Fe based alloys are commonly used in almost every sector of human life. For different reasons, the surfaces of the real parts are prepared using different methods, e.g., mirror-like polishing, grit-blasting, etc. The purpose of the present work is to answer the question how the surface preparation influences the oxidation behavior of Fe-based alloys. To answer this question, a high purity model alloy, Fe 5 wt Al, was isothermally oxidized in a thermogravimetrical furnace. The post-exposure analysis included SEM/EDS (WDS) and XRD. The surface roughness was determined by a contact and laser profilometer. The obtained results demonstrate that the mechanical surface preparation influences oxidation kinetics as well as the microstructure of the oxide scale formed on the alloy at both studied temperatures. Namely, polishing and grinding caused local formation of Fe-rich nodules and sub-layer of protective Al2O3. In contrast, gritblasting leads to the formation of a thick outer Fe-oxide and internal aluminum nitridation. A significant increase in the oxidation rate of the material after gritblasting was attributed to grain refinement in the near-surface region, resulting in an increase in easy diffusion paths, namely grain boundaries.
We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy $\mathrm{Co_{2}FeAl}$, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time ($τ_{M}$) in films of $\mathrm{Co_{2}FeAl}$ is almost independent of varying structural order, and that it is similar to that in elemental 3d ferromagnets. In contrast, the slower process of magnetization recovery, specified by $τ_{R}$, is found to occur on picosec…
This work reports about electronic structure calculations for the Heusler compound Co$_2$FeAl$_{1-x}$Si$_x$. Particular emphasis was put on the role of the main group element in this compound. The substitution of Al by Si leads to an increase of the number of valence electrons with increasing Si content and may be seen as electron-doping. Self-consistent electronic structure calculations were performed to investigate the consequences of the electron doping for the magnetic properties. The series Co$_2$FeAl$_{1-x}$Si$_x$ is found to exhibit half-metallic ferromagnetism and the magnetic moment f…
FeAl-based MIL composites of various iron alloys were fabricated with an innovative 'multiple-thin-foil' configuration and 'two-stage reaction' strategy.
We present results of extensive theoretical studies of Co$_2$FeAl$_{1-x}$Si$_x$ Heusler alloys, which have been performed in the framework of density functional theory employing the all-electron full-potential linearized augmented plane-wave scheme. It is shown that the Si-rich alloys are more resistive to structural disorder and as a consequence Si stabilizes the $L2_1$ structure. Si alloying changes position of the Fermi level, pushing it into the gap of the minority spin-band. It is also shown that the hyperfine field on Co nuclei increases with the Si concentration, and this increase origi…
Resonant photoemission spectroscopy has been used to investigate the character of Fe 3d states in FeAl alloy. Fe 3d states have two different character, first is of itinerant nature located very close to the Fermi level, and second, is of less itinerant (relatively localized character), located beyond 2 eV below the Fermi level. These distinct states are clearly distinguishable in the resonant photoemission data. Comparison between the results obtained from experiments and first principle based electronic structure calculation show that the origin of the itinerant character of the Fe 3d states…
The boosting effect of Cr on the growth of the protective alumina scale on Fe-Al alloys is investigated by x-ray photoelectron spectroscopy. Using low oxygen pressure the surface chemistry of the alloys is monitored starting from the first moments of oxidation. Chromium effect on the Fe-Al surface-bulk exchange is clearly detected by analyzing the measured surface concentrations within the atomic concentration models. Previous ab initio calculations agree well with the present experiments.
