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•Zr-rich IMCs were induced in Fe-36Ni Invar alloy with the addition of Zr.•There were fine grain and high dislocation density in the Zr-treated alloy.•Strength better than Zr-free ...alloy was obtained by Zr treatment.•Zr treatment modified the coefficient of thermal expansion in Fe-36Ni Invar alloy.
Nowadays, Invar alloy combining low thermal expansion behavior with improved mechanical properties is highly required. This study investigated the influence of Zr-rich intermetallic compounds (IMCs) on microstructure, thermal expansion behavior, and mechanical properties in Fe-36Ni Invar alloy. Zr-rich IMCs, namely Ni7Zr2 and Ni2Zr were introduced into Fe-36Ni Invar alloy by the addition of 0.22 wt% Zr. The microstructure of Zr-containing alloy was remarkably refined and accompanied by high dislocation density, which was associated with the resistance of recrystallization by the introduction of Zr-rich IMCs. The Invar alloy displayed a sufficiently low coefficient of thermal expansion (CTE) in the temperature interval of 20 °C ∼ 100 °C and 20 °C ∼ 200 °C after Zr treatment, with a CTE constant of 0.66 × 10-6 /°C and 2.07 × 10-6 /°C, respectively. The existence of residual stress and refinement of grain were responsible for an obvious decrease in CTE. Meanwhile, the yield strength increased from 256 MPa to 401 MPa after Zr treatment. The strengthening contribution mainly arose from dislocation strengthening and grain refinement strengthening.
In this study, the evolution of statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) during tensile tests was investigated in CoCrFeMnNi high-entropy alloys (HEAs) ...with 0 and 0.52 at. % nitrogen. The microstructure characterization indicated that the plastic deformation of the alloys was dominated by dislocation slip below 20% true strain. Nitrogen alloying increased the total dislocation density and the strain-hardening rate during deformation, which could be explained by the interaction between nitrogen atoms and dislocations. The higher fraction of soft orientation zones for 0.52 N HEA was conducive for strain accommodation, resulting in a reduced GND density. Hence, SSDs contributed primarily to the increase of the total dislocation density and the higher strain-hardening rate of nitrogen-doped CoCrFeMnNi.
•Nitrogen addition significantly enhanced the yield strength and strain-hardening rate of CoCrFeMnNi high-entropy alloy.•The evolution of SSD and GND densities of CoCrFeMnNi HEAs was quantitatively investigated.•Nitrogen alloying weakened the texture intensity and favored the strain accommodation.
The double loop electrochemical potentiokinetic reactivation method was modified and applied to detect the susceptibility to intergranular corrosion of super austenitic stainless steel S32654 aged at ...1000 °C. The precipitation behavior was clarified, and the corresponding intergranular corrosion mechanism of aged S32654 was proposed. The microstructure demonstrated that σ phase, Cr2N and π phase nucleated at the initial stages of aging and gradually grew larger with increasing aging time. The degree of sensitization increased rapidly in the early stage of aging and then showed a slow increasing rate during long term aging stage, and no self-healing has been observed. Such modified double loop electrochemical potentiokinetic reactivation method could successfully characterize the relationship between microstructure evolution and intergranular corrosion susceptibility of S32654. The mechanism of intergranular corrosion was attributed to the Cr and Mo depletion adjacent to σ phase and π phase and the Cr depletion around Cr2N.
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•Precipitation behavior of S32654 at nose temperature (1000 °C) is investigated.•A modified DL-EPR method is established to detect IGC susceptibility of S32654.•The degree of sensitization increases with aging time and no self-healing occurs.•Cr-depleted zone and Mo-depleted zone are observed adjacent to the precipitates.•The IGC of S32654 is caused by the Cr- and Mo-depleted zones.
