Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.     

Engineering On‐Surface Spin Crossover: Spin‐State Switching in a Self‐Assembled Film of Vacuum‐Sublimable Functional Molecule

The realization of spin‐crossover (SCO)‐based applications requires study of the spin‐state switching characteristics of SCO complex molecules within nanostructured environments, especially on surfaces. Except for a very few cases, the SCO of a surface‐bound thin molecular film is either quenched or heavily altered due to: (i) molecule–surface interactions and (ii) differing intermolecular interactions in films relative to the bulk. By fabricating SCO complexes on a weakly interacting surface, the interfacial quenching problem is tackled. However, engineering intermolecular interactions in thin SCO active films is rather difficult. Here, a molecular self‐assembly strategy is proposed to fabricate thin spin‐switchable surface‐bound films with programmable intermolecular interactions. Molecular engineering of the parent complex system [Fe(H₂B(pz)₂)₂(bpy)] (pz = pyrazole, bpy = 2,2′‐bipyridine) with a dodecyl (C₁₂) alkyl chain yields a classical amphiphile‐like functional and vacuum‐sublimable charge‐neutral Fe^II complex, [Fe(H₂B(pz)₂)₂(C₁₂‐bpy)] (C₁₂‐bpy = dodecyl[2,2′‐bipyridine]‐5‐carboxylate). Both the bulk powder and 10 nm thin films sublimed onto either quartz glass or SiO_x surfaces of the complex show comparable spin‐state switching characteristics mediated by similar lamellar bilayer like self‐assembly/molecular interactions. This unprecedented observation augurs well for the development of SCO‐based applications, especially in molecular spintronics.     

Unraveling the origin of strain-induced precipitation of M23C6 in the plastically deformed 347 Austenite stainless steel

In this research, the phenomenon of strain-induced precipitation of M₂₃ (M = Cr, Fe, Mo) C₆ precipitates in 347 austenite stainless steel was systematically studied. During annealing at 650 °C, the experimental results exhibited significantly earlier formation of M₂₃C₆ precipitates in the plastically strained specimen, than in the no-strain specimen. The various microstructural characterizations showed that the accelerated formation of M₂₃C₆ precipitates occurred around deformation bands that were generated by deformation twinning. Extensive investigation suggested that the strain-induced precipitation of M₂₃C₆ is attributed to the formation of strain-induced α′ martensite during plastic deformation near deformation bands.     

Abrasive wear of FeCr (M7C3–M23C6) reinforced iron based metal matrix composites

Abstract

The effects of volume fraction, particle size, and sintered porosity of FeCr (M₇C₃–M₂₃C₆) particulates on the abrasive wear resistance of powder metallurgy (PM) Fe alloy metal matrix composites have been studied under different abrasive conditions. It was seen that the abrasive wear rate of the composites increased with an increase in the FeCr volume fraction in tests performed with 80 grade SiC abrasive paper, but it decreased for tests conducted with 220 grade SiC abrasive paper. Furthermore, the wear rates decreased with an increase in FeCr size for composites containing the same amount of FeCr. Hence it is deduced that Fe alloy composites reinforced with larger size FeCr particles are more effective against abrasive wear than those reinforced with smaller ones. At the same time the results show that the beneficial effects of hard FeCr particulates on wear resistance far outweighed the detrimental effects of sintered porosity in the PM metal matrix composites. In addition, the fabrication of composites containing soft particles such as graphite or copper favours a reduction in the coefficient of friction, and increases the matrix hardness of the composite. For this reason graphite and copper were used in the matrix in different amounts to test their effect on the wear resistance. Increase in graphite and copper volume fraction allowed the formation of additional phases, which had high hardness and wear resistance. It was also found that the wear rate of the composites decreased considerably with graphite and copper addition.     

