Diffusion Of Cr Research Articles

Diffusion of chromium of Cr-coated Zircaloy accident tolerant fuel cladding: Model development and experimental validation

A mechanistic model that calculates Cr diffusion for Cr-coated Zircaloy cladding based on Finite Difference Method (FDM) has been developed and validated. The developed model has been incorporated into TRANOX 2.0. The Cr-coated cladding was divided into multilayers (Zr, ZrCr2 and Cr), and chromium diffusion coefficients of each phase were obtained by experiments. The prediction of the model was validated with Cr distribution measured by EPMA through extensive high temperature annealing experiments conducted in the temperature range of 1000–1300 °C. The developed model can predict diffusional loss coating thickness of Cr-coated Accident Tolerant Fuel.

Effect of laser shock peening and surface mechanical attrition treatment on the oxidation resistance of a 20Cr-25Ni-Nb stainless steel

In this study, the effects of laser shock peening (LSP) and surface mechanical attrition treatment (SMAT) were comparatively investigated on the oxidation behaviors of 20Cr-25Ni-Nb stainless steel in a high-temperature (650 °C) CO2 environment up to 1000 h. The results show that the oxidation resistance of stainless steel can be improved by LSP but deteriorated by SMAT. LSP promoted the formation of a thin and uniform Cr-rich oxide layer on the 20Cr-25Ni-Nb stainless steel instead of a thick duplex oxide layer with Fe-rich outer layer and Cr-rich inner layer which appeared on the untreated sample. For the SMAT-treated sample, besides the oxide scale, a thick internal oxidation zone (IOZ) and oxides spallation were also observed. The formation of a Cr-rich oxide layer was facilitated by the enhancement of the Cr diffusion within the LSP-induced hardened layer. The unsatisfying oxidation performance of the SMAT-treated sample is due to the surface micro-cracks and significant grain refinement which synergistically contributed to the visible carburization and oxidation in a high-temperature CO2 environment.

From promoting aggregation to enhancing obstruction: A negative feedback regulatory mechanism of alleviation of trivalent chromium toxicity by silicon in rice

Trivalent chromium [Cr(III)] is a threat to the environment and crop production. Silicon (Si) has been shown to be effective in mitigating Cr(III) toxicity in rice. However, the mechanisms by which Si reduces Cr(III) uptake in rice are unclear. Herein, we hypothesized that the ability of Si to obstruct Cr(III) diffusion via apoplastic bypass is related to silicic acid polymerization, which may be affected by Cr(III) in rice roots. To test this hypothesis, we employed hydroponics experiments on rice (Oryza sativa L.) and utilized apoplastic bypass tracer techniques, as well as model simulations, to investigate 1) the effect of Si on Cr(III) toxicity and its obstruction capacity via apoplastic bypass, 2) the effect of Cr(III) on silicic acid polymerization, and 3) the relationship between the degree of silicic acid polymerization and its Cr(III) obstruction capacity. We found that Si reversed the damage caused by Cr(III) stress in rice. Si exerted an obstruction effect in the apoplast, significantly decreasing the share of Cr(III) uptake via the apoplastic bypass from 18% to 11%. Moreover, Cr(III) reduced silica particles' radii and increased Si concentration in roots. Modeling revealed that a 5-fold reduction in their radii decreased the diffusion of Cr(III) in apoplast by approximately 17%. We revealed that Cr(III) promoted silicic acid polymerization, resulting in the formation of a higher number of Si particles with a smaller radius in roots, which in turn increased the ability of Si to obstruct Cr(III) diffusion. This negative feedback regulatory mechanism is novel and crucially important for maintaining homeostasis in rice, unveiling the unique role of Si under Cr(III) ion stress and providing a theoretical basis for promoting the use of Si fertilizer in the field.

