Search Results (26)

To enhance the molten salt corrosion resistance of Ni200 alloy plasma arc welds, the welds were subjected to tensile deformation followed by heat treatment. The grain boundary character distribution (GBCD) was analyzed using electron backscatter diffraction (EBSD) in conjunction with orientation imaging microscopy (OIM). A constant-temperature corrosion test at 900 °C was conducted to evaluate the impact of GBCD on the corrosion resistance of the welds. Results demonstrated that after processing with 6% tensile deformation, and annealing at 950 °C for 30 min, the fraction of low-ΣCSL grain boundaries increased from 1.2% in the as-welded condition to 57.3%, and large grain clusters exhibiting Σ3ⁿ orientation relationships were formed. During the heat treatment, an increased number of recrystallization nucleation sites led to a reduction in average grain size from 323.35 μm to 171.38 μm. When exposed to a high-temperature environment of 75% Na₂SO₄-25% NaCl mixed molten salt, the corrosion behavior was characterized by intergranular attack, with oxidation and sulfidation reactions resulting in the formation of NiO and Ni₃S₂. The corrosion resistance of Grain boundary engineering (GBE)-treated samples was significantly superior to that of Non-GBE samples, with respective corrosion rates of 0.3397 mg/cm²·h and 0.8484 mg/cm²·h. These findings indicate that grain boundary engineering can effectively modulate the grain boundary character distribution in Ni200 alloy welds, thereby enhancing their resistance to molten salt corrosion. Full article

In this experiment, a novel designed Mn-N-bearing, nearly Ni-free, TRIP-aided economical 19Cr (Fe-18.9Cr-10.1Mn-0.3Ni-0.26N-0.03C) duplex stainless steel (DSS) was prepared, and it exhibited a good combination of strength and toughness after suitable solution treatment, showing good application potential. The deformation mechanisms of ferrite and austenite are different during tensile deformation at room temperature: the ferrite phase was deformed by a dislocation slip mechanism and formed a cell structure due to its higher stacking fault energy; the lower stacking fault energy of austenite resulted in a strain-induced martensite phase transformation mechanism. With an increase in aging time from 1 h to 7 h at 750 °C in air, the σ phase precipitates in the ferrite triple grain boundary junction, which leads to an increase in ultimate tensile strength, acts as an obstacle to the dislocation motion and decreases the ductility, deteriorating the pitting corrosion resistance in 3.5 wt.% NaCl solution at the same time. The σ phase precipitation behavior does not alter the deformation mechanism of the phases of the solution-treated TRIP-aided economical DSS. Full article

Grain boundary engineering (GBE) was carried out on a nickel-based alloy (GH3535, Ni-16Mo-7Cr-4Fe), which intrinsically has many strings of primary molybdenum carbides. The strings induce inhomogeneous grain size distributions and increase the difficulties in achieving a GBE microstructure. In this work, the effects of the primary carbide distribution on the grain boundary network (GBN) evolution were investigated. A higher proportion of Σ3ⁿ grain boundaries (GBs) associated with extensive multiple twinning events was achieved in the specimen with more dispersive and finer primary carbides, which are the results of cross-rolling, i.e., cold rolling with a changed direction. In a starting microstructure with many strings of primary carbides, the dense and frequent occurrence of particle-stimulated nucleation (PSN) around the carbides induced more general high-angle GBs into the GBN, and the inhibition of GB migrations by the carbide strings suppressed the formation of large-sized highly twinned grain clusters. As a consequence, the Σ3ⁿ GBs could not be effectively enhanced. Full article

We investigated the improvement of mechanical properties of biograde Co–28Cr–6Mo–0.11N alloy prepared by electron beam melting through grain refinement via multiple reverse transformations. While the effects of single and double reverse transformation treatments on the microstructure have been investigated in previous studies, we investigated the effects of multiple reverse transformation heat treatments. The particle size was refined to 1/4, and the yield strength, tensile silence strength, and elongation were enhanced to 655 MPa, 1234 MPa, and 45%, respectively, satisfying ASTM F75 standards. Moreover, a mixed phase of ε and γ was observed to provide higher yield strength than a single γ structure. The dominant behavior in the γ → ε phase transformation at 1073 K was obvious. Grain growth was suppressed by the grain-boundary pinning effect of the Cr₂N phase during reverse transformation to the γ phase. Because no fracture was caused by precipitates such as σ, η, and Cr₂N phases, the influence of the precipitates on the tensile properties was small. Full article

