Superalloys with excellent high-temperature resistance and oxidation resistance have been widely used in aviation and energy fields. The new Ni-Co base superalloy is considered a candidate for the next generation of turbine discs due to its higher performance of mechanical properties and microstructure stability at high temperatures. However, tungsten inert gas (TIG) welding, metal inert gas (MIG) welding, and other welding techniques are not suitable for welding the new Ni-Co base superalloy because the Al + Ti content of the alloy reaches 7.5%, while traditional welding techniques (electron beam welding, friction welding, and diffusion welding) also have some disadvantages. For example, friction welding has certain requirements on the shape of the sample, and it is not suitable for welding large-volume alloys. Diffusion welding requires a long heat retention period and a harmful precipitation phase exists at the interface. A new welding method is applied in this study to solve the problem of welding nickel-based superalloy, achieving a better bonding effect. The Gleeble 3500 thermal simulator was used to study the plastic deformation bonding of Ni-Co base superalloys in a temperature range of 1000-1200^oC and a strain range of 5%-40% with a strain rate of 0.001 s^-1. The recrystallization behavior of the interface was studied by OM, EBSD, and TEM, and the bonding mechanism of the interface was clarified. The results showed that the resistance to deformation of the alloy was low when the plastic deformation bonding was performed at 1150^oC, and there was no risk of cracking of the alloy. Plastic deformation bonding experiments with different deformations had shown that the alloy can achieve complete bonding of the interface under the condition of 40% deformation, and its mechanical properties can reach the same level as the matrix. The tensile fracture analysis showed that the fracture profile of the 40% deformed joint was consistent with the base material, showing a ductile fracture pattern. The results of EBSD and TEM showed that the coarse grains near the interface were first refined during the plastic deformation. With the increase of deformation, the refined grain removed the original interface by the migration of the interfacial grain boundaries with the assistance of a continuous dynamics recrystallization process and ultimately led to the bonding of the interface.

High Cr ferrite heat-resistant steel has excellent geometric structure stability, low radiation swelling rate, and good corrosion resistance of liquid metal. TP347H austenitic heat-resistant steel is based on the traditional 18-8 austenitic steel with the addition of a certain amount of Nb and a small amount of N to precipitate MX-type carbonitride, which results in superior high-temperature properties. Steam with high temperature and pressure flowing through supercritical thermal power units may exhibit heterogeneous connections between high Cr ferrite and austenitic heat-resistant steel components in the supercritical thermal power units. In this study, the vacuum diffusion-bonding of dissimilar materials between high Cr ferritic and TP347H austenitic heat-resistant steel was performed, the effects of diffusion-bonding time and post weld heat treatment (PWHT) process on the microstructural evolution and mechanical properties of the diffusion-affected zone was examined. The results indicated that with the extension of diffusion-bonding time, the interfacial bonding rate gradually increased. The interaction due to the difference in deformation storage energy and dislocation slips resulted in dynamic recrystallization, and the fine grains formed at the diffusion-bonding interface evolved into a serrated interface. Fine and dispersed MX and M₂₃C₆ phases were precipitated in the austenite grain boundaries and at the grain boundaries of the diffusion-bonding zone. After PWHT, the grains in the diffusion-bonding zone were further refined, dislocations were stable, dislocation density reduced, small-angle grain boundaries increased, and element diffusion was more sufficient. Tensile tests at different temperatures showed that the fractured sites were all in the matrix, which indicates that high-quality diffusion-bonding joints of dissimilar materials were achieved.

Lightweight automobiles have a lower impact on the environment and save energy; therefore, they have become a focus within the automobile industry. Hot stamping parts made of high-strength steel have been widely used in car bodies. To study the mechanical properties and constitutive model of high-strength steel after hot stamping, the samples containing full martensite, full bainite, and martensite/bainite dual phases structure were obtained by controlling the tool's temperature and holding time during hot stamping. Then the load-displacement curves of different microstructures were obtained using nanoindentation tests. Subsequently, the modulus of elasticity, yield stress, strain hardening exponent, and other mechanical properties of these microstructures were calculated by reverse algorithms using dimensional analysis. Further, the power-law elastoplastic constitutive models of different microstructures were derived using these parameters. The errors of yield strength obtained using the reverse algorithm and tensile tests in full martensite and full bainite samples are -1.15% and 3.38%, respectively. The yield strength of the martensitic/bainite sample obtained using the reverse algorithm is 16.62%, 24.17%, and -11.78% different from that obtained by the tensile test, showing that the mechanical properties are different under macroscopic and microscopic conditions to some extent. Simultaneously, the average yield strength of the three points is only -1.41% different from that obtained using the tensile test. Finally, the derived constitutive models were verified by simulating the finite element nanoindentation. The results show that the constitutive model obtained using the inverse algorithm can accurately describe the mechanical properties of the main microstructures of high-strength steel after hot stamping.