Effect of inertia friction welding speed on microstructure and mechanical properties of 2205/316L stainless steel joints

In this paper, inertia friction welding (IFW) is used to enable a high-quality and efficient joint between 2205 duplex stainless steel and 316 L stainless steel. Specifically, stainless steel bars (2205/316 L) with a diameter of 35 mm and length of 120 mm are used. In addition, the effect of IFW speed on the microstructure and properties of welded joints is assessed using scanning electron microscopy, electron backscattered diffraction, energy dispersive X-ray spectroscopy, microhardness test, and tensile tests. The results indicate that dynamic recrystallization of the microstructure of joints occurs during IFW welding, which causes grain refinement. The diffusion of Ni from the 316 L base metal to the 2205 base metal, as well as the diffusion of Cr from the 2205 base metal to the 316 L base metal, results in a strong metallurgical bond between the two materials. In addition, the microhardness of the welding area exceeds that of the base metal (the closer to the weld zone, the greater the microhardness value). The tensile strength of the joints initially increases with increasing IFW speed, but then it starts decreasing beyond a certain point. The optimal welding parameters for achieving high-quality joints between 2205 duplex stainless steel and 316 L stainless steel are as follows: axial pressure = 5 MPa, rotational inertia = 4.17 kg·m2, and IFW speed = 2700 r/min.

Microstructural insights into EUROFER97 batch 3 steels

Extensive analytical electron microscopical analyses were carried out from the micrometer scale down to the nanometer scale to characterize three variants of the 9% reduced activation ferritic martensitic (RAFM) steel EUROFER97/3. No huge microstructural differences were observed between the three grades. Electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM) was used to determine prior austenite grain (PAG) and lath sizes of the martensite matrix. The PAG size varied between 4.5 µm and 6.5 µm depending on the reconstruction algorithm. Furthermore, the martensitic lath sizes determined by SEM-EBSD are only half or 1/3 of that determined manually from transmission electron microscopy (TEM) images, which might be related to the limited statistics in this type of TEM data evaluations. The SEM-EDX shows that M23C6-type phases are preferentially located on lath and grain boundaries due to preferential diffusion of elements like Cr, W, and C to and along grain boundaries, which agrees with TEM-EDX measurements. TEM techniques like STEM-EDX and high-resolution TEM were used to describe the occurring precipitates i.e., M23C6, VN, TaC morphologically, structurally, and chemically. In addition, the thermodynamic calculations were carried out to explain phase formation, phase fraction and phase composition. The results are in good agreement with the experimentally determined values. These results will provide a profound basis to explain the mechanical performance of these materials. Furthermore, it will lay a good reference basis of comparison for the material after neutron irradiation.

Oxidation Property of a Fourth-Generation Powder Metallurgy FGH4108 Nickel-Based Superalloy

Isothermal oxidation kinetics of a fourth-generation powder metallurgy FGH4108 nickel-based superalloy is investigated at 800 °C to 1100 °C by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). At 800 °C and 900 °C, the oxidation kinetic curves of the FGH4108 superalloy follow parabolic law. At 1000 °C, the oxidation kinetic curve follows cubic law. At 1100 °C, the oxidation kinetic curve has two distinct parts: the first part follows a parabolic law, and the second one obeys a linear law. Cross-sectional morphologies and elemental distributions show that the oxide film consists of two parts at 800 °C: the outer layer is a continuous dense protective Cr2O3 oxide film, and the inner layer is a discontinuous Al2O3 oxide layer. At 900–1100 °C, the oxides consist of three layers: the outermost is the oxides of Cr2O3 and TiO2, the middle is a continuous oxide of Al2O3, and the innermost is dotted oxides of TiO2. The thickness of the inner TiO2 oxide layer increases with the increase of oxidation temperature. On this basis, the oxidation behavior of the FGH4108 superalloy at high temperatures is confirmed to be controlled by the diffusion of Cr, Al, Ti, and O. From the aspect of oxidation resistance, the long-term service temperature of the FGH4108 superalloy should not exceed 1000 °C.