FeCrAl alloys are proposed and being intensively investigated as alternative accident tolerant fuel (ATF) cladding for nuclear fission application. Herein, the influence of major alloy elements (Cr and Al), reactive element effect and heating schedules on the oxidation behavior of FeCrAl alloys in steam up to 1500°C was examined. In case of transient ramp tests, catastrophic oxidation, i.e. rapid and complete consumption of the alloy, occurred during temperature ramp up to above 1200°C for specific alloys. The maximum compatible temperature of FeCrAl alloys in steam increases with raising Cr a…
This paper experimentally investigated the effects of silicone oil on the tribological behaviour of AZ31B/86CrMoV7 contact pair under a pin-on-disk configuration. A commercial silicone oil was used as an oxidation inhibiter for the AZ31B at elevated temperatures to 300 oC. The wear mechanism of the contact pair was explored by SEM morphologies of worn surfaces. The analysis revealed that silicone oil can effectively minimize surface oxidation. Compared with dry sliding, silicone oil can considerably lower down the friction coefficient and specific wear rate. Under lubrication, the abrasive wea…
The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heat…
Double ring formation on Co2MnSi (CMS) films is observed at electrical breakdown voltage during local anodic oxidation (LAO) using atomic force microscope (AFM). Corona effect and segregation of cobalt in the vicinity of the rings is studied using magnetic force microscopy and energy dispersive spectroscopy. Double ring forma-tion is attributed to the interaction of ablated material with the induced magnetic field during LAO. Steepness of forward bias transport characteristics from the unperturbed region of the CMS film suggest a non equilibrium spin contribution. Such mesoscopic textures in m…
Although of practical importance, there is no established modeling framework to accurately predict high-temperature cyclic oxidation kinetics of multi-component alloys due to the inherent complexity. We present a data analytics approach to predict the oxidation rate constant of NiCr-based alloys as a function of composition and temperature with a highly consistent and well-curated experimental dataset. Two characteristic oxidation models, i.e., a simple parabolic law and a statistical cyclic-oxidation model, have been chosen to numerically represent the high-temperature oxidation kinetics of c…
A catalyst surface may be exposed to strain due to application of load or interfacing with a substrate with large lattice mismatch. In order to test the effect of strain on catalytic efficiency, we use Density Functional Theory +U (DFT+U) to model water oxidation on expanded and contracted surfaces of the Ni1-xFexOOH alloy, one of the best known catalysts for water splitting. We find that a low amount of iron content has a similar effect as of applying compressive strain. Due to high oxidation state of Fe4+ at the active site, the Fe-O bond is shorter than in pure FeOOH, which is beneficial fo…
High entropy materials, which include high entropy alloys, carbides, and borides, are a topic of substantial research interest due to the possibility of a large number of new material compositions that could fill gaps in application needs. There is a current need for materials exhibiting high temperature stability, particularly oxidation resistance. A systematic understanding of the oxidation behavior in high entropy materials is therefore required. Prior work notes large differences in the thermodynamic favorability between oxides formed upon oxidation of high entropy materials. This work use…
The internal oxidation of Sn was investigated to understand the oxidation kinetics of monophase CuSn alloys. SnO2 particles were characterized by analytical transmission electron microscopy. The orientation relationship between SnO2 and Cu was determined with a special emphasis on the atomic scale structure of Cu/SnO2 interfaces (misfit dislocations and chemical structure). Habit planes with a pure oxygen plane terminating the SnO2 phase are greatly favored and large misfits promote the growth of plate shaped precipitates.
In recent times,there has been a significant volume of work on Eutectic High Entropy Alloys (EHEAs) owing to their remarkable castability combined with excellent mechanical properties,which aids in clearing obstacles for their technological applications. One of the most common EHEAs,which has been of enormous interest at present, primarily owing to its solidification and tensile behavior, is AlCoCrFeNi$_{2.1}$. However,to aim for high-temperature applications,oxidation behaviour of material is one of the major aspects that needs to be extensively investigated.To this end,the present work aims …