On precipitation in an Fe-Cr-Ni-Mn-Mo-W-V-Nb-N alloy,following prolonged ageing at 650°C

The main existing intergranular phase in the alloy observed is thick lamellar and small plate-like M₆C. Owing to the presence of M₆C precipitates, intergranular M₂₃C₆ nucleates at these locations and coherently precipitates to form a thin lamellar and large-sized filmy single crystal. The misfit of M₂₃C₆ with M₆C obtained from the moiré pattern is 3%. Intragranular M₆C and M₂₃C₆ twin along their own planes (111), and the resulting twins keep a coherent relationship with austenite. The misfit of VC precipitated along dislocations with the matrix is smaller than that of dispersed VC with the matrix. Intragranular Laves phase together with intragranular M₆C and VC strengthens the alloy grains.     

A Single‐Molecule Propyne Trap: Highly Efficient Removal of Propyne from Propylene with Anion‐Pillared Ultramicroporous Materials

Propyne/propylene (C₃H₄/C₃H₆) separation is a critical process for the production of polymer‐grade C₃H₆. However, optimization of the structure of porous materials for the highly efficient removal of C₃H₄ from C₃H₆ remains challenging due to their similar structures and ultralow C₃H₄ concentration. Here, it is first reported that hybrid ultramicroporous materials with pillared inorganic anions (SiF₆^2? = SIFSIX, NbOF₅^2? = NbOFFIVE) can serve as highly selective C₃H₄ traps for the removal of trace C₃H₄ from C₃H₆. Especially, it is revealed that the pyrazine‐based ultramicroporous material with square grid structure for which the pore shape and functional site disposition can be varied in 0.1–0.5 Å scale to match both the shape and interacting sites of guest molecule is an interesting single‐molecule trap for C₃H₄ molecule. The pyrazine‐based single‐molecule trap enables extremely high C₃H₄ uptake under ultralow concentration (2.65 mmol g^?1 at 3000 ppm, one C₃H₄ per unit cell) and record selectivity over C₃H₆ at 298 K (>250). The single‐molecule binding mode for C₃H₄ within ultramicroporous material is validated by X‐ray diffraction experiments and modeling studies. The breakthrough experiments confirm that anion‐pillared ultramicroporous materials set new benchmarks for the removal of ultralow concentration C₃H₄ (1000 ppm on SIFSIX‐3‐Ni, and 10 000 ppm on SIFSIX‐2‐Cu‐i) from C₃H₆.     

A study of microstructure and performance of cast Fe–8Cr–2B alloy

The effects of quenching temperature on microstructure and hardness of cast Fe–8Cr–2B alloy containing 0.3 wt% C, 2.0 wt% B, 8.0 wt% Cr, 0.6 wt% Si, and 0.8 wt% Mn were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), Rockwell hardness and Vickers microhardness testers. The experimental results indicate that the as‐cast microstructure of cast Fe–8Cr–2B alloy consists of M₂B (M = Fe, Cr), M₇(C, B)₃, α‐Fe, and γ‐Fe. The dendritic matrix composed of lath martensite mixed with a small amount of retained austenite, and the netlike boride M₂B distribute in the grain boundary. After quenching between 950 °C and 1100 °C, the netlike eutectic boride are broken up and a new phase‐M₂₃(C, B)₆ which is distributed in the shape of sphere or short rod‐like are precipitated from the matrix. Both the macrohardness and microhardness of specimens increase with the increasing quenching temperature. At about 1050 °C, the hardness reaches the maximum value. However, when the temperature exceeds 1050 °C, the hardness will decrease slightly. With the increase of tempering temperature, the hardness of cast Fe–8Cr–2B alloy quenching from 1050 °C decreases gradually and its impact toughness increases slightly. Crusher hammer made of cast Fe–8Cr–2B alloy quenching from 1050 °C and tempering from 300 °C has good application effect, and its service life improves by 150–180% than that of high manganese steel hammer.     

Microstructural characterization of a carbonitrided heat resisting alloy using focused ion beam-based techniques

Abstract

An austenitic Fe–25Cr–20Ni (wt.%) alloy was first nitrided in N₂–H₂ then carburised at 1000°C. The nitridation produced lamellar internal precipitates of Cr₂N which grew by discontinuous precipitation at a recrystallised austenite boundary formed at the precipitation front. Subsequent carburisation converted the Cr₂N to a mixture of chromium-rich M₇C₃+CrN, and released nitrogen into the austenite matrix. The submicron CrN precipitates were stabilized by a matrix supersaturated with nitrogen and exhibited a strong orientation relationship with the surrounding austenite. The rejected nitrogen diffused deeper into the alloy to form new, more finely spaced Cr₂N lamellae. Carbon diffusion overtook the nitride structure and precipitated finely spaced, lamellar M₂₃C₆ by a discontinuous precipitation process. This process is available to the carbides only when a prior boundary is present. This phenomenon, and the ability of nitridation to form a boundary and the inability of carburisation to do so, are interpreted using the energetics of nucleation.     