Interaction between SOFCs interconnect Cr-free multicomponent spinel coating materials and chromia

Excellent Cr diffusion resistance and interface stability is essential for coatings applied on the surface of ferrite stainless steel interconnect for SOFCs. Four kinds of materials, the equimolar (MnFeCoNiCu)3O4 (Eq), non-equimolar (Mn0·272Fe0·272Co0·272Ni0·092Cu0.092)3O4 (Non-Eq), MnCo2O4 (MC) and the Quaternary (MnFeCoNi)3O4 (Quaternary), are combined with chromium oxide to form diffusion couples in order to study the interfacial reaction mechanism. The constitutes and morphologies of the samples are tested. Two different composition reaction layers are found in equimolar/Cr2O3 diffusion couple. A slight Cr inward diffusion is detected in Quaternary/Cr2O3 diffusion couple.

Corrosion behavior of a new alumina-forming duplex stainless steel with different surface treatment in supercritical carbon dioxide

The corrosion behavior of an alumina-forming duplex stainless steel with different surface treatment was investigated in supercritical carbon dioxide. The results showed that polished materials had higher weight gain than the ground ones. An unprotective multilayer oxide scale was formed above FCC phase, whereas protective and continuous Cr/Al-rich oxide layers were formed above BCC phase on polished materials. Similar protective Cr/Al-rich oxide layers were formed, regardless of the FCC or BCC phase on ground materials. Grinding leads to the faster diffusion of Cr and Al, which causes the easier formation of the continuous Cr/Al-rich oxide layers.

High-Temperature Oxidation Behaviors of 321 Steel with Y or Nb Micro-Alloying

The effects of Y or Nb addition on the oxidation behavior of 321 steel at high temperatures were investigated by scanning electron microscopy (SEM), energy spectroscopy (EDS) and X-ray diffractometer (XRD). At the same time, the oxidation mechanism and oxidation kinetics of rare earth Y or Nb addition are explored. The results show that temperature greatly influences the high-temperature oxidation resistance of the alloys, and the oxidation phenomenon of the alloy becomes more obvious as the temperature increases. Adding 0.5 wt.% Nb or 0.045 wt.% Y elements can effectively improve the oxidation resistance of 321 stainless steel at high temperatures. The addition of rare earth Y can promote the diffusion of Cr in the matrix, leading to increased Cr content in the oxide film and the eventual formation of a dense Cr2O3 film, which effectively hinders the continuation of the oxidation reaction. As a result of the Nb addition, the outward diffusion of Cr elements can be effectively inhibited, Cr and O ion bond can be strengthened, the oxidation rate can be reduced, the adhesion rate of oxide film can be increased, and the oxidation resistance of 321 stainless steel can be improved.

Selective oxidation of rare metal oxide insulation layers on particle substrates for optimizing the performance of FeSiCr-based soft magnetic composites

This study introduces an approach to fabricating soft magnetic composites (SMCs) that effectively address core loss while maintaining high magnetic conductivity. During the sintering process in a water vapor environment, the CeO2 insulation layer facilitates the selective oxidation of Si while simultaneously impeding the outward diffusion of Fe and Cr. Consequently, a CeO2·SiO2 composite insulation layer forms within the FeSiCr-based SMCs. The FeSiCr-based SMCs with the CeO2·SiO2 composite insulation layer achieve a saturation magnetization of 153.1 emu/g, which is 10.9% lower than that of FeSi3.7Cr4.5 compacts, and exhibits a deterioration rate lower than previously reported values. Moreover, the permeability of the FeSiCr-based SMCs remains highly stable, demonstrating outstanding insulation characteristics. The total core loss and core loss separation results further corroborate that incorporating a CeO2·SiO2 composite insulation layer leads to a more pronounced reduction in interparticle eddy current loss than using a CeO2 insulating layer alone. This performance enhancement can be attributed to the selective oxidation of a rare metal oxide insulation layer on a ferromagnetic metal particle substrate, a technique applied to SMCs for the first time. These findings hold significant implications for the design of high-performance SMCs in miniaturized and lightweight electromagnetic devices.