Automobile industries require advanced high-strength steels (AHSSs) that possess high strength and good formability and are light; automobiles manufactured using AHSSs have reduced fuel consumption and enhanced safety compared to automobiles manufactured using traditional materials. Al-containing medium Mn steels, which are a typical example of 3rd generation AHSSs, have attracted much research attention with an aim of meeting these requirements as they possess extraordinary work hardening ability, which leads to high strength and excellent elongation at a low density. However, such steels often exhibit low yield strength resulting from the formation of coarse δ-ferrite grains due to the high Al content. In this study, the influence of tempering temperatures on the microstructure and mechanical properties of hot-rolled medium Mn steel containing 15% (volume fraction) δ-ferrite due to the addition of 3%Al (mass fraction) is studied. δ-ferrite with a length of 300 μm was refined and divided into a large number of bamboo-like grains having a length of about 3 μm due to dynamic recrystallization caused by hot rolling. The grain size of these refined δ-ferrite grains remained unchanged when the tempering temperature was increased to 700^oC. In the case of tempering at 400-500^oC, although the dislocation density in martensite decreased, the precipitation of fine cementite and nanosized VC particles compensated for this effect, leading to high yield strengths, which was almost the highest among all the tempering temperatures. Meanwhile, many C atoms could be partitioned from martensite to austenite, leading to the steel acquiring the enhanced chemical stability of austenite, which contributed to the higher work hardening rate and more durable strain hardening. Finally, the best mechanical combination consisting of a yield strength of about 1500 MPa, an ultimate tensile strength of 1800 MPa, and a total elongation of 14% was achieved after tempering at 400-500^oC. The resultant yield strength is much higher than that of other medium Mn steels having similar Al content because it is dependent on the tempered martensitic matrix rather than δ-ferrite. This is due to two factors: first, δ-ferrite in the studied steel is strengthened due to precipitation, dislocation, and grain refinement hardening; second, δ-ferrite grains have a small fraction of 15% and a refined size of 3 μm; thus, they are actually embedded in the martensite matrix as isolated islands. These results open a path for the designing and manufacturing of new low-density steels having high yield strengths.

Ultrahigh-strength steels have been widely used in critical engineering structures in military and civilian applications owing to the combination of ultrahigh strength and excellent toughness. The martensitic transformation start temperature (M_s) is an important parameter for designing alloys; it describes the thermodynamic stability and transformation behavior of austenite, affecting the strength and toughness of the alloy. To explore the influence of dilatational strain energy during martensitic transformation on M_s and calculate M_s in the Fe-C-Ni system, the dilatational curves of Fe-C-Ni alloys are measured using a dilatometer. Three tangents method is used to calculate M_s and austenitic transformation start temperature. The influence of composition on microstructure and lattice parameters after martensitic transformation was analyzed using OM and XRD. The dilatational strain energy model in the nonchemical driving force of martensitic transformation is modified considering the interaction between C and Ni components. The M_s of Fe-C-Ni system was calculated using a thermodynamic model in which the sum of martensitic transformation chemical driving force (the difference of Gibbs free energy between fcc and bcc phases) and nonchemical driving force (shearing strain energy of austenite, dilatational strain energy of austenite, dislocation stored energy of martensite, and interfacial energy of austenite and martensite) is zero. These results show that increasing C and Ni contents promote lattice expansion of the bcc phase after transformation whereas increasing Ni content reduces the martensite lath. The average proportion of dilatational strain energy of austenite in nonchemical driving force is approximately 41.3% in Fe-C-Ni alloys with atomic fractions of C < 1.0% and Ni < 20%. The prediction error of M_s in the Fe-C-Ni system is 4.1% using the modified model.

