Inconel 690 alloy is a nickel-based alloy with a high chromium content that provides excellent oxidation and corrosion resistances. The high oxidation resistances of the alloy is attributed to the protective Cr₂O₃ that forms during oxidation, which prevents the outward diffusion of alloy elements and the inward diffusion of oxygen. However, when the temperature exceeds 1000^oC, the volatilization and spallation of the Cr₂O₃ oxide scale severely reduce the oxidation resistance of the Inconel 690 alloy. The addition of trace active elements is an effective way to improve the oxidation resistance of superalloys; however, the effects of these elements on the oxidation behavior and mechanism of the Inconel 690 alloy remain unclear. In this study, the oxidation behavior of Inconel 690 alloys with varying contents of Al and Ti elements was systematically studied through oxidation kinetics, morphology observation, and element analysis. In addition, the effects of Al, Ti, and their coadditions on the oxidation behavior and mechanism of the Inconel 690 alloy were investigated. The results indicate that the addition of Al reduces the oxidation mass gain and improves the oxidation resistance of the Inconel 690 alloy. Moreover, the addition of Al and Ti accelerates the oxidation rate of the alloy at 850^oC, but retards it at 1000 and 1200^oC. The positive influence of Al addition can be attributed to the fact that Al₂O₃ particles precipitated at the grain boundary hinder the diffusion of Cr³⁺ along the grain boundary. The slow diffusion of Cr³⁺ inhibits the growth of the Cr₂O₃ oxide scale and reduces the number of holes in the alloy. As a result, the oxidation resistance of the alloy increases owing to the decrease in the oxidation rate and the increase in adhesion between the oxide scale and the substrate. When Al and Ti are added, Ti⁴⁺ , which acts as a high-valence ion, is doped into the Cr₂O₃ scale, promoting the outward diffusion of Cr³⁺ and accelerating the oxidation rate of the alloy at lower temperatures (850^oC). However, during oxidation, Ti tends to converge toward the surface of the Cr₂O₃ scale and form a nonvolatile Ti-rich oxide layer. The formation of this layer inhibits the volatilization and peeling of Cr₂O₃, thereby increasing the oxidation resistance of the Inconel 690 alloy at 1000 and 1200^oC.

Ni base superalloys have been extensively used in advanced aeroengine. With the development of modern aviation industry, high demands are being placed on the comprehensive performance of Ni base superalloys at high temperatures. To meet this demand, numerous refractory alloy elements are added to Ni base superalloys. However, in this way, the temperature bearing capacity of the alloy is enhanced, while the microstructure stability is reduced. Therefore, some scholars proposed to develop single crystal blades with various orientations through the anisotropy of a single crystal alloy. The stress rupture anisotropies of a Ni base single crystal superalloy DD432 under 760^oC, 810 MPa and 1000^oC, 280 MPa have been investigated in this study. The stress rupture properties of Ni base single crystal superalloy DD432 that deviated from <001>, <011>, and <111> with certain degrees were measured. It is discovered that rafting does not occur in specimens with three orientations, and the stress rupture life anisotropy of specimens is visible at 760^oC and 810 MPa. Furthermore, the stress rupture life of specimens with <111> orientation is the best, and as the orientation deviation degree increases, the stress rupture life gradually decreases. The stress rupture property of specimens with <001> orientation is lower than that of specimens with <111> orientation, and the stress rupture life increases with increasing orientation deviation degree. The creep resistance of specimens with <011> orientation is the lowest, and the stress rupture property is also enhanced as the orientation deviation degree increases. Rafting occurred in all the specimens with three orientations under the conditions of 1000^oC and 280 MPa, and the stress rupture property anisotropy of specimens decreased, but still existed. The stress rupture property of specimens with <111> orientation was slightly better than that of specimens with <001> orientation, and the stress rupture property of specimens with <011> orientation was still the worst, although it improved. A difference still exists in the stress rupture life as the degree of orientation deviation changes. However, there is no visible linear tendency.