Influence mechanism of cerium addition on precipitation behaviour of super austenitic stainless steel S32654

The influence mechanisms of Ce addition on the precipitation behaviour of σ phase and nitrides were systematically investigated. The results demonstrated that the addition of Ce significantly promoted the nucleation and slightly inhibited the growth of intergranular and intragranular σ phases. Meanwhile, Ce addition also accelerated the precipitations of π phase and Cr2N. Ce promoted the nucleation of intergranular σ phase mainly through: (i) leading to a linear increase in the driving force for σ phase and Cr activity; (ii) resulting in apparent grain refinement and providing more favorable nucleation sites. Besides increasing driving force and Cr activity, Ce addition also promoted the nucleation of intragranular σ phase by: (i) leading to lattice distortion, which not only caused local element concentration, but also reduced σ/γ interface energy, jointly increasing nucleation sites; (ii) providing other nucleation sites (Ce-bearing inclusions) for σ phase; (iii) enhancing Cr and Mo diffusion coefficients, which accelerated the supply of elements required for nucleation. Furthermore, Ce addition enhanced the driving force for Cr2N, Cr activity as well as Cr and N diffusions, and it also promoted the formation of more favorable nitride nucleation sites around σ phases. These combined actions accelerated the precipitations of π phase and Cr2N.

The Optimized Homogenization Process of Cast 7Mo Super Austenitic Stainless Steel.

Super austenitic stainless steels are expected to replace expensive alloys in harsh environments due to their superior corrosion resistance and mechanical properties. However, the ultra-high alloy contents drive serious segregation in cast steels, where the σ phase is difficult to eliminate. In this study, the microstructural evolution of 7Mo super austenitic stainless steels under different homogenization methods was investigated. The results showed that after isothermal treatment for 30 h at 1250 °C, the σ phase in steels dissolved, while the remelting morphologies appeared at the phase boundaries. Therefore, the stepped solution heat treatment was further conducted to optimize the homogenized microstructure. The samples were heated up to 1220 °C, 1235 °C and 1250 °C with a slow heating rate, and held at these temperatures for 2 h, respectively. The elemental segregation was greatly reduced without incipient remelting and the σ phase was eventually reduced to less than 0.6%. A prolonged incubation below the dissolution temperature will lead to a spontaneous compositional adjustment of the eutectic σ phase, resulting in uphill diffusion of Cr and Mn, and reducing the homogenization efficiency of ISHT, which is avoided by SSHT. The hardness reduced from 228~236 Hv to 220~232 Hv by adopting the cooling process of "furnace cooling + water quench". In addition, the study noticed that increasing the Ce content or decreasing the Mn content can both refine the homogenized grain size and accelerate diffusion processes. This study provides a theoretical and experimental basis for the process and composition optimization of super austenitic stainless steels.

Effect of cold rolling reduction on the precipitation behavior of σ phase in 2906 duplex stainless steel

The effect of cold rolling reduction on the precipitation of σ phase at 800 °C in the 2906 DSS was investigated. The results show that the precipitation rate of the σ phase is improved with the increase of rolling reduction, and the maximum volume fraction is limited to 15%–20% due to the exhaust of ferrite. Improving the rolling reduction could promote the diffusion of Cr, Ni, Mo departing from ferrite and Ni, Mo departing from austenite during the following aging process. The contents of Cr, Ni and Mo increased suddenly and then keep stable in the σ phase, which preferentially nucleated around the narrow ferrite regions. The increase of cold rolling reduction is favorable for σ phase precipitation due to the promoted diffusion of Cr, Ni and Mo elements, as well as the decrease in thickness of ferrite/austenite lamellar interface. The σ phase/austenite interface is incoherent without orientation relationship between the σ phase and austenite, the orientation relationship between ferrite and austenite illustrates (02-2)γ//(-13-2)α and (33-1)γ//(-33-2)α, and the σ phase becomes coarsen and continuous along the ferrite/austenite interface.