Effect of quenching on microstructure and wear‐resistance of Fe–10Cr–1.5B–2Al Alloy

Cast Fe–10Cr–1.5B–2Al alloy was quenched at different temperatures. The effects of quenching temperature on microstructure and hardness and wear‐resistance of Fe–10Cr–1.5B–2.0Al alloy were investigated by means of the optical microscopy, the scanning electron microscope, X‐ray diffraction, energy dispersive spectrometer, Vickers hardness and Rockwell hardness tester, and the MM‐200 block‐on‐ring wear testing machine under dry friction condition. The results indicate that the as‐cast microstructure of Fe–10Cr–1.5B–2.0Al alloy consists of ferrite, pearlite and netlike eutectics which are distributed in the grain boundary. The eutectics mainly include herringbone M₂B and chrysanthemum M₇(C, B)₃. The matrix gradually turns into single martensite with the increase of the quenching temperature. The type of borocarbides has no obvious change after quenching. The netlike boride almost totally fractures and transforms from the fish‐bone structure to the graininess. There is some retained austenite in the quenched structures when the quenching temperature is more than 1100 °C. When the quenching temperature is in a range of 1000 °C to 1100 °C, the hardness and wear resistance show a sharp increase with an increase of temperature, and show a slight decrease after surpassing 1100 °C.     

Record‐High Superconductivity in Niobium–Titanium Alloy

The extraordinary superconductivity has been observed in a pressurized commercial niobium–titanium alloy. Its zero‐resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record‐high pressure up to which a known superconducting state can continuously survive. Remarkably, at such an ultra‐high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (T_C) increases to ≈19.1 K from ≈9.6 K, and the critical magnetic field (H_C2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both the T_C and the H_C2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high‐pressure superconducting properties observed in the niobium–titanium alloy not only expand the knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.     

Time dependence at 600 and 650° C of M23C6 precipitate composition in AlSl 304 stainless steel

The composition of M₂₃C₆ precipitates in thermally treated AlSl 304 stainless steel was investigated as a function of ageing time at T=600 and 650° C using X-ray fluorescence induced by electron bombardment. With increasing ageing time, it appears that the M₂₃C₆ precipitates become progressively enriched in Cr and depleted in Fe. However, a tendency towards saturation appears, the Cr content being higher at T=650° C.     

Formation of Re-containing Carbides in a Second Generation Directionally Solidi¯ed Ni Base Superalloy

The precipitates at grain boundary in a directionally solidified Ni base superalloy after heat treatment, aging at 975°C, and creep rupture test have been characterized. Besides the primary MC carbides and fine particles of μ phase, the Re-containing M₂₃C₆ was observed. The precipitation kinetics revealed that the formation of M₂₃C₆ was associated with the dissolution of μ phase and MC carbides. TEM image shows that the continuous precipitation of M₂₃C₆ particles effectively hinders the dislocation movement and strengthens the grain boundaries. The high strength of the alloy suggests that M₂₃C₆ carbides are beneficial to the properties although Re as an important matrix strengthening element was consumed.     

Time dependence at 550 and 700°C of M23C6 precipitate composition in AISI 304 stainless steel

The time dependence at 550 and 700°C of M₂₃C₆ precipitate composition in AISI 304 stainless steel has been investigated using the X-ray fluorescence induced by electron bombardment. By increasing the ageing time, M₂₃C₆ precipitates become enriched in Cr and depleted in Fe.     