Nickel-based superalloys, especially single-crystal (SC) ones, have long been recognized as important materials for turbine blades used in aerospace and gas engines. Static magnetic fields are effective at controlling the material forming. The use of static magnetic fields during solidification has evolved as a sophisticated approach for efficiently controlling the microstructures and mechanical performance of metallic materials. In recent years, studies have shown that static magnetic fields have a complex effect on dendrites in SC superalloys. However, the mechanism of static magnetic fields regulating stray grains on remelted interface needs to be investigated further. This work studied the generation of stray grains near the seed remelted zone and the evolution mechanism during the directional solidification of the SC superalloy assisted by a magnetic field by tracing the solidification microstructure. The stray grains of large orientation that deviated from the <001> direction appeared on the remelted zone interface of the solidification microstructure when the magnetic field was applied, accompanied by the formation of a large-angle grain boundary (LAGB). Most of the stray grains were distributed at the sample edge. The increase in magnetic field intensity and pulling speed increased the number of stray grains and the length of the LAGB. As the solidification progressed, the large-orientation stray grains and the LAGBs were eliminated at a fast speed and evolved into small-orientation dendrites. During the following solidification, the orientation of the dendrites became even smaller and the evolution speed decreased sharply. The increase in withdrawal speed intensified the evolution process. The stray grains formed in the remelted zone can be attributed to the twisting dendrite by the thermoelectric magnetic force. The distribution of more stray grains around the sample was caused by the circulation from thermoelectric magnetic convection at the macroscopic scale.

The dual-cluster composition formula of the widely-used α + β Ti-6Al-4V and α-{[Al-Ti₁₂](AlTi₂)}₁₂ + β-{[Al-Ti₁₄](V₂Ti)}₅, reported in our previous work, indicates that all Ti alloys are composed of α and β units. In this study, Ti-(3.19-7.45)Al-(0-12.03)V (mass fraction, %) alloys are designed following composition formula of [Al-Ti₁₂](AlTi₂)}_{17 - n} + β-{[Al-Ti₁₄](V₂Ti)}_n by changing n value (number of β cluster units). The alloys as prepared by copper mould suction-casting cover microstructures ranging from pure α to pure β. In the as-cast state, as the n value increases, the microstructure changes from single α phase (α' martensite), via α + β dual-phase, and finally to single β phase. The morphology of α' martensite gradually changes from plate-like to lamellar and needle-like. Ti-6Al-4V alloy corresponds to n = 5, where β phase begins to appear. When n = 8, needle-like α' martensite shows the highest content. When n = 12, α phase disappears completely and is replaced by β phase. Correspondingly, the strength of the alloys increases first and then decreases, while the plasticity changes inversely, due to the presence of fine-needle α' martensite. Among all the compositions, Ti-5.28Al-6.14V alloy (n = 8) shows the highest strength (about 90 MPa higher than Ti-6Al-4V), with tensile strength of 1019 MPa, yield strength of 867 MPa. Its specific strength and hardness of 230 kN·m/kg and 0.76 GPa·cm³/g increased by 9% and 5%, respectively, are both superior to Ti-6Al-4V.