Focal research has been conducted on medium-entropy alloys (MEAs) that exhibit a balanced combination of strength and plasticity. In this study, the microstructure, dynamic mechanical properties, and texture evolution of an as-cast medium-entropy CoCr_0.4NiSi_0.3 alloy were investigated through dynamic compression tests at strain rates ranging from 2100 to 5100 s⁻¹ using the Split Hopkinson Pressure Bar in order to elucidate the underlying dynamic deformation mechanism. The results revealed a significant strain rate effect with dynamic compressive yield strengths of 811 MPa at 2100 s⁻¹, 849 MPa at 3000 s⁻¹, 919 MPa at 3900 s⁻¹, and 942 MPa at 5100 s⁻¹. Grains were dynamically refined from 19.73 to 3.35 μm with increasing strain rates. The correlation between adiabatic temperature rise induced by dynamic compression and dynamic recrystallization was examined, revealing that the latter is not associated with adiabatic heating but rather with phase transition triggered by the dynamic stress during compression. The proportion of Σ3ⁿ (1 ≤ n ≤ 3) grain boundaries in deformation specimens increases with increasing strain rates during dynamic compression. The formation of specific three-node structures enhances both strength and plasticity by impeding crack propagation and resisting higher mechanical stress. In the as-cast state, significant anisotropy was observed in the MEA. As strain rates increased, it transited into a stable {111}<112> F texture. The exceptional dynamic properties of strength and plasticity observed in the as-cast state of the MEA can be attributed to a deformation mechanism involving a transition from dislocation slip to the formation of intricate arrangements, accompanied by interactions encompassing deformation nanotwins, stacking faults, Lomer–Cottrell locks, stair-rods, and displacive phase transformations at elevated strain rates. Full article

This study aimed at the mechanical characterization, on a nanometric scale, of the constituents obtained for different fractions in duplex stainless-steel plates subjected to 850, 950, 1000, and 1150 °C heating treatments via hardness measurements and determining their influences on the fretting wear behavior of the studied steel. The obtained ferrite (α)-, austenite (γ)-, and sigma (σ)-phase fractions were determined using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The mechanical characterization was carried out using hardness measurement and fretting wear techniques via nanoindentation. For comparison purposes, the Vickers microhardness was also characterized to determine the effect of the σ phase, which eventually formed, on the obtained microstructure properties as a whole. Two distinct behaviors were observed, depending on the eventual formation of σ phase as a function of the treatment temperature: (i) specimens treated at 850 and 950 °C showed a hardening effect (HV_0.5 values of 333 ± 15 and 264 ± 13, respectively) due to σ-phase precipitation (hereafter termed ‘as-aged’), and (ii) specimens treated at 1000 and 1150 °C (with HV_0.5 values of 240 ± 13 and 249 ± 4, respectively) showed no σ-phase precipitation (hereafter termed ‘as-solubilized’). The increases in the microhardness values for the as-aged specimens were attributed to the hardness of the σ-phase precipitates (which showed nanohardness values varying in the 8.0–8.5 GPa range), which was approximately twice that of the austenite and ferrite grains (both phases showed nanohardness values in the 3.6–4.1 GPa range, on average). When formed (for fractions on the order of 8% and 3% at 850 and 950 °C, respectively), σ phase was mainly observed at the α/γ grain interfaces or boundaries. Fretting wear tests, using a diamond sphere with a radius of 10 μm as the counter body and a load of 20 mN, revealed the same wear mechanisms in the α/γ matrix for all studied conditions. However, as-solubilized specimens (heat-treated at 1000 and 1150 °C) displayed higher resistance to fretting micro-wear in the austenitic grains compared to the ferritic grains, indicating lower plastic deformation in the respective wear scars on the obtained tracks. In particular, as-aged specimens (heat-treated at 850 and 950 °C) exhibited lower coefficients of friction due to their higher surface resistances. The localized wear at σ-phase grains was much less pronounced than at ferrite and austenite grains. Overall, this study provides valuable insights into the mechanical behavior of microstructural changes in duplex steel at the nanometric scale. Full article

Silver products with high relief have become popular in the silver decoration industry. However, it is difficult to obtain these products through conventional processing at ambient temperature. The aim of this work is to solve this problem by increasing the deformation temperature. Detailed studies were conducted on the evolution of microstructure characteristics in bulk high-purity silver by electron backscatter diffraction (EBSD) to achieve high-relief applications at elevated temperatures. The high temperature sample is mainly composed of recrystallized and substructured grains, exhibiting a more stable state than the ambient temperature sample. More than 70% annealing twins are observed in the hot-working sample. They are characterized by the amount of Σ3ⁿ-type triple grain boundary junctions within large grain clusters formed by multiple twinning. These particular boundaries improve the intergranular corrosion resistance and degradation, which is significantly essential for high-purity silver jewelry exposed to sweat and air. The closed multi-coining processes at different temperatures were conducted subsequently. The performance of workpieces demonstrates that increasing the deformation temperature is a viable alternative for producing durable high-relief silver products. Full article