Due to its superior mechanical, processing, and service properties, oxide dispersion-strengthened (ODS) alloy (particularly Fe base ODS steel) has emerged as the most promising future candidate for advanced reactor structural materials. However, there are some problems with hot isostatic pressing sintering ODS steels, such as higher sintering temperature, longer sintering time, and relatively coarse grains. In this work, 14Cr-ODS steels were prepared by ultra-high pressure sintering with a sintering pressure of 3, 4, and 5 GPa, respectively. The microstructure and mechanical properties of the ultra-high pressure sintered 14Cr-ODS steel samples were characterized by density test, SEM, TEM, hardness test, and tensile test. Based on the contrast analysis of microstructure and mechanical properties, the effect of sintering pressure on the microstructure and mechanical properties of ultra-high pressure sintered 14Cr-ODS steel was investigated, and the effect mechanism was thoroughly analyzed. Analysis results show that the main oxide precipitate of ultra-high pressure sintered 14Cr-ODS steel is Ti₂O₃, and the average grain size of 14Cr-ODS steel prepared by ultra-high pressure sintering is less than 300 nm, which is approximately 5% of the average grain size of 14Cr-ODS steel prepared by conventional hot isostatic pressing sintering. The average grain size of samples prepared by ultra-high pressure sintering decreased initially and then increased as sintering pressure increased. The Vickers hardness of ODS steel samples sintered at 4 GPa can reach 604 HV, and the tensile strength is approximately 1.5 GPa, which is 1.6 times than that of 14Cr-ODS steel samples with a similar composition prepared by conventional hot isostatic pressing sintering. Ultra-high pressure sintered 14Cr-ODS steel with good sintering formability and a density greater than 99% can be obtained at lower sintering temperatures and shorter sintering times. Its excellent performance can be mainly associated with the comprehensive influence of the plastic deformation effect produced by ultra-high pressure sintering on grain nucleation and atom diffusion in sintered samples.

Nitrogen-alloyed austenitic stainless steel QN1803 (2.0%Ni-3.5%Ni) has been developed to replace the conventional 304 stainless steel (8%Ni). Both the microstructure and the corrosion resistance of both types of stainless steels in the annealed state have been extensively studied, whereas those in the cold strained state have not been studied sufficiently. The aforementioned stainless steels often undergo cold forming processes during industrial applications, such as straightening, leveling, and bending, etc., which may lead to the changes of microstructure and corrosion resistances as well as the performance. In this study, the tensile tests were performed with different tensile strains for both nitrogen-alloyed QN1803 and the conventional 304 stainless steels. The microstructure and strengthening, as well as the toughening mechanisms were investigated using EBSD, XRD, and TEM. The corrosion resistance and its mechanism under different tensile strains were evaluated and analyzed via electrochemical workstation, acid corrosion tests, OM, and SEM. Notably, the microstructures of both QN1803 and 304 stainless steels are changed from dislocation plugging to α' martensite with the increase in tensile strain. The yield strength of QN1803 stainless steel is 26% higher, while its elongation is 6.6% lower than that of 304 stainless steels, respectively. This could be attributed to both the higher nitrogen content and the lower transition temperature of 50% martensite induced by 30% strain (M_d30) for QN1803 stainless steel as compared with that for 304 stainless steel. As a result, the volume fraction of the strain-induced martensite for QN1803 stainless steel under tensile strain is lower, leading to the lower toughening effect than that of 304 stainless steel. Interestingly, the experiments show that the tensile strain has a minor effect on the intergranular corrosion whereas noticeably negative effect on both the pitting and the sulfuric acid corrosion resistances of both stainless steels. Not surprisingly, 304 stainless steel undergoes a more remarkable decrease in both the pitting and the sulfuric acid corrosion resistances with the increase in tensile strain as compared with QN1803 stainless steel. This could be well understood since martensite could lead to the destruction or deterioration of the passive film on the stainless steel surface, producing an unstable dissolution-generation state during corrosion, thus reducing the corrosion resistance after tensile deformation. In conclusion, the yield strength is enhanced, ductility is slightly impaired, and both the pitting and the sulfuric acid corrosion resistances under tensile deformation are improved by nitrogen alloying. Based on these technical advantages, together with the nickel saving effect, QN1803 stainless steel has been applied in various industrial areas, such as building, construction, and home appliance, etc.