Oxide dispersion-strengthened (ODS) steels with nano-scale Y₂O₃ or Y-Ti-O oxides have been considered as potential structural materials used in advanced nuclear systems. In this work, a novel 9Cr-ODS steel, namely, MX-ODS steel, was designed by decreasing carbon content to eliminate conventional M₂₃C₆-type carbides and by increasing the content of nitrogen and vanadium to form MX-type precipitates. In addition, the MX-ODS steel was synergistically strengthened by nano-scale MX precipitates and oxides. After fabrication by powder metallurgy, microstructural observation, and mechanical property tests were conducted after different heat treatments. The density of the prepared materials using hot forging instead of hot isostatic pressing was about 98%. Results of the microstructure observation of the MX-ODS steel indicated that after normalizing and tempering, the tempered martensitic structure dominated, and the mean effective grain size was approximately 1 μm. Moreover, the preferential orientation of coarse-grained and fine-grained mixed grains was not detected. By diminishing carbon content, M₂₃C₆-type carbides precipitated at the grain and sub-grain boundaries were eliminated. By contrast, MX-type precipitates with a diameter of approximately 30-200 nm were formed in the matrix. Furthermore, nano-scale Y-rich oxides with an average size of approximately 3.0 nm were dispersed in the matrix, and a number density can reach to approximately 1.9 × 10²³ m^-3. Furthermore, “core-shell” structure precipitates were found, which were spherical in shape with a diameter ranging from 10 to 20 nm. Such precipitates also contained Y, Ta, and O as the core and V as the shell. The mechanical properties indicate that microhardness decreased from 372 to 320 HV with the increase of normalizing temperature from 980^oC to 1200^oC. In addition, microhardness decreased significantly after tempering but initially increased and then decreased with the increase of tempering temperature from 700^oC to 800^oC, with a peak microhardness at approximately 750^oC. Excellent strength and ductility were obtained after normalizing at 1100^oC for 1 h and then tempering at 750^oC for 1 h. Yield strength, ultimate tensile strength, and total elongation were 1039 MPa, 1103 MPa, and 20.5% when tested at room temperature and 291 MPa, 333 MPa, and 16% at 700^oC, respectively.

Controlling nonmetallic inclusions in steels is critical during the steelmaking process. Temperature affects the chemical equilibrium between steel and the inclusions, the composition of the inclusions changes with changes in temperature. During the solidification and cooling processes, the cooling rate is a significant factor affecting the temperature. Therefore, the composition of nonmetallic inclusions transforms during the solidification and cooling of steels. To study the evolution of the inclusion composition in pipeline steel at cooling rates of 800, 600, 400, 200, 100, and 5^oC/min, high-temperature confocal scanning laser microscopy was employed to accurately control the temperature during the cooling process. The thermochemical software FactSage was employed to reveal the theoretical basis of the transformation of the inclusion composition. A kinetic model for the evolution of the inclusion composition in pipeline steel during the cooling process was established, and the effect of inclusion diameter and cooling rate on the transformation was analyzed. The results revealed that with the decrease in the cooling rate, the Al₂O₃ content in the inclusions increased from 66.33% to 75.06%, the CaS content increased from 1.07% to 10.55%, and the CaO content decreased from 28.27% to 11.24%. Further, the MgO content decreased from 4.33% to 3.15% during the cooling process. The number densities of the inclusions were 76.15 and 15.28 mm^-2 at cooling rates of 800 and 5^oC/min, respectively. As the cooling rate decreased, the average diameter of the inclusions first decreased from 2.09 to 1.62 μm and subsequently increased. The thermodynamic equilibrium composition of the inclusions in the molten steel was 41.71%CaO-50.76%Al₂O₃-6.50%MgO-1.03%SiO₂. With a decrease in temperature, inclusions transformed from Al₂O₃-CaO-MgO to CaS-Al₂O₃-MgO-(CaO). The cooling rate had little effect on the MgO and Al₂O₃ contents in the inclusions. The inclusion diameter and cooling rate had an apparent influence on the CaO and CaS contents in the inclusions. The critical cooling rate at which the CaS content became greater than the CaO content was impacted by the inclusions' diameter. The critical cooling rates for inclusions with diameters of 1 and 2 μm were approximately 400 and 100^oC/min, respectively, whereas the rates were much smaller than 1^oC/min for inclusions with diameters larger than 5 μm.