Effect of heat treatment on microstructure and property of Fe‐Cr‐B alloy

In this article, the effect of heat treatment in different quenching temperature on microstructure and hardness of Fe‐Cr‐B alloy was studied, by contrast with boron‐free Fe‐Cr alloy. The results indicated that microstructure of boron‐free Fe‐Cr alloy consisted of the martensite and a few (Cr, Fe)₇C₃ type carbide. The microstructures had no obvious change with the increase of quenching temperature, but its hardness increased from 51.5 HRC to 60.8 HRC. When boron element was added into the Fe‐Cr alloy, the netlike eutectic structure began to break and spheroidizing after quenching, in which the borocarbide turned into spherical groups and network Fe₂B phase was broken. Moreover, the portion of martensite increased, and the amount of secondary carbide decreased, and the size of secondary carbide began to largen after quenching. When the quenching temperature reached 1100°C, secondary carbide particles dissolved in the matrix wholly. The hardness of Fe‐Cr‐B alloy increased with the increase of quenching temperature below 1050°C. The hardness of sample containing 2.0% B and quenching at 1050°C reached 66.7 HRC. The hardness of Fe‐Cr‐B alloy had no obvious change when quenching temperature continued to increase. After tempered at 200°C, the microstructure of Fe‐Cr‐B alloy had no significant change and its hardness had slight decrease. The hardness of sample containing 2.0% B tempered at 200°C reached 63.9 HRC.     

A study of heat treatment of high‐boron high‐speed steel roll

High‐boron high‐speed steel (HSS) is a cheap roll material. In the paper, the authors research the effect of heat treatment on the microstructure and properties of high‐boron high‐speed steel HSS roll containing 0.54% C, 1.96% B, 3.82% W, 7.06% Mo, 5.23% Cr and 2.62% Al by means of the optical microscopy (OM), the scanning electron microscopy (SEM), X‐ray diffraction (XRD) and hardness test. The results showed that as‐cast structure of boron‐bearing high‐speed steel HSS consisted of martensite, pearlite, M₂(B, C), M₃(B, C) and M₂₃(B, C)₆ type borocarbides. After quenching, the matrix transformed into the lath martensite, and M₃(B, C) dissolved into the matrix. When quenching temperature is lower than 1050°C, the hardness is increased with the increase of quenching temperature under oil cooling, while quenching temperature excels 1100°C, the hardness will decrease with the increase of quenching temperature. Under the condition of salt bath and air cooling, the effect of quenching temperature on the hardness is similar to the above law, but the quenching temperature obtaining the highest hardness is higher than that of oil cooling. The highest hardness is obtained while tempering at 525°C. The hardness of high‐boron high‐speed steel HSS roll is 66.5 HRC, and its impact toughness excels 13.1 J/cm². Using in pre‐finishing stands of high‐speed hot wire‐rod rolling mill, the wear rate of high‐boron HSS rolls is 0.26 mm/one thousand tons steel. However the manufacturing cost of high‐boron HSS rolls is obviously lower than that of powder metallurgy hard alloy rolls, it is only 28% of that of powder metallurgy (PM) hard alloy rolls.     

Locking and Unlocking the Molecular Spin Crossover Transition

The Fe(II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)] (pz = pyrazol‐1‐yl, bipy = 2,2′‐bipyridine) can be locked in a largely low‐spin‐state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO₂ and Al₂O₃, or in powder samples by mixing with the strongly dipolar zwitterionic p ‐benzoquinonemonoimine C₆H₂(—? NH₂)₂(—? O)₂. Remarkably, it is found in both cases that incident X‐ray fluences then restore the [Fe{H₂B(pz)₂}₂(bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light‐ or X‐ray‐induced excited‐spin‐state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.     

Effect of aluminum content on solidification microstructure and properties of Fe‐Cr‐B‐Al alloys

The microstructures and mechanical properties of eight kinds of Fe‐Cr‐B‐Al alloys containing X wt.%Al‐0.35 wt.%C‐10.0 wt.%Cr‐1.4 wt.%B‐0.6 wt.%Si‐0.8 wt.%Mn (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0) were studied by means of optical microscopy (OM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), Rockwell hardness and Vickers micro‐hardness testers. The results indicate that the as‐cast microstructure of aluminium‐free sample consists of the martensite, austenite and eutectic borocarbides, and the eutectic borocarbides are the mixture of (Fe, Cr)₂B and (Cr, Fe)₇(C, B)₃, and its hardness reaches 65 HRC. When a small amount of aluminium element (Al ? 1.0 wt.%) is added, the phase composition has no significant change, and the hardness excels 65 HRC. When the concentration of aluminium reaches 1.5 wt.%, the matrix of Fe‐Cr‐B‐Al alloy becomes pearlite and δ‐ferrite, leading to a sharply decrease of the hardness. The proportion of ferrite goes up along with increasing aluminium concentration, and the hardness of Fe‐Cr‐B‐Al alloy has slight decrease.     

