48Al alloy processed by mechanical alloying and spark plasma sintering

Bulk Ti-48Al alloy samples were prepared by the high energy ball milling (HBM) of elemental powders, followed by spark plasma sintering (SPS) of the HBM processed powders. The microstructure, phase evolution and mechanical properties of the bulk alloy were studied. The resulting TiAl + Ti3Al two phase alloy possessed an equiaxed fine grain structure, unlike the usual lamellar structure produced by arc melting. The process parameters of HBM and SPS, e.g., milling speed, milling time and sintering temperature were used to tune the phase fraction, microstructure, and grain size. A very high nanohardness of up to ∼12 GPa was obtained, ∼2.4 times higher than the corresponding value of the as-cast counterpart. The combined influence of powder size reduction during HBM, high Ti3Al phase fraction and microstructural development during SPS resulted in higher hardness, wear resistance and yield pressure. Thus, a HBM+SPS processing approach is a promising processing route for the manufacture of high hardness bulk TiAl alloys.

Structural decomposition retarded crystal growth in the undercooled liquid of Zr70Al12.5Fe17.5 metallic glass-forming alloy

The recent attention on crystallization of undercooled glass-forming metallic liquids has been directing to the intriguing viscosity behaviors. The present study discloses that structural decomposition of the grown-out crystalline products plays a part in the crystal growth kinetics. In the supercooled liquid of Zr70Al12.5Fe17.5 metallic glass, the polymorphically crystallizing metastable τ2-Zr6Al2Fe phase decomposes into τ2 + τ2′ (space group:Amm2, a = 6.895 Å, b = 3.368 Å, c = 5.119 Å), and retards crystal growth. The structure decomposition also occurs in the solidified bulk alloys.

Comparison between bulk and particle solder alloy on the performance of low-melting solder joints

The influence of uniformity and small size solder particles on microstructure evolution and mechanical performances of low temperature soldering joints was investigated. First, experiments were set up to obtain uniform and small-size solder particles, which were compared to bulk solder joints by using reflow at temperatures of 100 °C and 120 °C. The preparation process was carried out on the solder alloy using sono-chemical cooling process method to synthesize the uniform and small-size solder particles. Secondly, scanning electron microscopy (SEM) was utilized to obtain the morphology of solder particles, and x-ray diffraction (XRD) was used to prove that the prepared solder particles had good purity and crystallinity. We found that the size effect and uniformity of the particles would refine the morphology of the solder joints, and reduce the intermetallic compound (IMC) thickness by 17%. Thirdly, the shear strength of solder particles/Cu joints at different reflow temperatures of 100 °C and 120 °C was found to be greater than that of bulk solder. Also, the liquid solder has good flowability since the particles have extremely low surface energy, resulting in the reduction of contact angle. Furthermore, the bulk solder/Cu joints were found to fracture partially between IMC and the solder, mainly between the solder matrix, but in the solder particles/Cu joints, it fractured completely inside the solder matrix, which suggests that the uniformity of small-size particles has changed the fracture mode of the joint.

Corrosion of Synthetic Intermetallic Compounds and AA7075-T6 in Dilute Harrison’s Solution and Inhibition by Cerium(III) Salts