The microstructure morphology and evolution of mechanical properties are investigated in this study. The results show that the phases displayed no clear change after thermal exposure at 250 °C for 200 h. The tensile strength of the as-cast alloy showed a downward trend in different degrees with the increase in the tensile temperature, while the influence of elongation was opposite to the tensile strength. In addition, the tensile strength tended to be stable after thermal exposure at 250 °C for 100 h. The main creep mechanism of the as-cast alloy at a low temperature and low stress (T ≤ 250 °C; σ ≤ 40 MPa) is grain-boundary creep. The Monkman–Grant empirical formula was used to fit the relationship between the creep life and the minimum creep rate, and the fitting results are:$t_r·{\dot{\epsilon }}_{min}^{0.95}=0.207$. Full article

The shoulder line is fundamental to geomorphic evolution and erosion monitoring research on the Loess Plateau, which represents the boundary between positive terrain (intergully) and negative terrain (inner gully). The existing extraction methods mainly suffer the problems of unclear geological significance, poor landform application, and low efficiency of algorithms. This paper proposes a new loess shoulder line automatic extraction method, in which topographic feature points (ridge and valley points) were used as endpoints to generate continuous profiles, and two parameters, analysis operator size (L) and filter threshold (σ), were created for shoulder point extraction from each profile. This method can be applied to complex landforms such as the continuous shoulder lines of terraces and extracts. Herein, three typical areas on the Dongzhi Plateau were selected to assess the performance of the method, and a digital elevation model (DEM) with a resolution of 5 m was used as source data. The accuracy assessment index was the Euclidean distance offset percentage (EDOP), and the original evaluation method was improved based on Structure from Motion–Multiview Stereo (SfM-MVS) technology. The experimental results showed that the average accuracy of the proposed method in the three test areas reached 89.3%, which is higher than that of the multidirectional hill-shading and P-N methods. Via testing in different areas, it could be concluded that the extraction efficiency was less affected by the area of the test region, and the approach exhibited a suitable robustness. Simultaneously, the optimal values of parameters L and σ were examined. This study increases the possibility of accurate shoulder line extraction in the large area of the Loess Plateau. Full article

The optimization of grain boundary character distribution (GBCD) is of great significance to improve the GB-related properties for heavy-gauge austenitic stainless steels worked in harsh environments such as reactors of nuclear power, which can usually be realized by regulating the thermomechanical process. In this paper, special solution annealing processes for a hot-rolled nuclear grade 316H plate were designed to introduce different character distribution of Σ3ⁿ boundaries (1 ≤ n ≤ 3) and random high-angle GBs (RHAGBs), and the regulation principle among them were clarified. It was worked out that the optimized GBCD by characterization of large twin related domains, abundant interconnected Σ3ⁿ boundaries and interrupted topology network of RHAGBs could be effectively facilitated through solution annealing with a long time period at lower temperature or short time period at higher temperature, in which the recrystallization, grain growth and GB migration during heat treatment process played key roles. Moreover, the length fraction of Σ3ⁿ boundaries were found to be hardly changed when they reached about 77%, but their character distribution could be continuously optimized. Full article

In this study, 2205 duplex stainless steel with 12 mm thickness was welded by alternating magnetic field-assisted laser arc hybrid welding. The effect of an alternating magnetic field on the proportion distribution of two phases of the welded joint was investigated. The texture distribution, grain boundary misorientation, and grain size of welded joints were analyzed and characterized. The uniform distribution of alloying elements in the two phases was improved by a 20 mT alternating magnetic field. The diffusion dissolution of Ni and N elements into the γ phase was promoted, which was conducive to the transition from the α to γ phase and reduced the precipitation of Cr₂N, such that the ratio of γ to α was 43.4:56.6. The ratio of the two phases of the weld was balanced by the alternating magnetic field of 30 mT, such that the ratio of γ and α was 44.6:55.4 and the texture dispersion was weakened. The Σ3 twinning boundary of the austenite phase in the weld was transformed to HABs, the ferrite phase underwent dynamic recrystallization, and the austenite phase had a cube texture, copper texture, and goss texture. Full article