In the present paper, the analysis of polishing, grinding and sand-blasting processes on oxidation behavior of nickel, copper and nickel-copper alloy has been investigated. Surface roughness has been measured using two methods: by contact profilometer and by fractal analysis. It has been proved that sand-blasting process influences the surface roughness the most but also contaminates the surface with alumina particles. The oxidation tests were performed in thermogravimetrical furnace. The obtained mass gain plots, SEM microphotographs of cross-sections and SEM EDS analysis of the samples prove…
NiTi-based high-temperature shape memory alloys (HTSMAs) such as NiTiHf have been utilized in a broad range of applications due to their high strength and work output, as well as, their ability to increase the transformation temperatures (TTs). Recently, additive manufacturing techniques (AM) have been widely used to fabricate complex shape memory alloy components without any major modifications or tooling and has paved the way to tailor the manufacturing and fabrications of microstructure and critical properties of their final parts. NiTi alloys properties such as transformation temperatures …
Hot salt stress corrosion cracking in Ti 6246 alloy has been investigated to elucidate the chemical mechanisms that occur. Cracking was found to initiate beneath salt particles in the presence of oxidation. The observed transgranular fracture was suspected to be due to hydrogen charging; XRD and high-resolution transmission electron microscopy detected the presence of hydrides that were precipitated on cooling. SEM-EDS showed oxygen enrichment near salt particles, alongside chlorine and sodium. Aluminium and zirconium were also involved in the oxidation reactions. The role of intermediate corr…
Ab initio calculation results for the electronic structure of disordered bcc Fe(x)Al(1-x) (0.4<x<0.75), Co(x)Al(1-x) and Ni(x)Al(1-x) (x=0.4; 0.5; 0.6) alloys near the 1:1 stoichiometry, as well as of the ordered B2 (FeAl, CoAl, NiAl) phases with point defects are presented. The calculations were performed using the coherent potential approximation within the Korringa-Kohn-Rostoker method (KKR-CPA) for the disordered case and the tight-binding linear muffin-tin orbital (TB-LMTO) method for the intermetallic compounds. We studied in particular the onset of magnetism in Fe-Al and Co-Al systems a…
In the present work, it is shown that the surface preparation method used on two Ni-based superalloys prior to aluminizing chemical vapor deposition (CVD) is one of the most important factors determining the oxidation resistance of aluminized Ni-based superalloys. It was found that grit blasting the substrate surface negatively affects the oxidation resistance of the aluminized coatings. For grit blasted and aluminized IN 625, a thicker outer NiAl coating was formed compared to that of IN 738. In contrast, no effect on NiAl coating thickness was found for grit blasted and aluminized IN 738. Ho…
Plasma electrolytic oxidation (PEO) is a process for obtaining oxide coatings on valve metals. Mostly PEO is done in an aqueous solution electrolyte which limits the size of treated parts due to the system heating up. In presented work an alternative method of PEO processing applied in aluminum 1050 alloy in nitrate molten salt was investigated. The morphology, phase and chemical compositions, micro-hardness, and corrosion resistance were examined using. The obtained results showed that formed coating contains from two sub-layers, outer soft layer with the thickness of 4 micrometer and inner, …
The corrosion, mechanical degradation and biological performance of cold-drawn WE43 Mg wires were analyzed as a function of thermo-mechanical processing and the presence of a protective oxide layer created by continuous plasma electrolytic oxidation (PEO). It was found that the corrosion properties of the non-surface-treated wire could be optimized by means of thermal treatment within certain limits, but the corrosion rate remained very high. Hence, strength and ductility of these wires vanished after 24 h of immersion in simulated body fluid at 37$^\circ$C and, as a result of that rather quic…
Regularities and features in the behavior of electrical and magnetic properties of Co$_2$FeZ (Z = Al, Si, Ga, Ge, Sn, Sb) half-metallic ferromagnetic Heusler alloys
本文测量 Co$_2$FeZ 系列 Heusler 合金的电阻率、磁化和 Hall 效应，揭示其随 Z 元素原子序数变化的电磁关联及不同温区的电阻散射规律。
It has been widely accepted that the introduction of titanium carbides into titanium-based alloys can significantly enhance the oxidation resistance due to their superior physicochemical stability at elevated temperatures. The present study reported for the first time that the ordered C vacancies within titanium carbides could lead to an uncommon phenomenon particularly at the very initial stage of oxidation. The intrinsic micro-to-macro oxidation mechanisms were systematically clarified with the aids of transmission electron microscope and ab-initio molecular dynamics simulation.
