Diffusion Of Cr Research Articles

Unique strengthening effect in additively manufactured bimetallic Mild Steel and Stainless Steel

We fabricated a unique bimetallic cylinder by Directed Energy Deposition 3D printing. The inner part of the cylinder was built using Mild Steel, while Stainless Steel was used as the perimeter. The tensile properties and microstructures were compared with those of cylinders made from a single material. The bimetallic structure showed an ultimate tensile strength of 757 MPa, which was ∼150–300 MPa higher than that of cylinders using single Mild Steel or Stainless Steel. The strengthening was attributed to a fully martensite microstructure in the Mild Steel-side, and the martensitic formation was caused by the diffusion of Cr and Ni. Although the bimetallic structure caused a reduction in ductility owing to the formation of twins and debonding of the interface, the moderate elongation to failure of 12.1 % was maintained.

Dynamic strain ageing of austenitic Ni-based alloy during cyclic loading at 350 °C: Mechanism and its evolution

Dynamic strain ageing (DSA) in an austenitic Ni-based alloy was investigated by symmetric/asymmetric low cycle fatigue tests with different strain amplitudes at 350 °C. A quantitative analysis of DSA activity was provided in terms of the characteristic parameters (DSA transient and serration length), and associated microstructural features were explored. Particular attention was paid to the comparison in the DSA domain between Ni- and Fe-based austenite with different strain ratios. Results showed that DSA existed throughout life and led to the continuous cyclic hardening in austenitic Ni-based alloy. The DSA activity first decreased and remained almost constant under symmetric loadings. A gradual increase in DSA activity was observed for the asymmetric loadings before the final failure. In addition, a physical model was presented to explore the DSA mechanisms of austenitic Ni-based alloys in different temperature ranges. Specifically, at 350 °C, DSA is caused by C diffusion in austenite, while at higher temperatures, it may be caused by Cr diffusion. Moreover, for a given mechanism, the DSA occurred at a higher temperature range in Ni-based alloy than in Fe-based alloy.

Effect of Ta on selective oxidation behavior and oxide film adhesion of alumina-forming austenitic alloys

The effects of Ta on the selective oxidation behavior and scale adhesion of Fe-45Ni-25Cr-4Al-1.5Nb-0.4C-xTa (x=0.6, 0.9, 1.2, 1.5 wt%) austenitic alloys at high temperature under low oxygen pressure were investigated. The FCC, MC-(Ta, Nb)C, M23C6 and B2-NiAl phases were found in the annealed alloy. Addition of Ta significantly promoted the formation of an external Al2O3 layer and inhibited growth of Cr2O3 and spinel at 850 °C under oxygen pressures of 10−18 atm and 10−24 atm. Similar oxidation behavior was observed in two-step oxidation process (650 °C for 8 hours and 1000 °C for 16 hours). Ta and Nb preferentially segregated at the interface between the oxide scale-matrix as well as grain boundary within the oxide scale during oxidation, and inhibited outward diffusion of Al and Cr. A large oxygen activity gradient was formed in the oxide film, resulting in the formation of columnar grains. Due to a significant decrease in oxygen activity at the front of the reaction, selective oxidation of alumina was promoted. Furthermore, the scratch test technique was used to quantify the adhesion of oxide scales. Peg-like structures formed by Ta and Nb positively influenced the adhesion properties of the oxide scale.

Influence of Cobalt and Cobalt–Manganese Oxide Coating Thickness Deposited by DLI-MOCVD as a Barrier Against Cr Diffusion for SOC Interconnect

AbstractThe influence of cobalt and cobalt–manganese oxide coating thickness on its ability to be a good diffusion barrier against Cr outward diffusion was investigated for stainless steel interconnects (AISI 441) of a solid oxide cell (SOC). The coatings were all synthesized using a DLI-MOCVD (Direct Liquid Injection-Metal Oxide Chemical Vapor Deposition) hot wall reactor. The study shows that a minimum cobalt oxide thickness of 300 nm was needed to be a good diffusion barrier against Cr for the 500-h exposure test. This observation was linked to the Mn concentration reached in the cobalt spinel during exposure. Indeed, during exposure at high temperature, Mn diffused from the substrate into the cobalt coating and transformed cobalt spinel into Co-Mn spinel. Whereas pure cobalt spinel was a good Cr diffusion barrier, cobalt-manganese spinel, Co3-xMnxO4, was not when x > 2. The thickness of the cobalt coatings must be chosen so that the Mn quantity coming into it from diffusion from the substrate does not degrade the protectiveness of the coating.