A large amount of air is drawn into the high-temperature plasma jet during the atmospheric plasma spraying (APS) process because it operates in an atmospheric environment, thus oxidizing metal-spray particles. The oxide inclusion resulting from in-flight droplet oxidation inhibits the metallurgical bonding between lamellae in the coating, which limits the applications of plasma-sprayed metal coatings. In this study, a novel approach to create oxide-free molten droplets is proposed by adding B to the CuNi powder to achieve sacrificial oxidation of B in the high-temperature droplet and protect the alloy elements from oxidation. Two powders of CuNi2B and CuNi4B were prepared to deposit the coatings via APS. The effect of B content and spray distance on the microstructure, as well as the O content of CuNi coating, was studied using methods such as SEM, EDS, XRD, and inductively coupled plasma-optical emission spectrum (ICP-CES). The results show that the droplet can be heated to more than 1900^oC, and the introduction of B in the powder can inhibit the oxidation of alloy elements in the high-temperature droplet during flight, thus reducing the oxygen in the CuNi coating. Moreover, the deoxidizing effect is affected by the B content of the droplet. Using 4%B CuNi alloy powder and increasing spray distance, the oxide in the coating is reduced. The oxygen in the coating is introduced via oxidation after droplet deposition, and the oxygen content of the coating prepared using the optimized spraying process is reduced to 0.43%, which is considerably lower than 3.5% of CuNiIn coating. An increase in the spray distance and a reduction in B content of CuNi powder, which contains 1.83%B, to 0.5% is insufficient to inhibit the oxidation of the alloying elements of the in-flight particles. The result yields a critical B content of approximately 0.5% for high-temperature droplet oxidation protection. The increase in the B content decreases the melting point, as well as the oxidation of the alloy, thus enhancing the metallurgical bonding between CuNi particles and improving the compactness of the coating. In addition, with the increase in the B content of the coating through the powder composition design and process parameters control from 0.26% to 3.61%, the microhardness of CuNi coating increases from 151 HV_0.2 to 457 HV_0.2.

The high-entropy-amorphous alloy (also called a pseudo-high-entropy alloy), as one of the categories of high-entropy alloys, is being developed as a new type of functional and structural high-strength material due to its advantages of high entropy and amorphousness. To further develop a new high-entropy-amorphous alloy, based on the results of previous studies, (Fe_0.33Co_0.33Ni_0.33)_{84 - x} Cr₈Mn₈B _x (x = 10-18, atomic fraction, %) alloys were synthesized (hereafter, alloys 10B, 11B, 13B, 15B, and 18B). The ribbons with a thickness of 20-30 μm and a width of about 1.5 mm were produced by single-roller melt-spinning. The alloy ribbons were annealed at different temperatures and then characterized by XRD, DSC, TEM, SEM, OM, universal tensile testing machine, and Vickers hardness tester. The results show that the addition of B improves the glass-forming ability and thermal stability of these alloys. The ribbons are fully amorphous at the low B content of 11%. Two exotherms were observed based on the DSC curves for the 11B-18B as-spun ribbons. The crystallization behaviors of 11B-18B alloy ribbons are as follows during heating: 11B and 13B, [am]→[am′ + bcc]→[am″ + bcc + fcc]→[bcc + fcc + M₂₃B₆]→[fcc + M₂₃B₆]; 15B, [am]→[am′ + fcc + bcc]→[am″ + bcc + fcc + M₂₃B₆]→[bcc + fcc + M₂₃B₆]→[fcc+ M₂₃B₆]; and 18B, [am]→[am′ + fcc]→[am″ + fcc + M₂₃B₆]→[fcc + M₂₃B₆] (The am′ and am″ are the residual amorphous phases after the first and the second exotherms, respectively)

The microstructure and composition distribution of U_30.03Zr_28.83Ti_9.66Ni_7.00Cu_8.75Be_15.73 uranium-based amorphous composites were characterized using TEM and SEM in this study. The composite is a biphasic composite of α-U and amorphous phases dominated by zirconium and titanium. Both phases form a multilevel nested structure in the form of spherical precipitated phases. During cooling of the composite material, the phase separation of uranium and copper phases dominated by the nucleation-growth mechanism occurs first, and the other alloying elements are then preferably distributed according to the mixing enthalpy. The subsequent solidification is a two-step process: α-U solidification and amorphous transformation. A special structure with a transition layer is formed in the area where the amorphous phase is the spherical precipitated phase. These results provide new insights for the composition design and heat treatment adjustment of amorphous composite materials.