Rare earth metals have strong affinity with O and S in molten steel. However, due to high density of rare earth inclusions close to that of molten steel, rare earth inclusions are not easy to float ...up and it is not conducive to take away O and S. In this paper, the influence of rare earth magnesium alloy on the evolution mechanism of inclusions and deoxidization and desulfurization in H13 steel was analyzed, and the thermodynamic calculation was developed. The research results indicate that the composite inclusions of low-density MgO attaching or wrapping on the surface of high-density Ce inclusions are formed after adding Ce–Mg alloy to H13 steel. Compared to Ce inclusions alone, composite inclusions have lower average density and larger diameter, so that they have a faster floating rate according to Stokes law and are easier to float up. Therefore, rare earth magnesium treatment is beneficial to effectively remove the impurity elements such as O and S in the steel.
•Ce addition could modify MgO and MnS into Ce2O3 and Ce2O2S in S32654.•Ce addition leads to noticeable refinement of both dendrite structure and σ phase.•Effective Ce-bearing inclusions could serve ...as nucleation cores of γ and σ phase.•Solute Ce markedly enhances undercooling degree, further refining dendrite structure.•Solute Ce promotes σ phase nucleation, while dendrite refinement limits its growth.
The influence of Ce addition on the solidification structure and σ phase of super austenitic stainless steel S32654 was systematically investigated via microstructural characterization and thermodynamic calculation. The results indicate that a small addition of Ce could modify MgO and MnS into Ce-bearing inclusions Ce2O3 and Ce2O2S. Ce addition led to noticeable refinement of both the dendrite structure and σ phase. The refinement mechanism could be attributed to the combined actions of effective Ce-bearing inclusions and solute Ce. Effective Ce-bearing inclusions could serve as heterogeneous nucleation cores of austenite as well as σ phase, which provided a favorable prerequisite for their refinement. Solute Ce significantly enhanced the undercooling degree of the system, further promoting dendrite structure refinement. Meanwhile, solute Ce improved the eutectic precipitation conditions of σ phase and further promoted its nucleation, while the dendrite refinement limited its growth space. Finally, more fine and dispersed σ phase particles formed in S32654 with Ce addition. The refinement of dendrite structure and σ phase will reduce the temperature and time required for high-temperature homogenization, which is beneficial to the hot working of this steel.
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Ultra-high strength steels are the crucial irreplaceable materials in the fields of aerospace, national defense and military industries. Unfortunately, it is challenging to achieve the requirements ...of vital components because of the strength-toughness trade-off of ultra-high strength steels. In this review, three types of conventional ultra-high strength martensitic steels are systematically summarized. These are: (i) low alloy ultra-high strength steels; (ii) ultra-high strength maraging steels; and (iii) Co–Ni secondary hardening steels. The main influencing factors of mechanical properties of ultra-high strength steels and the exploration on strengthening/toughening are discussed. In particular, the design concept based on the synergistic precipitation of low lattice misfit nano-particles and the formation of martensite matrix with high-density dislocations is a promising approach to develop novel ultra-high strength steels. Finally, some suggestions on the traditional design methods and future development of ultra-high strength steels are put forward and an outlook on future work is offered. This provides guidance for the development of novel ultra-high strength steels with excellent comprehensive performance.
The effects of Cr and Mo on the precipitation behavior and associated intergranular corrosion susceptibility of S32654 were investigated by microstructure characterization, thermodynamic calculation ...and electrochemical measurement. Increasing Cr and Mo contents significantly promoted the formation of σ phase and inhibited the precipitation of R, π phase and Cr2N. Mo has stronger promotion effect on the formation of σ phase than Cr. The effects of Cr and Mo on the precipitation behavior were mainly achieved by affecting the driving force for precipitation and the activity of elements. The specific influence mechanism has been fully discussed in this paper. Additionally, the promotion effect of Cr and Mo on precipitation further expanded the area of Cr- and Mo-depleted zones around various precipitates, enhancing the intergranular corrosion susceptibility of S32654.
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•Effects of Cr and Mo on driving force and element activity were clarified.•Increasing Cr and Mo contents significantly promotes the precipitation of σ phase.•Raising Cr and Mo contents remarkably enhances the intergranular corrosion susceptibility.•Mo has stronger promotion effect on precipitation and intergranular corrosion susceptibility.•Intergranular corrosion is mainly caused by Cr- and Mo-depleted zones around precipitates.
