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The CoCrNiMox (x = 0, 0.1, and 0.2 in molar ratio) medium entropy alloys (MEAs) were fabricated by vacuum arc melting, followed by cold rolling and annealing treatments. The X-ray diffraction (XRD), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM) were employed to characterize the microstructures. It has been shown that the CoCrNi MEA has a single FCC phase and the Mo-containing MEAs contain (Cr, Mo)-rich σ precipitates. In addition, the Mo addition caused significant grain refinement, due to the fact that the presence of σ phase exerts a strong pinning effect on the grain boundary migration. The hardness testing results indicate an increment in Vickers hardness from 187.5 ± 4.5 Hv of CoCrNi alloy to 309.5 ± 10.3 Hv of CoCrNiMo0.2 alloy. The yield strength and ultimate tensile strength also increase from 339 ± 2 to 644 ± 5 MPa and from 810 ± 5 to 1071 ± 17 MPa, respectively, but the elongation drops from 88.4 ± 4.0% to 29.5 ± 7.6%. The grain refinement and the precipitation of σ phase make synergistic contribution to the reinforcement of Mo-containing CoCrNi-based MEAs. The details and explanations in this study may guide the future design and research of the CoCrNi-based quaternary alloys with enhanced properties.

CoCrFeNiAl x Ti y high-entropy alloys were produced by the induction melting method and their oxidation behavior investigated when exposed to 1000°C for different durations. One or more body-centered cubic phases were found in all alloys, except CoCrFeNiTi 0.5 . In the CoCrFeNiTi 0.5 alloy, two different face-centered cubic phases and one tetragonal sigma phase were detected. Scanning electron microscopy elemental analysis showed that all the alloys exhibited homogeneous microstructure. Energy-dispersive x-ray spectroscopy analysis revealed that Cr and Fe elements were enriched in one phase and Al-Ni-Ti elements in another. The presence of Ti negatively affected the oxidation behavior. According to the oxidation test results, dominant Al 2 O 3 formation was observed in the CoCrFeNiAl 0.5 and CoCrFeNiAlTi 0.5 alloys. As a result, these two alloys exhibited the best performance among the five high-entropy alloys in terms of mass gain and oxide thickness.

Many engineering structural parts, such as internal gear pumps, worked in high-salt and high-temperature environments. The material (40Cr) of the parts in the pump failed due to severe wear and corrosion. To improve the wear resistance and corrosion resistance of 40Cr, the FeCoNiCrMox (x = 0, 0.3, 0.6, 0.9, 1.2) HEAs coatings were deposited on the surface of 40Cr by laser cladding technology, and the phase structure, microstructure and hardness of the coatings were investigated. In addition, friction and wear tests, electrochemical tests and neutral salt spray tests were conducted to investigate the effect of Mo content on the wear resistance and corrosion resistance of the coatings. The results showed that the phase structure of the FeCoNiCrMo0 coating was the FCC phase, and the phase of the FeCoNiCrMox (x = 0.3, 0.6, 0.9, 1.2) coatings were composed of the FCC phase and σ phase. The Mo element was enriched in interdendrite. With the increasing content of Mo elements, solid solution strengthening, fine grain strengthening, and second phase strengthening occurred in the coatings, which led to the increased hardness and wear resistance of the coatings. The FeCoNiCrMo1.2 coating had the best wear resistance. In the 3.5%NaCl solution, the corrosion resistance of the coatings was better than that of the substrate. With the increase of Mo content, the corrosion potential of the coatings drifted positively, the corrosion current density decreased, and the corrosion resistance increased. However, with excessive Mo element content, the σ phase will precipitate, leading to galvanic coupling corrosion of the coatings and reducing the corrosion resistance. Because the FeCoNiCrMo0.9 coating had the highest corrosion potential and the lowest corrosion current density. Therefore, it had the best corrosion resistance. The neutral salt spray test result also showed that the corrosion resistance of the FeCoNiCrMo0.9 coating was the best.