TiO$_2$ and WO$_3$ are two of the most important earth-abundant electronic materials with applications in countless industries. Recently alloys of WO$_3$ and TiO$_2$ have been investigated leading to improvements of key performance indicators for a variety of applications ranging from photo-electrochemical water splitting to electrochromic smart windows. These positive reports and the complexity of the ternary W-Ti-O phase diagram motivate a comprehensive experimental screening of this phase space. Using combinatorial thermal oxidation of solid solution W$_{1-x}$Ti$_{x}$ precursors combined wi…
In our work, a novel processing strategy for the continuous fabrication and surface modification of wires from Magnesium alloy WE43 by means of plasma-electrolytic oxidation (PEO) is presented. In the first step, wires with a strong basal texture and small grain size ($\approx$ 1 $μ$m) were manufactured by combined cold drawing and in-line stress-relief heat treatment steps that optimized the mechanical properties (in terms of strength and ductility) by means of annealing. In a second step, and to the best of our knowledge for the first time ever, the wires were continuously surface-modified w…
Open-porous scaffolds of WE43 Mg alloy with a body-center cubic cell pattern were manufactured by laser powder bed fusion with different strut diameters. The geometry of the unit cells was adequately reproduced during additive manufacturing and the porosity within the struts was minimized. The microstructure of the scaffolds was modified by means of thermal solution and ageing heat treatments and was analysed in detail by means of X-ray microtomography, optical, scanning and transmission electron microscopy. Moreover, the corrosion rates and the mechanical properties of the scaffolds were meas…
This progress report summarizes recent studies of electrochemical oxidation to modulate the interfacial tension of gallium-based alloys. These alloys, which are liquid at ambient conditions, have the largest interfacial tension of any liquid at room temperature. The ability to modulate the tension offers the possibility to create forces that change the shape and position of the metal. It has been known since the late 1800s that electrocapillarity-the use of potential to modulate the electric double layer on the surface of metals in electrolyte-lowers the interfacial tension of liquid metal. Ye…
Wear resistant coatings were produced on a permanent mould cast MRI 230D Mg alloy by (a) PEO in silicate based electrolyte, (b) PEO in phosphate based electrolyte, (c) hybrid coatings of silicate PEO followed by laser surface alloying (LSA) with Al and Al2O3, and (d) hybrid coatings of phosphate PEO followed by LSA with Al and Al2O3. Microstructural characterization of the coatings was carried out by scanning electron microscopy (SEM) and X(ray diffraction. The tribological behavior of the coatings was investigated under dry sliding condition using linearly reciprocating ball-on-flat wear test…
The durability of passivable metals and alloys is often limited by the stability of the surface oxide film, the passive film, providing self-protection against corrosion in aggressive environments. Improving this stability requires to develop a deeper understanding of the surface structure and initial surface reactivity at the nanometric or atomic scale. In this work we applied scanning tunneling microscopy to unravel the surface structure of a model stainless steel surface in the metallic state and its local modifications induced by initial reaction in dioxygen gas. The results show a rich an…
The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallisation scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high temperature events or its behaviour when exposed to air. Here, a combined soft and hard X-ray photoelectron spectroscopy (XPS) characterisation approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behaviour of a 300~nm-thick TiW film.…
Effect of a quenching medium (water, liquid nitrogen and block of brass) on a short-range ordering in Fe(100-x)Cr(x) (x less than 19) alloys was studied with the Moessbauer spectroscopy. The distribution of Cr atoms was expressed in terms of the Cowley-Warren short-range order (SRO) parameters: alpha1 for the first neighbor-shell, alpha2 for the second neighbor-shell and alpha12 for both neighbor-shells. It was revealed that none of the quenching media resulted in a random distribution of atoms, yet the degree of randomness was the highest for the samples quenched onto the block of brass. The …
Part of developing new strategies for fabrications of nanowire structures involves in many cases the aid of metal nanoparticles (NPs). It is highly beneficial if one can define both diameter and position of the initial NPs and make well-defined nanowire arrays. This sets additional requirement on the NPs with respect to being able to withstand a pre-growth annealing process (i.e. de- oxidation of the III-V semiconductor surface) in an epitaxy system. Recently, it has been demonstrated that Ag may be an alternative to using Au NPs as seeds for particle-seeded nanowire fabrication. This work bri…