The viscosity of 30%CaO-30%SiO2-15%Al2O3-5%MgO-10%Na2O-10%CaF2-xCe2O3 (mass%, x = 0, 5, 10, 15) were measured by the rotating column method, and then the viscosity and the Tbr (break temperature) of ...the mold flux were analyzed based on the results. Meanwhile, the structural characteristics of the mold flux were investigated using Raman spectroscopy and XRD (X-ray diffraction). The results show that Ce2O3 predominantly destroys the silicate network structure at high temperature, reduces the polymerization degree of the mold flux, simplifies the high-temperature structure of the mold flux, and reduces the friction resistance of viscous flow. From the apparent phenomenon, the viscosity of the mold flux decreases at high temperature. In addition, with the increase of Ce2O3 content in the mold flux, Ca4Si2O2F changes to Ce9.33(SiO4)6O2, which enhances the crystallization ability of the mold flux and increases the Tbr of the mold flux.
Super-austenitic stainless steel S32654 sheets with 2.4mm thickness were successfully welded by friction stir welding (FSW) at the rotational speeds of 300 and 400rpm with a constant traverse speed ...of 100mm/min using W-Re tool. The sound joints with almost no nitrogen loss were successfully produced. The microstructure evolution was characterized by optical digital microscope (ODM), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), electron backscatter diffraction (EBSD) and transmission electron spectroscopy (TEM). The results suggest that the grain structure evolution in stir zone (SZ) is dominated by continuous dynamic recrystallization (CDRX). The strain rate plays a dominated effect on obvious grain refinement. The band structures containing W and Re are generated due to the wear between tool probe and steel in SZ. Furthermore, the microhardness measurements and transverse tensile tests indicate that the grain refinement combining with high density dislocations and substructures improves the hardness and strength, but greatly reduces the plastic deformation capacity of joints. The more suitable welding parameters are determined as 300rpm and 100mm/min for this steel.
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•Friction stir welding was successfully applied to super-austenitic stainless steel S32654.•Grain structure evolution in stir zone is dominated by continuous dynamic recrystallization.•Strain rate plays a dominated effect on grain refinement.•Grain refinement, high density dislocations and substructures together improve hardness and strength, but reduce elongation.•The more suitable welding parameters are determined as 300 rpm and 100 mm/min for this steel.
In this work, time temperature transformation (TTT) diagram, thermodynamic calculations and microstructure observations were performed to investigate the precipitation behavior of hyper duplex ...stainless steel UNS S32707 aged at nose temperature for various aging time up to 48h. The corresponding secondary phase transformation mechanisms were proposed. The results demonstrate that the precipitation kinetics of σ phase in UNS S32707 is very fast due to its higher Cr and Mo contents at nose temperature of 950°C. At the initial stage of aging, σ phase preferentially formed along the α/α and α/γ phase boundaries and then penetrated into α phase, resulting from the eutectoid reaction α→γ2+σ. Meanwhile, a few σ phases nucleated at the γ/γ phase boundaries. With the aging time prolonging, some σ phases nucleated within γ phase due to the decomposition of the supersaturated γ phase. In addition, a cellular structure consisting of lamellar σ and γ2 phase was detected during short term aging, and it gradually grew with aging time via cell boundary migration. After 30min and 6h aging, the lamellar σ phases in the cellular structure broke into small lamellas or into bending bars and evolved into blocky shapes. Most of rounded Cr2N particles formed at the σ/γ2 interfaces and were surrounded by σ and γ2 phase. A few rod-like Cr2N particles nucleate at γ/γ phase boundaries due to the low energy interfaces between Cr2N and γ phase. No Cr2N was detected in α phase, which was attributed to the fact that σ phase has much faster precipitation kinetics than that of Cr2N.
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•Precipitation behavior and phase transformation of S32707 at 950°C are investigated.•Precipitation kinetics of S32707 is fast due to high alloy contents.•σ phase nucleate α/α, α/γ and γ/γ phase boundaries and within primary γ phase.•The transformation mechanism of cellular structure containing σ and γ2 is discussed.•Rounded Cr2N forms at σ/γ2 interfaces and rod-like Cr2N nucleate at γ/γ interfaces.
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