In this paper, the chemical composition of 7Mo-alloyed super austenitic stainless steel was optimized and the principle of composition designing of Fe-24Cr-(22-x)Ni-7Mo-xCo (x = 0, 2, 4) alloys was discussed. Thermodynamic calculation, as well as microstructure observation, was carried out to investigate the precipitation behavior of Fe-24Cr-(22-x)Ni-7Mo-xCo alloys aged at 700–1100 °C. The results demonstrated that none of second phase was detected at 700 °C aging and at least three different types of precipitates, σ phase, χ phase and M23C6 were identified after aging at 800–1100 °C for 2 h. At all aging temperature, the intermetallic σ was determined as the dominant second phase. For alloys of 22Ni, 20Ni2Co and 18Ni4Co, the area fraction of precipitates increased first and then decreased with temperature increasing from 700 to 1100 °C. The 1000 °C was determined as the most sensitive temperature for this alloy. Furthermore, the area fractions of second phase of 20Ni2Co (x = 2) at all temperatures were lower than that of 22Ni (x = 0) and 18Ni4Co (x = 4). In addition, the characteristics of cellular precipitates growing have been disclosed in this paper. Compared with nickel-based alloy, the addition of cobalt did not affect the misfit between σ phase and austenite matrix, which is probably due to the high content of nitrogen in Fe-24Cr-(22-x)Ni-7Mo-xCo alloys.

In this study, the alloying and phase separation behaviors of AlCoCrFeNi-based high-entropy alloys (HEAs) were investigated. The valence electron concentration (VEC) of the AlCoCrFeNi HEA was modified by adding specific elements (Mg, Ti, Mn, Cu, and Zn) to produce biphasic HEAs. These HEAs were prepared by mechanical alloying for 30 hours, followed by the consolidation of the powders at 1000 °C. The results demonstrated the formation of a body-centered cubic (ordered BCC/B2) phase in AlCoCrFeNi–Mg and AlCoCrFeNi–Ti, while a dual-phase face-centered cubic (FCC) phase and minor BCC phases were observed in AlCoCrFeNi–Cu and AlCoCrFeNi–Zn. AlCoCrFeNi and AlCoCrFeNi–Mn exhibited the precipitation of a σ phase in the BCC matrix and a minor FCC phase. The AlCoCrFeNi–Mn HEA exhibited the highest compressive strength among itself, AlCoCrFeNi–Cu, and AlCoCrFeNi–Zn HEAs, owing to the precipitation of a harder σ phase and a higher ordered BCC/B2 fraction. In addition, the AlCoCrFeNi–Cu and AlCoCrFeNi–Zn HEAs exhibited the maximum fracture strain and absorption energies. We propose that a controlled VEC approach by the addition of suitable elements can be used to tailor the microstructure and phase stability of AlCoCrFeNi HEAs.

The oxidation structure, microstructure and mechanical properties of Super304H and HR3C are investigated after running at 630 °C/3 MPa for 40,000 h. After service, a duplex oxide is formed on the steam side of heat-resistant steel, and it is possible that the surface treatment has caused surface deformation of Super304H, leading to an increase in corrosion resistance. For Super304H, a chain-like distribution of M 23 C 6 has been formed at the grain boundaries, causing creep cavities, NbCrN and ɛ -Cu particles have precipitated inside the grains. For HR3C, there is a significant aggregation of Cr at the grain boundaries, with only a small amount of M 23 C 6 and σ phase, and a large number of nanoscale NbCrN particles appear within the grains. The stable and dispersed precipitates inside the grains hinder the movement of dislocations, allowing heat-resistant steel to maintain higher strength than its as-received condition after long-term service. The microstructure evolution at grain boundaries leads to a decrease in toughness and plasticity of heat-resistant steel, and the continuous evaporation of Cr in high-temperature steam will cause a gradual decrease in Cr concentration and a deterioration in oxidation resistance of heat-resistant steel.