Decoherence in superconducting qubits has long been attributed to two level systems arising from the surfaces and interfaces present in real devices. A recent significant step in reducing decoherence was the replacement of superconducting niobium by superconducting tantalum, resulting in a tripling of transmon qubit lifetimes (T1). One of these surface variables, the identity, thickness, and quality of the native surface oxide, is thought to play a major role as tantalum only has one oxide whereas niobium has several. Here we report the development of a thermodynamic metric to rank materials b…
Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the major detrimental factors to the performance of a Magnetic Tunnel Junction (MTJ), is difficult to avoid during the fabrication process of the MTJ's tunnel barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs, over-oxidation of the tunnel barrier results in the formation of a CoO antiferromagnetic (AF) interface layer which couples with the bottom FM electrode to form a typical AF/FM exchange bias (EB) system. In this work, surface oxidation of the CoFe and CoFeB bottom electrodes was detected via …
We have studied the surface of pure and oxidized Pt$_3$Zr(0001) by scanning tunneling microscopy (STM), Auger electron microscopy, and density functional theory (DFT). The well-annealed alloy surface shows perfect long-range chemical order. Occasional domain boundaries are probably caused by nonstoichiometry. Pt$_3$Zr exhibits ABAC stacking along [0001]; only the A-terminated surfaces are seen by STM, in agreement with DFT results showing a lower surface energy for the A termination. DFT further predicts a stronger inward relaxation of the surface Zr than for Pt, in spite of the larger atomic …
The imaging of active nanoparticles represents a milestone in decoding heterogeneous catalysts dynamics. We report the facet resolved, surface strain state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent x-ray diffraction imaging under catalytic reaction conditions. Density functional theory calculations allow us to correlate the facet surface strain state to its reaction environment dependent chemical composition. We find that the initially Pt terminated nanoparticle surface gets Rh enriched under CO oxidation reaction conditions. The local composition is facet orientatio…
A new class of crack-resistant nickel-based superalloy containing high $γ^\prime$ fraction is studied for the laser-powder bed fusion (L-PBF) process. The effects of the (Nb+Ta)/Al ratio is emphasised, a strategy that is shown to confer excellent low-temperature strength whilst maintaining oxidation resistance at high temperatures via stable alumina scale formation. The processability of the new alloys is characterised with respect to defect assessment by micro-focus x-ray computed tomography; use is made of a prototype turbine blade geometry and the heritage alloy CM247LC as a benchmark. In a…
Liquid metals exhibit remarkable mechanical properties, in particular large surface tension and low viscosity. However, these properties are greatly affected by oxidation when exposed to air. We measure the viscosity, surface tension, and contact angle of gallium (Ga) and a eutectic gallium-indium alloy (eGaIn) while controlling such oxidation by surrounding the metal with an acid bath of variable concentration. Rheometry measurements reveal a yield stress directly attributable to an oxide skin that obscures the intrinsic behavior of the liquid metals. We demonstrate how the intrinsic viscosit…
Plasma electrolytic oxidation (PEO) is a surface processing for oxide coatings formation with advanced properties. Mostly, PEO is performed in an aqueous solution electrolyte which limits the dimension of treated parts due to the system heating up. Therefore, treatment of large surfaces cannot be applied in aqueous electrolyte. In current work an alternative approach of PEO treatment applied in aluminum 1050 alloy in nitrate molten salt was investigated. The structure, phase and chemical compositions, and micro-hardness were examined using XRD, SEM, EDS and micro-hardness tests. The obtained r…
Segregation, precipitation, and phase separation in Fe-Cr systems is investigated. Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy are used. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods has to be used. Using Exact Muffin-Tin Orbitals method the effective chemical potential as a function of Cr content (0-15 at.% Cr) is calculated for a surface, second atomic layer and b…
The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1wt% Mn and one containing 0.5wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protect…
This study aimed to explore and develop a new material with high cost-effectiveness, excellent strength, light weight, high hardness, great wear resistance, corrosion resistance, and favorable oxidation resistance. Structural analysis suggested that, with the change in Co addition amount, the surface morphology and structure of the alloy system changed. XRD analysis indicated that, the alloy system was the FCC+BCC mixed structure. In addition, metallographical demonstrated that, with the increase in Co content, the dendritic crystal transformed from big block to dendritic structure, then to sn…