This study addresses the behavior of an aluminum alloy and its components in conditions simulating the presence of atmospheric salts. The corrosion of synthetic intermetallic compounds (IMCs) Al2Cu, Al2CuMg, Al7Cu2Fe, MgZn2, and bulk aluminum alloy 7075-T6 was studied in dilute Harrison’s solution (DHS, 0.35 wt% (NH4)2SO4 + 0.05 wt% NaCl). For IMCs, electrochemical measurements were performed using a microcell, and a standard electrochemical cell was used to study the bulk alloy. Separately measured cathodic and anodic potentiodynamic polarization curves were recorded, and prolonged immersion was characterized using electrochemical impedance spectroscopy. Samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Bulk AA7075-T6 was less susceptible to corrosion in DHS than in chloride solution stemming from the beneficial presence of sulfate ions and lower chloride concentration. The susceptibility of IMCs to corrosion in DHS increased in the order Al2Cu < Al7Cu2Fe < Al2CuMg < MgZn2 due to the increased dissolution of Mg in the presence of ammonium ions. The relative nobility of IMCs was determined based on the galvanic current density. Further, the possibility of using cerium chloride, nitrate and acetate salts as corrosion inhibitors in DHS was evaluated. Ce salts acted as inhibitors for the alloy during 14 d testing, forming a Ce hydroxide layer. The degree of protection depended on the type of anion, with acetate Ce salt giving the most efficient protection. For the IMC, however, inhibition by Ce salts did not occur during short measurements in the microcell, indicating the importance of galvanic interaction with the alloy matrix in the inhibition mechanism, which was confirmed by long-term measurements of the alloy.

Structural, Crystallization Kinetics and Physical Properties of Se85Te15−xAgx Chalcogenide Glasses

In this study, Se85Te15−xAgx (x = 3, 6, 9 and 12) chalcogenide glasses were examined for their structure, crystallization kinetics, and physical characteristics. The kinetics of crystallization in these glasses were studied using various methods. By using the melt quenching process, Se85Te15−xAgx bulk alloys were created. The amorphous nature of the alloys was confirmed using High Resolution X-Ray Diffraction (HRXRD). The crystallization kinetics of the Se85Te15−xAgx glasses were studied using non-isothermal differential scanning calorimetry (DSC) measurements at heating speeds of 5, 10, 15, 20 and 25 K/min. The different characteristic temperatures, including the glass transition (Tg) and on-set crystallization (Tc) temperatures, have been determined from a variety of DSC thermograms. Using the Kissinger and Moynihan techniques, the activation energies of the glass transition (ΔEt) were computed.

Transport and thermoelectric properties of crystalline Cu2−xAgxSe alloys prepared by facile method

Crystalline alloys of Cu2−Se containing a transition metal, Earth-abundant and eco-friendly element, are studied in this article as promising eco-friendly thermoelectric materials. Thermoelectric and transport properties of Cu2−xAgxSe (x = 0.00, 0.01, 0.03, 0.05, 0.07) bulk alloys were studied in the temperature range 273–473 K. Crystal structure and surface morphology were tested by X-ray diffraction analysis and scanning electron microscope. Electrical conductivity measurements showed metallic conduction behaviour besides a tremendous decrease in its values with increasing Ag content (x). In contrast, Ag-doping increased the Seebeck coefficient to a large extent. Calculations of the electronic thermal conductivity showed that phonon scattering is increased due to the Ag-doping. The incorporation of Ag-atoms resulted in existence of point defects, introduced secondary phases which significantly reduced the thermal conductivity. Such effects resulted in enhanced ZT, specially at higher Ag-content. The largest ZT value was recorded at 0.25, achieved for Cu1.93Ag0.07Se at 473 K, which is about two times of that observed for the pristine Cu2Se sample. The overall findings of the current study clearly demonstrates that the increasing of the Ag content helps to obtain good TE properties of the cubic Cu2−xAgxSe system, specially at higher temperatures.

Magnetic Hyperthermia and Antibacterial Response of CuCo2O4 Nanoparticles Synthesized through Laser Ablation of Bulk Alloy