Novel titanium matrix composites reinforced by NiCoCr medium entropy particles

Titanium matrix composites (TMCs) reinforced with 2–8.3 wt.% NiCoCr medium-entropy alloy particles (NiCoCrp/Ti) were synthesized through an in-situ reaction between NiCoCr and Ti powders during a spark plasma sintering process at 850 °C and 1000 °C. At 850 °C, there are obvious interdiffusion layers with Ti3(Ni, Co) intermetallic around NiCoCr particles. At 1000 °C, the sufficient diffusion of Ni, Co and Cr away from the prior NiCoCr particles results in clusters of granular intermetallic (Ti3(Ni, Co) and Ti2(Ni, Co)) in the Ti matrix. Besides, the addition of NiCoCr particles leads to a significant improvement in tensile strength and hardness. Ti-2wt.%NiCoCr sintered at 1000 °C has a combination of high tensile strength (788 MPa) and good ductility (23.4%). Furthermore, the mechanical properties are mainly correlated to the amount and distribution of the intermetallic compounds in TMCs.

Investigation of CuxMn3-xO4 coatings for solid oxide fuel cell interconnect applications

In order to investigate the performance of Cu–Mn spinel coatings used for solid oxide fuel cell interconnect, a series of CuxMn3-xO4 (x = 0.6, 0.8, 1.0, 1.1, 1.2, and 1.3) coatings are prepared using supersonic spraying via subsequent sintering. The chemical composition and distribution, lattice structures, morphology, and electrical properties are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and area-specific resistance (ASR) measurements. The experimental results show that with the increase of Cu concentration in the coatings, the crystalline structure gradually changes from inverse spinel into normal spinel, especially when 0.6 ≤ x ≤ 1.0. An improvement in electrical conductivity accompanies the crystalline structural transformation. The coatings show high electrical conductivity when 1.0 ≤ x ≤ 1.3, especially from 600 to 800 °C. However, the scale (mainly Cr2O3) is reactive to the CuxMn3-xO4 coatings (especially when 1.1 ≤ x ≤ 1.3) at elevated temperatures. During the long-term experiment, the reaction corrodes the coatings, consumes the materials in coatings and scale, and forms new phases (mainly (Cu,Mn,Cr)3O4 and (Cr,Mn)3O4). This phenomenon (Cr-corrosion) aggravates the Cr diffusion, causes a quick increase of ASR, and weakens the oxidation resistance. The adverse effects of Cr-corrosion can be alleviated by appropriately lowering the operating temperature. Among the coatings, Cu1.0Mn2.0O4 exhibited good electrical conductivity and soundness against the Cr-corrosion and can be considered a competitive candidate for SOFC applications.

Microstructural mechanism of local dynamic recrystallization around cavity during tertiary creep in directionally solidified superalloy CM247LC

This study investigated the mechanism of nucleation and growth of dynamic recrystallization (DRX) locally occurring around a cavity depending on the creep strain at 982 °C under 180 MPa in directionally solidified superalloy CM247LC. Misorientation increased when the creep strain reached 15%, and low-angle boundaries along rafted γ–γ’ around the cavity formed a high-angle boundary, which is assumed to be the initial nucleation mechanism of recrystallization. Further deformation increased the dislocation density in the recrystallized grains, resulting in post-DRX via fast dynamic recovery. The γ matrix was coarsened by the atomic diffusion of Co and Cr along the dislocation line in the γ’ phase of the recrystallized grains.