Interdiffusion mechanism between EB-PVD deposited CoCrAlY bond-coat and superalloy substrate under high-temperature oxidation service conditions in gas turbine

The interdiffusion mechanism between EB-PVD deposited CoCrAlY coating and superalloy substrate under high-temperature oxidation atmosphere in actual gas turbines was investigated both experimentally and through thermodynamics and kinetics simulation. The interdiffusion behavior differs at different regions of blade due to the different service temperatures. The elemental interdiffusion and phase transformation behavior were investigated and discussed. Needle-like topologically close-packed phase and σ-CrCoNi phase have precipitated within the secondary reaction zone due to the γ→γ’ transition and the uphill diffusion of Cr, W, and Mo. Decreasing the Co content in the MCrAlY bond-coat can inhibit the interdiffusion behavior to a certain extent.

Accelerated formation of M2C carbides by proton irradiation inhibits molten salt corrosion in Ni-based alloy

The effect of displacement damage on the corrosion behavior in a molten FLiBe salt environment was investigated. The element distribution and microstructure around the grain boundaries (GBs) after corrosion were characterized. The results show a decrease in corrosion thickness with increasing irradiation dose and the presence of intergranular corrosion. Nanoscales M2C carbides were observed to be distributed, with a denser and thicker distribution in samples with higher irradiation doses. Their distribution depth is related to the Cr depletion region, inhibiting Cr diffusion toward the GBs and surface. Furthermore, the nucleation mechanism of M2C carbides along the GBs and in irradiated regions was revealed, attributed to the combined effects of thermal influences, element preferential dissolution due to corrosion, and irradiation-induced segregation.

Influence of surface plastic deformation on oxidation behavior for low Cr content nickel superalloy: Focus on the Cr diffusion

The surface plastic deformation induced by shot-peening on the oxidation behavior for GH3535 alloy was investigated. The variation in microstructure and mechanical property of GH3535 with different shot peening intensities was compared, followed by exposure to air at 950 °C for 200 h. The structure and elemental distribution of the oxide film were analyzed by combining SEM, EBSD and TEM. The result shows that shot peening significantly improves the oxidation resistance of GH3535 alloy. This treatment promotes the formation of fine grains on the surface of the alloy, and the grain boundaries act as fast diffusion channels of Cr leading to the Cr effective diffusivity of shot peening samples increase, the Cr effective diffusivity value is 0.235, 4.8 × 10−3, 3.8 × 10−3, 3.7 × 10−3, respectively. It promotes the formation of protective scales for the continuous Cr2O3 layer. Additionally, the effect of grain size on the diffusion of scale-forming elements (Cr, Si and Mn) in the GH3535 alloy was analyzed, with a particular focus on the diffusion of Cr and the synergistic effects of scale-forming element.

Microstructure Evolution of Vacuum Diffusion-Bonded 304 Stainless Steel/20 Carbon Steel Bimetallic Interface after Solution Treatment.

In this study, a 304 stainless steel (304 SS)/20 carbon steel (20 CS) bimetal was prepared by vacuum diffusion bonding, with 20 CS as the substrate and 304 SS as the cladding layer, and the interfacial microstructure and bonding strength before and after solution treatment were studied. The 304 SS and The 20 CS formed a strong metallurgical bond after being held at 1380 °C for 60 min without defects such as unbonded regions. Diffusion of Cr and Ni atoms from the 304 SS to the 20 CS occurred, and the C atoms diffused from the 20 CS to the 304 SS, forming a carburized region. A pearlitic region with an average width of approximately 20 μm was formed on the 20 CS side. After solution treatment, austenitization was formed in the carburized region, which was accompanied by the formation of twin crystals. The interfacial bonding strength of the bimetal was measured to be 485 MPa, which increased to 547 MPa after the solution treatment.

High temperature corrosion of wrought and wire arc additively manufactured 316L stainless steel in a simulated boiler environment

In this decade, the working temperature of the power plants significantly increased to above 700 °C to enhance efficiency. The corrosive species deposits on the hot section components were prone to corrosion damage at elevated temperatures. This study investigates the microstructure and high-temperature corrosion characteristics of the wrought and wire-arc additive manufactured (WAAM) 316L stainless steel in an aggressive molten Na2SO4 + 25% NaCl salt and air environment at 750 °C. The corrosion rate of both wrought and WAAM-built 316L was higher in the molten salt (MS) environment compared to air due to the chloride and sulfate deposits. The wrought 316L was severely prone to corrosion damage with spallation and cracking, which was attributed to the dissolution of the non-protective Fe2O3 scale by Cl. The WAAM-built 316L showed the lower oxidation and depth of corrosion attack in both air and MS environments than the wrought steel due to the fine dendrite grains, resulting in the outward diffusion of more Cr. The accelerated degradation occurred on the WAAM and wrought 316L SS in MS condition due to the dissolution of Cr2O3 and the faster inward diffusion of Na+. The detailed oxide growth, internal corrosion attack, and oxide failure mechanisms of the steels were explored in the air and MS conditions.