In recent years, ultra-high strength steel (UHSS) has been widely utilized in engineering structures, mining machinery, and military equipment. However, UHSS is prone to brittle fracture and fatigue failure due to high strength and relatively low plasticity. Moreover, residual stress induced by welding process affects both brittle fracture and fatigue failure. In this work, a single-pass butt-welded joint was fabricated by metal inert-gas welding. The base metal was 1600 MPa grade UHSS with a 5 mm thickness, and the filler metal was ER307Si. The distributions of welding residual stress and hardness of the butt-welded joint were measured using the hole drilling method and a microhardness tester, respectively. Based on measured values of hardness in the heat-affected zone (HAZ) and softening zone (SZ), SYSWELD software was used to develop an advanced computational approach with consideration of “thermal-metallurgical-mechanical” coupling behaviors. In addition to the strain hardening and annealing effects of weld metal, the established computational model accounted for both the solid-state phase transformation (SSPT) of HAZ and softening effect of SZ. The temperature field and residual stress distribution of the UHSS single-pass butt-welded joint were simulated. Furthermore, the simulated results were compared with the corresponding measured data. The simulation results revealed the effect of SSPT and softening on welding residual stress. The numerical results indicated that SSPT has a strong influence on both the magnitude and distribution of the longitudinal residual stress; however, it has a limited effect on transverse residual stress. Meanwhile, the softening effect drastically affects the peak values of the longitudinal residual stress, while it hardly influences transverse residual stress. When both SSPT and softening effects are simultaneously considered in the numerical model, the computed results of welding residual stress are in good agreement with the experimental measurements.

Controlling the macrosegregation induced by electropulsing is of high commercial importance. This study investigates the macrosegregation of the primary Si phase in casting Al-Si hypereutectic alloys via the different solidification stages and components of alloys treated with electropulsing. Experimental results show that a serious gradient macrosegregation of the primary Si phase occurs, and four types of primary Si regions are formed: coarse plate-like, refined plate-like, and fine polygon primary Si, as well as a eutectic structure. The wider the solidification temperature range, the more serious the macrosegregation. A near-eutectic structure occurs at the center of ingots when the solidification temperature range exceeds a certain threshold. With an increasing current density of electropulsing, the segregation degree of primary Si increases initially and then decreases for different Al-Si hypereutectic alloys, but the current density with regard to the most serious segregation is closely related to the Si content. Furthermore, it is proved that the migration behavior of primary Si particles plays an important role in macrosegregation. A special casting experiment under the condition of limited heat flux along the radial direction was performed to clarify the macrosegregation mechanism of primary Si under electropulsing. After nucleation during the solidification process, the primary Si particles move to the front of the solid-liquid interface due to secondary flow in bulk liquid and then are easily captured due to the electromagnetic repulsive force or its component. The force flow in the bulk liquid and mush zone and the secondary flow in front of the solid-liquid interface make obvious solute redistribution and promote the growth of the primary Si phase, which is maintained until the solute concentration in the bulk liquid approaches the eutectic composition.

Due to its outstanding all-around performance, Fe-Co-Ni ultra-high strength steel (UHSS) is frequently used in crucial load-bearing components. The UHSS components will be significantly deformed during the welding and post-welding heat treatment operations, which makes the subsequent assembly to satisfy usage requirements a challenge. As a result, it is crucial to simulate the entire manufacturing process of UHSS components to investigate and comprehend the laws of stress and distortion in UHSS component weld joints throughout the manufacturing process. In this study, the “thermo-metallurgical-mechanical” coupled finite element model's accuracy is first verified, followed by the development of a heat source model for electron beam welding. The evolution of the microstructure of the weld joint, stress, and distortion in the weld joint of complex components are thus precisely predicted using the linked model throughout the production process of “electron beam welding-vacuum gas quenching”. The primary cause of the severe deformation of complex components is vacuum gas quenching. The solid-state phase transformation cannot be ignored in the simulation process of “electron beam welding-vacuum gas quenching” of the complicated components.

Several high-entropy alloys (HEAs), such as single-phase bcc HEA with high strength and fcc with high ductility have been developed over the past few decades. Eutectic HEA (EHEA), such as AlCoCrFeNi_2.1, consists of both fcc and bcc microstructure, imparting excellent mechanical properties. The recent research on AlCoCrFeNi_2.1 EHEA primarily focuses on its mechanical properties. However, corrosion resistance of AlCoCrFeNi_2.1 EHEA is rarely discussed, which is crucial for the application of new materials. This work investigates the corrosion behavior of AlCoCrFeNi_2.1 EHEA in 0.05 mol/L H₂SO₄ and 0.05 mol/L H₂SO₄ + 0.02 mol/L NaCl solutions using electrochemical evaluation, SEM, EDS, and XPS. The results indicate that Cl^- do not alter the semiconductor type of passive film on AlCoCrFeNi_2.1 EHEA, but they considerably affect the compactness. Cl^- change passive film properties by influencing the Al and Cr oxide contents; however, Ni is not affected by Cl^-. The Ni-Al-rich phase is preferentially dissolved in 0.05 mol/L H₂SO₄ solution, and pitting corrosion and selective dissolution occur in 0.05 mol/L H₂SO₄ + 0.02 mol/L NaCl solution.