Effect of induction preheating on microstructure and mechanical properties of friction stir welded dissimilar material joints of Inconel 718 and SS316L

Using an external heat source during friction stir welding (FSW) of high melting point materials is a promising approach to enhance the material flow and reduce the axial load, improving the tool life. In this study, conventional and induction heating-assisted friction stir welding (I-FSW) in joining dissimilar Inconel 718, and stainless steel (SS316L) alloy using the WC-10%Co tool was experimentally investigated. The welding was performed at a constant rotational speed of 300 rpm with welding speeds of 70 and 140 mm/min and 300 °C preheating temperature in the assisted FSW. The results show that the application of friction stir welding resulted in highly refined grains with deformed carbide particles in the stir zone of both materials, which increased mechanical properties. An increase in temperature during the I-FSW process led to a slight increase in the size of the particles and high atomic diffusion of Cr and Mo elements at the interface of the SS316L and Inconel 718 joint, which showed enhancement of the weld strength. In addition, highly dragged SS316L material into the advancing side of the Inconel 718 was observed, which was directly influenced by the preheating temperature and welding speed. The mechanical properties of the joint with preheating significantly improved at a high welding speed (i.e., at 140 mm/min). The tensile strength of the I-FSW joint at 140 mm/min was 683 MPa which was 99.8% of the base SS316L. Dynamic recrystallization resulted in grain refinement, which raised the hardness value. The results also show that preheating reduces downward axial force, enhancing the tool life.

Exploring the effects of Mn content in the Mn-Co spinel coating on its formation and slowing the outward Cr diffusion

Effects of Mn content on the performance of Mn-Co spinel coatings are clarified systematically. Two dense spinel coatings are fabricated on Crofer 22 H by microwave sintering and subsequent oxidation. After oxidation at 800 ℃ in air for 400 h, the thickness of the Cr-containing layer of MnCo2O4-coated sample is significantly lower than that of (Mn, Co)3O4-coated sample. The growth mechanism of Cr-containing layer is elucidated from aspects of stabilizing energy and configurational entropy. Controlling Mn3+ content reduces the outward diffusion channels for Cr and avoids the excessive growth of the Cr-containing layer, achieving a lower resistance as well.

The influence of preparation of nano-ZrO2/α-Al2O3 gradient coating on the corrosion resistance of 316L stainless steel substrate

Abstract Generally, 316L stainless steel instrumentation tubes working in a humid environment with a large amount of Cl− all the year round have serious corrosion problems, so the stainless steel substrate should be gradiently coated with nano-ZrO2/α-Al2O3 slurry. In this article, the slender 316L stainless steel tube was first ground by magnetorheological fluid and then coated with the slurry, which can not only increase the contact area between the coating and the substrate but also prevent the generation of new substances that have adversely affected the adhesion of the coating. The properties of the samples were characterized and analyzed; the results showed that the substrate ground by magnetorheological fluid is more favorable for bonding with coating under the grinding conditions that the speed of the tube is 210 rpm, magnetic induction intensity is 40.83 mT, and mass ratio of micron and submicron magnetic particles is 2.3. The coating prepared under the above conditions has uniform thickness, flat surface, and can better inhibit the diffusion of Cr of the substrate to its surface. It can be obtained from corrosion resistance analysis that the coating has the highest self-corrosion potential of −0.016 V and the lowest corrosion current density of 0.491 μA/cm2, which indicate that the coating has the strongest corrosion resistance. According to the composition analysis of the coating, the composition of the corroded coating is similar to that of the coating itself, but accompanied by a small amount of Fe, which indirectly indicates that the coating is relatively compact, the coating is well bonded with the substrate, and the coating can protect the substrate; thus, the service life of 316L stainless steel instrumentation tubes is extended.

Effect of microstructure on hot corrosion behavior of new superalloy Ad 730 in 75 wt% Na2SO4 + 25 wt% NaCl molten salt

The new superalloy Ad 730 samples of three different microstructures were prepared by heat treatment and then exposed to a 75 wt% Na2SO4 + 25 wt% NaCl salt at 973 K for 30 h. Compared with the samples with larger grain sizes and fewer γ’ phases, the initial sample (HT) had a denser oxide layer and suffered less hot corrosion damage. The fine-grained structure in HT accelerates the diffusion of Cr to form protective oxides and provides more oxide nucleation sites. The more γ’ phases in HT inhibit the diffusion of hot corrosion.