B and Ce composite microalloying for improving high-temperature oxidation resistance of 254SMO super-austenite stainless steel

Aiming at serious oxidation problem of 254SMO super-austenitic stainless steel during hot working, the influence of B and Ce composite microalloying on its oxidation behavior was comparatively investigated at 1050 and 1100 °C. The results demonstrated that the combination of B and Ce can significantly alter the composition of the oxide film in 254SMO. Particularly, B and Ce composite microalloying can effectively promote the diffusion of Cr to the surface, and form a dense Cr2O3 oxide film at a faster rate in the initial stage, which is more conducive to inhibiting the Mo volatilization and thus improving the oxidation resistance of 254SMO steels. Additionally, compared to the 0.005 wt% B (50B) and 0.005 wt% B together with 0.002 wt% Ce (50B + 20Ce) samples, the addition of 0.005 wt% B together with 0.005 wt% Ce (50B + 50Ce) had a more significant effect on improving high-temperature oxidation resistance of 254SMO. This research provides a valuable scholarly reference for improving the oxidation resistance of super-austenite stainless steels.

Exploring the effect of Si content in Cr coating on its microstructure and properties

Four CrSi coatings with varying Si content ranging from 2 at.% to 9 at.% were deposited on the both surface of Zr-4 substrate. This study systematically investigated the effect of Si content on microstructure, mechanical properties, corrosion resistance and high-temperature oxidation resistance. Due to the lower Si content, only the Cr phase was detected in the CrSi coating, with Si existing in solid-solution. Therefore, all CrSi coatings exhibited good mechanical and hydrophobic properties. The addition of Si in the Cr coating significantly reduces the surface roughness compared with Cr coating. During the long-term autoclave test (30-day, 60-day and 90-day) at 320 °C and 11.3 MPa, CrSi coatings demonstrated superior corrosion resistance attributed to the effective inhibition of further Si dissolution by the dense and continuous Cr2O3 layer forming on the surface. Additionally, after exposure to high-temperature oxidation at 1200 °C for 30 min, all CrSi coatings protect the substrate from oxidation. However, only the Cr0.91Si0.09 coating internally formed a dense and continuous in-situ Zr2Si diffusion barrier layer, which effectively reduce the Cr/Zr inter-diffusion, and sufficient Si prevent the outwards diffusion of Cr by forming Cr3Si boundary. The corrosion and oxidation mechanism have been studied in detail.

Phase transformation mechanisms occurring during spark plasma sintering elaboration of new duplex composite stainless steels

This study focuses on the elaboration of duplex stainless steels (DSS) from powder mixtures using spark plasma sintering (SPS). Different mass fractions of an austenitic 316L powder and a ferritic 410L one were blended and then sintered by SPS. Microstructural characterizations of the sintered samples obtained from different powder mixtures were performed. They were coupled with marking experiments of the powder particles’ surface. The results showed the formation of martensite within the ferritic powder and at the austenite/ferrite interfaces, following two different mechanisms. In addition, it was found that the width of the martensitic regions is mainly influenced by the diffusion of Cr and Ni from the austenitic to the ferritic powder during sintering. The characterizations revealed that the originality of this approach lies in the particular microstructure obtained after sintering. The characteristic size of the ferritic and austenitic domains in the final material is that of the initial powder particles (up to some hundred microns). Moreover, each domain is formed by equiaxed and isotropic grains, having a size ranging from some microns to some tens of microns. This particular microstructure justifies the use of the term “composite duplex stainless steels” (COMPLEX) for this kind of new DSS.