The wide variety of uses for nanoparticles (NPs) is due to their unique combination of features in a single assembly. The arc melted copper-cobalt ingot sample were qualitatively studied using laser induced breakdown spectroscopy (LIBS). Later, using the fabricated alloy as a target material for Nd:YAG laser ablation, CuCo2O4 NPs were synthesized. The magnetic properties of the synthesized NPs were studied using a vibrating sample magnetometer (VSM). To determine the composition and morphology of the synthesized NPs, X-ray diffraction (XRD), energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and dynamic light scattering (DLS) techniques were used. The TEM and DLS showed that particles were spherical in shape with an average size of 32 nm and 28 nm, respectively. The antibacterial activity of the synthesized NPs was studied against S. aureus and E. coli strains as positive and negative controls using a standard approach. CuCo2O4 nanoparticles exhibited non-mutagenic potential against S. typhimurium TA-98 and TA-100 strains. Furthermore, the magnetic hyperthermia study of CuCo2O4 nanofluid was examined using a lab-made apparatus. The specific absorption rates (SAR) of 4.57 and 5.17 W/g were determined for the magnetic field strength of 230 μT and 247 μT, respectively. The study shows antibacterial activity and magnetic hyperthermia potential of the synthesized nanoparticles.

Structural, magnetic and magnetocaloric properties of hexagonal MnCoGe-based thin films

The characteristic size of the magnetocaloric materials in magnetic refrigerators, thermomagnetic regenerators and energy harvesters is usually down to micrometer and nanometer length scales, which provides the largest possible contact surface area to the heat transfer liquid. Therefore, it is pivotal to investigate the structural, magnetic and magnetocaloric properties of magnetocaloric materials at micrometer and nanometer length scales. In the present study, we have successfully fabricated MnCoGe-based thin films using the pulsed laser deposition (PLD) technique. The as-deposited film shows a dense surface, homogeneous chemical distribution and a preferred orientation along the c-axis of the hexagonal Ni2In-type structure. In-situ temperature-dependent X-ray diffraction measurements on the as-deposited films reveal an irreversible structural degradation above 300 °C due to the considerable evaporation of the Mn atoms, which causes the formation of microscale pores in the films. The thermomagnetic behavior of the film annealed at 250 °C displays a reversible second-order ferromagnetic transition of the hexagonal phase at Curie temperature TC = 275 K, and a first-order hexagonal-orthorhombic structural transition below 200 K. The decoupling of the magnetic and structural transitions brings reversible magnetocaloric effect with the entropy change ΔS values of 2.5 J/(kgK) and the refrigeration capacity reaching 89 J/kg in Δμ0H = 5 T, which are comparable to other reported magnetocaloric thin films. Moreover, antiferromagnetic coupling in the low-temperature orthorhombic phase was observed, which has rarely been reported in the MnCoGe-based bulk alloys. The different magnetic ground states between the thin film and the bulk alloys may be originated from the strain effect imposed by the Al2O3 substrate. Consequently, this work not only provides insights into the structural, magnetic and magnetocaloric properties of the MnCoGe-based alloys at a nanometer length scale, but also offers a new idea for tailoring the multifunctional properties for this material family.

β Grain Size Inhomogeneity of Large Scale Ti-5Al-5V-5Mo-3Cr Alloy Bulk after Multi-Cycle and Multi-Axial Forging in α + β Field

In order to fabricate homogeneous large-scale Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy bulk with fine and equiaxial β grain, we performed a series of multi-axial α + β field forging with 62 forging cycles on the large-scale Ti-5553 billet by using 12.5 MN high-speed hydraulic press. The β-annealed microstructure was the starting microstructure of the billet. After the 6th forging cycle, β grain deformed dramatically, and the grain-boundary network developed within the irregular β grain. As the forging cycle increased to 44, the volume fraction of the fine and equiaxial β grain that is less than 20 μm, which is caused by dynamic recrystallization, increased gradually. However, the incomplete dynamic recrystallization region within the original β grain could not be eliminated. As the forging cycle further increased, the volume fraction of the fine and equiaxial β grain did not increase. In contrast, the abnormal grain growth of the β phase occurred during 50th~62nd forging cycle. Here, we attribute the formation of the incomplete dynamic recrystallization region and the abnormal grain growth of the β phase to the high deformation rate of the α + β forging. The refining behavior of β grain and the abnormal coursing β grain, which is found during the multi-cycle multi-axial forging of large-scale Ti-5553 alloy billet, are seldom reported in the isothermal compression of small-scale Ti-5553 alloy specimen. The findings of the paper are instructive for improving the sub-transus forging strategy that is used to fabricate the large-scale homogeneity Ti-5553 alloy billet with fine and equiaxial β grain.