Particulate reinforced aluminum matrix composites are widely used in various industrial fields owing to their high specific strength and modulus, low coefficient of thermal expansion, etc. In general, because stronger matrix alloys tend to produce stronger composites, composites with high-strength Al-Zn-Mg-Cu alloys as the matrix are paid considerable attention. However, the aging behavior of the SiC/Al-Zn-Mg-Cu composites has not been well understood. In the present work, SiC particles with a volume fraction of 15% reinforced Al-7.5Zn-2.8Mg-1.7Cu (mass fraction, %) composite and corresponding unreinforced alloy were fabricated using the powder metallurgy technique. The effects of the aging time on the hardness, electrical conductivity, and mechanical properties of the composite and corresponding matrix alloy were investigated. The T6 heat treatment process suitable for the composite was proposed. The nanoscale precipitates under different aging conditions were quantitatively analyzed using TEM and HRTEM. The results indicated that the SiC particles exhibited an obvious promoting effect on the aging process of the SiC/Al-Zn-Mg-Cu composite. The composite reached the corresponding maximum hardness 14 h earlier than the unreinforced alloy. The maximum hardness (238 HV) of the composite was 29 HV higher than that of the unreinforced alloy. The width of the precipitation-free zone of the composite was similar to that of the unreinforced alloy, but the number of the grain boundary phases in the composite increased. The formation of the grain boundary phases and interfacial reaction products could consume alloying elements and reduce the density of precipitated phases in the composite. Based on HRTEM, SiC particles did not change the aging precipitation sequence (SSS-GP zone-η'-η) of the Al-Zn-Mg-Cu alloy, and the η' phase was the major strengthening phase of the T6-treated SiC/Al-Zn-Mg-Cu composites.

The morphology of precipitates changes during coarsening regimes, thereby resulting in the modification of mechanical properties of metallic materials. Hence, understanding the morphological evolution in precipitates is critical to tailor the macroscopic properties of industrial alloys. In particular, the morphology of Fe-rich precipitates in Cu alloys is complex, and it evolves from sphere to cube to petal and finally splits, which has been observed during casting and furnace cooling. However, morphological changes in Fe-rich precipitates during isothermal treatment remain unclear; thus, revealing the mechanism of morphological evolution is necessary. In this study, the relationship among the morphological evolution behavior of Fe-rich precipitates in Cu-2.0Fe (mass fraction, %) alloy, temperature, and time under different isothermal-treated processes was analyzed using SEM and TEM coupled with phase-field modeling. Results show morphology changes from a sphere in nanoscale to a cube in submicron scale to a four-branched petal in the submicron scale, and to a multi-branched petal in micron scale during coarsening of Fe-rich precipitates in Cu-2.0Fe alloy isothermally treated at 924, 964, and 984^oC (i.e., the temperature range of the fcc Fe phase). The size of multi-branched petal-like Fe-rich precipitates and the number of branches increase with the increase of isothermal temperature and holding time. During coarsening of multi-branched petal-like precipitates, the surrounding small Fe-rich precipitates are engulfed, and thence the number density of the smaller ones in nano and submicron scales decreases when the temperature increases. The modeling result elucidates the multiple morphological evolution of Fe-rich precipitates, which is identical to the experiments, under the effects of interfacial energy, elastic energy, and chemical driving force. In particular, the combined effect of the latter two energies induces the initiation and growth of secondary branches out of primary branches in the four-branched petals, thereby producing multi-branched petal-like precipitates.