Morphological Evolution and Dealloying During Corrosion of Ni20Cr (wt.%) in Molten FLiNaK Salts

The dealloying corrosion behavior of the FCC Ni20Cr (wt%) in molten LiF-NaF-KF (FLiNaK) salts at 600 °C under varying applied potentials was investigated. Using in-operando electrochemical techniques and a multi-modal suite of characterization methods, we connect electrochemical potential, thermodynamic stability, and electro-dissolution kinetics to the corrosion morphologies. Notably, under certain potential regimes, a micron-scale bicontinuous structure, characterized by a network of interconnected pores and ligaments riched with the composition of the more noble (MN) element, becomes prominent. At other potentials both MN and less noble (LN) elements dealloy but at different rates. The dealloying process consists of lattice and grain boundary diffusion of Cr to the metal/salt interface, interphase Cr oxidation, accompanied by surface diffusion of Ni to form interconnected ligaments. At higher potentials, the bicontinuous porous structure undergoes further surface coarsening. Concurrently, Cr(II), Cr(III), and Ni(II) begin to dissolve, with the dissolution of Ni occurring at a significantly slower rate. When solid-state transport of Cr is exceeded by the interfacial rates, dealloying depths are limited.

Grain boundary diffusion and segregation of Cr in Ni [formula omitted] bicrystals: Decoding the role of grain boundary defects

Grain boundary diffusion of Cr in a near Σ11(1̄13)[110] Ni bicrystal is measured over a temperature interval between 503 K and 1203 K using the radiotracer technique. The grain boundary diffusion coefficients, Dgb, and the triple products, P=s⋅δ⋅Dgb, are determined in the C- and B-type kinetics regimes, respectively, with s being the segregation factor and δ the grain boundary width. Opposite to expectations, two distinct contributions to short-circuit diffusion along the nominally single interface are distinguished and related to the existence of two macroscopic facets with distinct grain boundary inclinations and, as a result, distinct structures. The experimental results indicate that the segregation factor of Cr in Ni is about unity, which is fully supported by ab initio calculations. Using classical atomistic simulations, Ni grain boundary self-diffusion coefficients are calculated for the symmetric and asymmetric facets. The computational simulations reveal accelerated self-diffusion kinetics along the asymmetric facet, attributing this phenomenon to the presence of disconnection-like defects. This elucidates the experimentally observed diffusion dynamics of chromium atoms, thereby corroborating the heterogeneous mechanisms governing atomic migration across distinct facets.

Effect of quenching-super intercritical quenching-tempering process on the evolution of microstructure and the yield ratio of Ni–Cr–Mo steel

In the present study, X-ray diffraction, electron probe microanalysis, and transmission electron microscopy, among other advanced characterization methods, were used to analyze the microstructure and mechanical properties of the 9Ni steel treated at different heat treatment conditions. The goal was to reveal the dual-optimal strengthening mechanism of “low yield ratio and ultra-cryogenic toughness” of the investigated steel. The experimental results indicated that the 9Ni steel, designed with a ''Ni–Cr–Mo'' multi-elemental composition, balanced its hardenability and corrosion resistance. After the Q-Q' (710 °C) -T process, the diffusion of Cr and Mo atoms in the steel accelerated, while the Fe displacement diffusion formed a more stable FeNi face-centered cubic structure with a lattice energy of −76.98 eV. At this stage, the microstructure of the investigated steel primarily consisted of tempered sorbite (with an average effective grain size of 13.1 μm), tempered martensite lath clusters, reversed austenite, and precipitated Cr23C6 and Mo2C particles. This composition resulted in a low yield ratio. The concentration of Ni near the lath clusters promoted the formation of the reversed austenite with a volume fraction of 10.2%, ensuring the cryogenic toughness of the investigated steel.

Compositional zoning and evolution of symplectite coronas in jadeitite

Abstract Multistage fluid activities play an important role in the interaction between jadeitite and symplectite coronas; therefore, we studied the compositional zoning and evolution of representative Myanmar jadeitite. Under the influence of multistage Ca-, Na-, and Si-rich fluid activity, some minerals in Myanmar jadeitite formed symplectite coronas with concentric rings and multilayer metasomatic reaction rim structures. Additionally, the concentrations of Cr and Fe decrease from the core to the peripheral jadeite minerals, whereas the concentration of Si markedly increases. There is almost no Si or Ca in the chromite core, and the concentrations of Si and Ca increase sharply in rims composed of uvarovite. Due to Cr diffusion, the edge of the jadeite adjacent to kosmochlor is Cr-rich and Al-poor. The different element concentrations indicate that the uvarovite formed from the presence of a chromite and jadeite interaction, Si in the kosmochlor after metasomatism or an external Ca-rich fluid. One possible explanation for the formation of kosmochlor is the interaction between chromite and a Na- and Si-rich fluid. Also, Ca-rich fluid could have first interacted with chromite and formed uvarovite; subsequently, a Na-rich fluid could have entered and become saturated with kosmochlor, leading to the formation of kosmochlor surrounding the uvarovite.