The structure and mechanical properties of as-cast and thermomechanicaly processed (cold rolling followed by annealing at 700 °C and 900 °C) Fe₄₀Mn₄₀Cr₁₀Co₁₀ (at.%) high-entropy alloys doped with different amounts (0, 0.5, and 2.0 at.%) of N were examined. The as-cast Fe₄₀Mn₄₀Cr₁₀Co₁₀ alloy (N0) contained σ-phase particles at the boundaries of the fcc grains. The addition of 0.5 (N0.5) and 2.0 (N2) at.% of nitrogen suppressed the formation of the σ-phase due to which the alloys consisted of only the fcc phase. Annealing after rolling resulted in the development of static recrystallization and precipitation of additional phases: the σ-phase was found in the N0 and N0.5 alloys, and hexagonal M₂N nitrides were found in the N2 alloy. The strength of the as-cast alloys at room temperature increased with increasing nitrogen concentration due to interstitial solid solution strengthening. Cold rolling and subsequent annealing resulted in considerable strengthening of the program alloys. The strength of the alloys increased with the N content and decreased with increasing annealing temperature. The best combination of mechanical properties at room temperature was attained in the N2 alloy after annealing at 700 °C, and at 77 K, was demonstrated by the N0.5 alloy after annealing at 900 °C. Full article

Within the grain boundary engineering (GBE) of alloys, a mixed grain boundary network with random grain boundaries interrupted by twin boundaries, contributes to enhancing the overall grain boundary-related properties. The higher density of twin boundaries is pursued herein. Furthermore, a two-stage deformation method, i.e., prior cold deformation followed by thermal deformation, was proposed for improving the mixed grain boundary network in the thermal deformation of Ni80A superalloy. The influence of prior cold deformation on the mixed grain boundary network was investigated through a series of two-stage deformation experiments. The analysis of the stress–strain curves shows that the critical strain for dynamic recrystallization (DRX) and peak strains decrease significantly under the effect of prior cold deformation. In comparison to the necklace-like microstructures that occur after a single thermal deformation, the microstructures apparent after a two-stage deformation are characterized by finer DRX grains with abundant Σ3ⁿ twin boundaries, with a significantly improved density of the Σ3ⁿ twin boundaries (BLD_Σ3ⁿ) by a factor of around nine. With increasing prior cold strain, the grain size, after a two-stage deformation, decreases continuously, while the BLD_Σ3ⁿ increases firstly and then decreases. The mechanisms for improving the mixed grain boundary network via two-stage deformation were uncovered. The sub-grain boundaries formed in prior cold deformation stimulate the nucleation of DRX grains and twins; meanwhile, the driving force for grain boundary migration is enhanced due to prior stored energy. Then, DRX is activated in advance and occurs more completely, thereby promoting the formation of Σ3ⁿ twin boundaries. Full article

In this study, based on the sensitivity of the chemical composition fluctuation to the thermodynamic parameter, which controls the level of the stacking fault energy (SFE), a series of high Cr–Mn–N twinning-induced plasticity (TWIP) stainless steels are designed by using a sublattice model, and their mechanical properties and micro deformation mechanism are analyzed The formation of mechanical twins (Mts) during the deformation makes the test steel show a perfect combination of strength and ductility after different solution treatments. Among them, after a solution treatment at 950 °C and 1050 °C, the 19Cr–0.7N and 19CrSi–0.7N samples have the maximum value with the product of the strength and plasticity reaching 60.7% and 64.6%, and 12Cr–CN has the maximum value after the solution treatment at 1200 °C, reaching 81.3%. The SFE values of the 19Cr–0.7N and 19CrSi–0.7N samples were relatively high, 48 mJ·m⁻² and 45 mJ·m⁻², respectively. The SFE of 12Cr–CN is 37 mJ·m⁻², and the Mts grow rapidly during the deformation and maintain the highest twinning density under the same strain conditions. The characterization of the tensile samples occurs under different deformations by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The results of the EBSD local misorientation difference angle analysis showed the Silicon element addition with a good Mts saturation rate. It is observed from the TEM that the nucleation process of the Mts with a high SFE is difficult, and the Mts emit and grow inward along the grain boundary during the tensile process and present a cross shape with the increase in strain. The contribution of the grain boundary strengthening (σ₀), dislocation strengthening (σ_f), and twinning strengthening effect (σ_t) under dynamic micro-refinement to stress were calculated. It is known that under a certain amount of strain, the ratio of σ_t and σ_f changes with increasing, and when the contribution of the twinning deformation to the stress exceeds about 25%, the reinforcement of the plastic deformation is dominated by the plane of σ_f. Full article

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be ${\mathsf{\sigma }}_{{10}^5}^{700}=$ 57.1 MPa and ${\mathsf{\sigma }}_{{10}^5}^{750}=$37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M₂₃C₆, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M₂₃C₆ preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M₂₃C₆ in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture. Full article