Powder Metallurgy Processing and Characterization of the χ Phase Containing Multicomponent Al-Cr-Fe-Mn-Mo Alloy

High entropy alloys present many promising properties, such as high hardness or thermal stability, and can be candidates for many applications. Powder metallurgy techniques enable the production of bulk alloys with fine microstructures. This study aimed to investigate powder metallurgy preparation, i.e., mechanical alloying and sintering, non-equiatomic high entropy alloy from the Al-Cr-Fe-Mn-Mo system. The structural and microstructural investigations were performed on powders and the bulk sample. The indentation was carried out on the bulk sample. The mechanically alloyed powder consists of two bcc phases, one of which is significantly predominant. The annealed powder and the sample sintered at 950 °C for 1 h consist of a predominantly bcc phase (71 ± 2 vol.%), an intermetallic χ phase (26 ± 2 vol.%), and a small volume fraction of multielement carbides—M6C and M23C6. The presence of carbides results from carbon contamination from the balls and vial during mechanical alloying and the graphite die during sintering. The density of the sintered sample is 6.71 g/cm3 (98.4% relative density). The alloy presents a very high hardness of 948 ± 34 HV1N and Young’s modulus of 245 ± 8 GPa. This study showed the possibility of preparing ultra-hard multicomponent material reinforced by the intermetallic χ phase. The research on this system presented new knowledge on phase formation in multicomponent systems. Moreover, strengthening the solid solution matrix via hard intermetallic phases could be interesting for many industrial applications.

Advanced characterization of bulk alloy and in-situ debris nanoparticles formed during wear of Fe–Nb–B ultrafine eutectic laser cladding coatings

The microstructure of Fe76·5Nb8·5B15 (at.%) ultrafine eutectic (UE) coatings were processed by laser cladding, then the bulk alloy coating and wear debris were evaluated by advanced electron microscopy techniques in order to understand the ultrafine matrix and nanoscale intermetallic sub-micron and nanoscale phases formed, as a result of rapid solidification. In addition, according to the laser cladding processing parameters used, the coatings presented nanoscale borides that increased mechanical properties, with the coating michohardness values reached being four times higher than those of the AISI mild steel substrate. Combined with the sliding speeds, it played an important role in the definition of different wear mechanisms. The wear performance of the coatings and substrate was evaluated by three-body wear tests using a pin-on-disc device. It can be seen at lowest sliding speed a plastic deformation and for higher slidding speed, an accumulation of debris on the laser clad coating surface. Different wear mechanisms were observed in the coatings samples: abrasive, adhesive, shearing and delamination. The presence of oxide particles, responsible for the formation of a tribofilm, was also observed for some coatings. Debris nanoparticles from 10 to 100 nm, resulting from fracture or breakage of the coatings during wear tests, were also be identified as Fe-based and oxide nanoparticles. Thus, the Fe–Nb–B ultrafine eutectics laser clad coatings showed wear resistance properties comparable to or even superior to commercial alloys and other materials processed by rapid solidification.

Manipulation of the microstructure and properties of La(Fe,Si)13 alloys via solidification kinetics

The microstructural features of as-cast alloys are predominantly influenced by the fabrication process, which eventually determine the properties. In the present work, we demonstrate that the microstructure (e.g., composition, phase constitution and distribution) and the magnetocaloric properties of the La(Fe,Si)13 alloys can be effectively tailored by tuning the solidification kinetics. La(Fe,Si)13 alloys with different dimensions were prepared by suction casting and jet casting. Finite element (FE) simulations reveal a difference in the temperature gradient along the solidification path of the rapid solidification process. The microstructure difference between the edge and center regions in the as-cast ingots along the solidification direction is increased for the samples with larger diameter. This can be attributed to the different solidification kinetics as predicted by the FE simulations. More homogeneous microstructure can be observed in the samples with a smaller size. The largest magnetic entropy change value of 16 J/kg·K can be obtained in thin plates, which is comparable to that of master bulk alloy. Our thorough study of solidification mechanisms during rapid solidification provides research basis for manipulating the microstructure homogeneity and the related properties, which also paves the way for the study of other rapid-solidificated materials.