Effects of alloying elements (Mo and W) on corrosion behavior of Ni-based alloys in NaCl-KCl-MgCl2 molten salt

The effects of elements (Mo and W) on the corrosion behavior of some Ni-based alloys (GH3535 and GH3539) with a low Cr content (6–7 wt%) in a NaCl-KCl-MgCl2 molten salt under Ar gas at 800°C were investigated by an immersion test combining with the microstructure analyses. GH3539 alloy suffered more severe corrosion attack than GH3535. The precipitates caused by the alloying elements affects the outward diffusion of Cr. W is more sensitive to corrosion than Mo in the chloride salt, resulting in the worse corrosion resistance of GH3539 compared to GH3535. The interactions among the elements also affect the corrosion.

Interfacial reaction characteristics and mechanisms during dissimilar friction stir lap welding of pure copper and Al0.1CoCrFeNi alloy

In order to explore the weldability and widen the applicability of high entropy alloys (HEAs), dissimilar friction stir lap welding of pure copper and Al0.1CoCrFeNi alloy was investigated in this research. The results indicate that a sound dissimilar weld without tunnels and voids is produced, where a ∼20 μm thick interfacial layer is evident at the Cu-HEA interface. The lap joint is fractured at the upper Cu sheet rather than the lap surface during tensile shear test, suggesting an effective bonding of the dissimilar materials. The lap weld roughly consists of three layers along the plate thickness direction, i.e. welds of Cu, HEA and Cu-HEA interface, respectively. The interface of weld, which has experienced the most severe Cu-HEA interaction during welding, is characterized by uniformly distributed and highly recrystallized ultra-fine grains (∼0.79 μm). Meanwhile, intermetallic compounds particles (Al13Cr2 and Al13Co4) ranging in size of 150∼400 nm are generated at the fine grain boundaries. Fast diffusion of Al, Cr and Co elements and easy segregation of these elements during welding result in the formation of the particles. The sufficient DDRX and the pinning effect of particles are responsible for the formation of ultra-fine grains in the weld interface. The present study lays a theoretical foundation for the joining of conventional Cu and the multi-component HEA, which contributes to the improvement of HEA availability in the field of manufacturing industry.

Early-stage diffusion and oxidation behavior of Cr-Nb coated Zr alloy accident tolerant fuel cladding materials at 1200°C–1500°C

Although the Cr-coated Zr alloys are considered as the most promising accident tolerant fuel cladding materials in recent years, the Cr-Zr interdiffusion problem is a major obstacle for the widely used of the Cr coating. In this study, Cr-Nb coated Zr alloy samples were prepared by magnetron sputtering. The Nb layer was designed as a barrier layer to retard the Cr-Zr interdiffusion. By using a fast-in and fast-out equipment, short time annealing was conducted in argon environment to study the coating evolution in high temperature environment. Results show that the Nb coating can retard the inward diffusion of Cr because of the low diffusion coefficient of Cr in the Nb layer. However, the complete miscibility between Nb and Zr lead to the fast consumption of the Nb coating. After 10 min annealing at 1200 °C, the 2.5 μm Nb coating disappeared, and a Cr-Zr-Nb mixed phase formed between the residual Cr coating and Zr substrate. The early-stage microstructure evolution of the sample was analyzed, and the barrier effect of the Nb coating was summarized.

Hot corrosion Type I of novel Co‐based γ/γ′ superalloys with and without Al‐ and Cr‐diffusion coatings

AbstractCo‐based γ/γ′ superalloys are in development as candidates to replace Ni‐based superalloys in turbine engines, where they will be exposed to hot corrosion. The present study investigates the resistance to high temperature Type I hot corrosion of four alloys of the Co–Ni–Al–W system, which differed in their Co to Ni ratio and their W content. In addition, the alloys are tested not only uncoated but also with an Al and a Cr diffusion coating. The exposures were performed at 950°C with a Na2SO4 deposit of 2.5 mg/cm2 in air containing 0.1% SO2 for 100 and 300 h. The degradation was investigated by material loss measurements. Both coatings proved mostly protective for 100 h, while the uncoated samples were severely degraded. After the longer exposure time, only the Cr‐coating remained partially protective and outmatched the Al‐coating. Further, a positive influence was attributed to a higher Co to Ni ratio (1.4 vs. 0.7) and a higher W content (5 vs. 3 at%).