The effects of Ti and Si addition on the phase equilibrium and mechanical properties of the equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) were investigated. The phase equilibrium at 1000 °C was determined from the result of X-ray diffraction and electron probe micro-analysis. Ti addition stabilizes the σ phase, A12 phase and C14-Laves phase, while Si addition stabilizes the A13 phase. The phase relationships were represented by projection onto (Co, Fe, Mn, Ni)–Cr–X(Ti or Si) isothermal ternary cross-section at 1000 °C in Co–Cr–Fe–Mn–Ni–X senary system. Tensile tests were conducted on Cantor-based fcc single solid solution alloys with Ti or Si dissolution at room temperature. The 0.2% yield strength and ultimate tensile strength increased with either element addition. The Ti-added alloy showed higher strength than the Si-added alloy. The difference in ductility in the alloys is related to their strain hardening behavior in the higher strain range. Graphic abstract

In this work, the effect of heat input and aging treatment on the microstructural characteristics and mechanical properties of similar AISI 317L austenitic stainless steel weldments used in the petroleum industry was investigated. The filler metal used was the AWS ER317L electrode at two different heat input levels (4 and 8 kJ/cm) in order to verify the influence of this parameter on the precipitation of deleterious phases. The specimens were aged at 700 °C for 50 and 100 h. Quantification and microchemical mapping of precipitated phases after welding and aging thermal treatment (ATT) were performed. Vickers hardness and tensile tests were used to evaluate the mechanical properties. It was observed that aging promoted a refinement of the base metal region, and all delta ferrite was transformed into sigma phase. The delta ferrite present in the fusion zone was completely transformed into sigma and chi phases. In the aged specimens for 100 h a lower occurrence of the secondary austenite phase (γ2) was identified, which indicates that with the increase of ATT time the dissolution of γ2 occurred in the already precipitated sigma phase. All welding conditions showed an increase in tensile strength, yield limit and hardness with the ATT.

Stainless steel, known for its excellent corrosion resistance, is widely used in various fields such as energy, chemical processing, and marine engineering. However, under certain environmental conditions—especially high temperature and high chloride environments—intergranular corrosion (IGC) remains one of the primary causes of stainless steel failure, posing serious safety risks in critical applications. This paper reviews recent advances in the study of IGC in stainless steels. It first introduces classical mechanisms such as the chromium depletion effect, and then discusses newly proposed mechanisms including sigma (σ) phase precipitation and impurity element segregation. Subsequently, the article summarizes detection methods for IGC, with a focus on techniques such as electrochemical potentiokinetic reactivation (EPR) and localized electrochemical methods (SVET/LEIS), which offer improved precision and broader applicability. Finally, strategies for enhancing IGC resistance are discussed, including alloy composition optimization, heat treatment processes, and data-driven service life prediction models. Future research directions include the development of innovative testing technologies and advanced corrosion-resistant stainless steel design to address challenges under complex service conditions.

Purpose For the purpose of saving nickel, this study aims to develop new duplex stainless steel cored wires suitable for wire arc additive manufacturing (WAAM) with the addition of nitrogen. Design/methodology/approach The effect of nitrogen content on the microstructure and mechanical properties of the thin-walled deposits is investigated in detail. Findings The microstructure of thin-walled deposits mainly consists of austenite, ferrite and secondary austenite. With increasing nitrogen content, the austenite in the deposited metals increases. The austenite proportion in the bottom region is more than that in the top region of the deposited metals. The χ phase is randomly distributed at the grain boundaries and within ferrite. The σ phase is mainly precipitated at ferrite and austenite grain boundaries. With increasing nitrogen content, the tensile strength of the deposited metals increases, but the impact toughness of the deposited metals deteriorates. Originality/value This study proposes new duplex stainless steel cored wires for WAAM, which realizes the objective of saving nickel.