Magnetic phase diagram mapping in Fe1−xRhx composition-spread thin films

We have fabricated high-quality polycrystalline Fe1−xRhx composition-spread thin films by cosputtering Fe and Rh, and investigated their structural and magnetic transformations as a systematic function of composition. With increasing Rh concentration, Fe1−xRhx thin film undergoes from an α׳ phase to a disordered γ phase and also shows a magnetic transition from a ferromagnetic phase to a paramagnetic phase. Vibrating-sample magnetometry and x-ray magnetic circular dichroism measurements show an antiferromagnetic-ferromagnetic transition in the range of 0.52 &amp;lt; x &amp;lt; 0.58 in the Fe1−xRhx composition gradient at room temperature. Based on our structural and magnetic property mapping, we construct a thin-film phase diagram of Fe1-xRhx. Compared to reported results in bulk alloys, the antiferromagnetic-ferromagnetic transition in the Fe1−xRhx thin films was found to occur at slightly higher Rh concentrations, while the boundary between the pure γ phase and the α׳/γ mixed phase region is shifted to the lower concentration Rh.

Hydrogen adsorption study on nanostructured Ag–Rh films grown by supersonic cluster beam deposition

Following recent literature work, an interesting result on hydrogen sorption by Ag–Rh clusters was reported which do not follow the common understanding of hydrogen absorption in bulk alloys. Thus, nanostructured Ag–Rh films with a composition of 75% Ag–25% Rh were grown by supersonic cluster beam deposition onto microbalance quartz crystals in order to measure hydrogen absorption. The films were exposed to H2 gas to study hydrogen absorption at different partial pressure. The Ag–Rh films were then characterized by XPS and AFM. The Ag/Rh atomic ratio was found to be 75/25 by XPS. Surface roughness was determined by AFM to be 3 nm RMS in all the films. In disagreement with the quoted literature report, no hydrogen absorption was observed in our Ag–Rh films up to 5.3×103Pa of hydrogen pressure. The response of the balance was checked and validated by measuring hydrogen absorption on 10 and 100 nm Pd films. The hydrogen absorption by Pd yielded an atomic H/Pd ratio of ∼0.65 in agreement with our prior measurements.

Magnetostructural transition and magnetocaloric effect in Mn0.5Fe0.5NiSi1−xAlx melt-spun ribbons (x = 0.055 and 0.060)

Melt-spun ribbons samples of the multicomponent alloy Mn0.5Fe0.5NiSi0.940Al0.060 were prepared and the magnetostructural transition (MST) and related magnetocaloric properties studied for as-solidified ribbons and ribbon samples annealed between 800 and 950 °C for 4 h. The results are compared with those reported in the literature for melt-spun ribbons with an Al content x = 0.055 and bulk alloys. It is shown that all samples undergo a first-order MST from a paramagnetic Ni2In-type hexagonal structure to a ferromagnetic TiNiSi-type orthorhombic one. Ribbons show broader isothermal entropy change ΔST(T) curves with moderate maximum values of |ΔST|max at 2 T (7.2–7.3 J kg−1 K−1) in comparison with the reported for bulk alloys. However, the average value of the magnetic hysteresis loss linked to the hexagonal-to-orthorhombic transition is low in comparison with the one reported for most magnetocaloric materials with first-order magnetostructural transitions. This work underlines the effectiveness of this rapid solidification technique to produce highly homogeneous ribbon samples of multicomponent alloys.

Tuning of structural, band gap and optical properties through Sb substitution in Fe doped GeTe bulk alloys

Tuning of structural, band gap and optical properties through Sb substitution in Fe doped GeTe bulk alloys