Room‐Temperature Ferromagnetic Insulating State in Cation‐Ordered Double‐Perovskite Sr2Fe1+xRe1−xO6Films

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room‐temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B‐site cations in the double perovskite Sr₂Fe_1+xRe_1‐xO₆ (?0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr₂FeReO₆, an FMI state with a high Curie temperature (T_c ≈ 400 K) and a large saturation magnetization (M_S ≈ 1.8 µ_B f.u.^?1) is found in highly cation‐ordered Fe‐rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe³⁺? Fe³⁺ and Fe³⁺? Re⁶⁺ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices.     

Corrosion Resistance Enhancement of Selective Laser Melted Cost-Effective Interstitial Medium-Entropy Alloy via Deep Cryogenic Treatment

An economically interstitial FeMnCrNiCu medium-entropy alloy (MEA), which exhibits preferable corrosion resistance in 3.5 wt% NaCl solution, is fabricated by selective laser melting (SLM). Compared with annealing, which is one of the posttreatments for additive manufacturing, deep cryogenic treatment (DCT) also contributes significantly to the alloy properties and is expected to improve corrosion resistance. Herein, the corrosion behavior of SLM-MEA after DCT in 3.5 wt% NaCl solution is investigated. DCT improves corrosion resistance by promoting the uniform distribution of precipitates in SLM-MEA and changing the cation ratio in the passive film. After DCT, the corrosion current density of the SLM-MEA in original and annealed state decreases by 27.07% and 62.69%, respectively. The annealed SLM-MEA exhibits the worst corrosion resistance as a result of the precipitation of continuous Cr-rich M₂₃C₆ carbides at grain boundaries. The face-centered cubic structure of MEA does not change after annealing or DCT. Significantly, the number, size, and distribution of the precipitates are greatly affected. DCT after annealing promotes the uniform diffuse distribution of M₂₃C₆ carbides and nanoscale Cu-rich particles while increasing the Cr+Ni/Fe+Mn ratio in the passive film, resulting in a significant improvement in corrosion resistance.     

Reductive Transformation of Layered‐Double‐Hydroxide Nanosheets to Fe‐Based Heterostructures for Efficient Visible‐Light Photocatalytic Hydrogenation of CO

Conversion of syngas (CO, H₂) to hydrocarbons, commonly known as the Fischer–Tropsch (FT) synthesis, represents a fundamental pillar in today's chemical industry and is typically carried out under technically demanding conditions (1–3 MPa, 300–400 °C). Photocatalysis using sunlight offers an alternative and potentially more sustainable approach for the transformation of small molecules (H₂O, CO, CO₂, N₂, etc.) to high‐valuable products, including hydrocarbons. Herein, a novel series of Fe‐based heterostructured photocatalysts (Fe‐x) is successfully fabricated via H₂ reduction of ZnFeAl‐layered double hydroxide (LDH) nanosheets at temperatures (x) in the range 300–650 °C. At a reduction temperature of 500 °C, the heterostructured photocatalyst formed (Fe‐500) consists of Fe⁰ and FeO_x nanoparticles supported by ZnO and amorphous Al₂O₃. Fe‐500 demonstrates remarkable CO hydrogenation performance with very high initial selectivities toward hydrocarbons (89%) and especially light olefins (42%), and a very low selectivity towards CO₂ (11%). The intimate and abundant interfacial contacts between metallic Fe⁰ and FeO_x in the Fe‐500 photocatalyst underpins its outstanding photocatalytic performance. The photocatalytic production of high‐value light olefins with suppressed CO₂ selectivity from CO hydrogenation is demonstrated here.