Tungsten (W) possess comprehensive physical and chemical properties that are suitable for aerospace and space nuclear power applications, including the highest melting temperature (3410^oC) among metals, high elastic modulus, thermal shock resistance, and high temperature strength. However, its poor ductility at room temperatures significantly hinders its fabricability and potential use in the above-mentioned fields. Accordingly, to improve the ductility of W, solid solution strengthening is the primary method considered besides grain refining and deformation strengthening. Experimental studies have shown that Ir is a brittle metal with an fcc structure, but it can greatly improve the ductility of W; however, the corresponding mechanism is still unclear. Thus, using the first principles method based on density functional theory together with phonon spectrum calculations, the effect of the addition of different contents of Ir on the structure, phase stability, mechanical properties, and thermodynamic properties of W were studied. The relation between the addition of different contents of Ir and above-mentioned properties of W-Ir alloys were theoretically investigated. It was found that Ir can induce instability in the W-Ir alloy in the ground state due to the occupation of its antibonding electrons below the Fermi level. When content of Ir added is less than 7.4%, the formation of the W-Ir alloy becomes stable in the ground state. With an increase in temperature and the content of Ir, the thermodynamic stability is improved, implying that Ir is suitable for incorporation with W for application at high temperature. The addition of Ir helps to improve the toughness of the W alloy, which is consistent with the experimental observation. Besides, Ir can simultaneously improve the planar shear resistance. Furthermore, the pCOHP analysis revealed that the inherent mechanism of the ductile effect of brittle Ir in W is attributed to their different modes of electron transition and overlapping. For Ir, electrons transfer from its higher energy orbital of {\mathrm{d}}_{x^{\mathrm{2}}-y^{\mathrm{2}}} to the lower energy d _xz and d _yz orbitals. In contrast, for W, the electrons transfer from its low energy orbital of {\mathrm{d}}_{z^{\mathrm{2}}} to the d _xzand d _yz orbitals. The d _xzand d _yzorbitals of Ir and W form a metallic bond, which is further enhanced with an increase in the content of Ir added. Therefore, Ir acts as a toughness-enhancing element in W-Ir alloys.

Inconel 718 alloy is an Fe-Ni based superalloy precipitation-strengthened by γ″ phase (Ni₃Nb) and γ′ phase (Ni₃(AlTi)). It has been widely used in aviation, energy, chemicals, and other fields because of its outstanding mechanical properties, resistance to high-temperature oxidation, and corrosion resistance. Because the mechanical properties of Inconel 718 alloy are primarily determined by the γ″ precipitates, Nb becomes one of the most important alloying elements. Due to the high content, large atomic radius, and small partition coefficient of Nb, Nb segregation occurs easily during the solidification process of casting, welding, and laser cladding. The segregation drastically degrades mechanical properties and increases the difficulty of subsequent heat treatment. The composition of Inconel 718 alloy comprises many elements, and some trace elements are inevitably introduced from the raw materials. The interaction of the elements has a certain effect on Nb segregation. In this study, the effect of the interaction of elements caused by doping of Cu on Nb segregation in Inconel 718 alloy was studied by first-principles calculation and experiment. The Ni-Fe-Cr-Nb supercell model was constructed with and without Cu doping. The enthalpy of formation, cohesive energy, state density, electron density difference, and population analysis was calculated. The calculation results show that the doping of Cu reduces the stability of the system. Doping will change the interaction between elements and affects the strength and density distribution ratio of the charged density between atomic bonds in the system. The addition of Cu increases the bond strength between the Fe atoms and Cr atoms and the repulsive force between Fe atoms and Nb atoms in the matrix. The experimental results show that the addition of 0.1%Cu (mass fraction) decreases the segregation of Fe and Cr, but promotes the segregation of Nb. Experimental results and first-principles calculations show that the increase in the repulsive force between the Nb atom and Fe atom, which is caused by the interaction between the alloying elements after doping with Cu, is the essential reason for Cu to promote Nb segregation.

The migration behavior of <111> symmetric tilt grain boundaries (GBs) having different misorientation angles was simulated using a molecular dynamics synthetic driving force method. The effect of temperature on the migration behavior was investigated in the temperature range of 300-800 K. The results demonstrated that the temperature dependencies of GB migration varied with the misorientation. GBs with a misorientation of 8.61°-21.79° exhibited antithermally activated migration, whereas those with a misorientation of 38.21°-60° exhibited thermally activated migration. For the GBs having a misorientation of 27.80°-32.20°, there was an apparent transition from thermally activated migration at low temperature to antithermally activated migration at high temperature. The mobility of the GBs having a misorientation of 8.61°-21.79° was much higher than that of other GBs, but the differences between them decreased with increasing temperature. The GB structures at different temperatures can be well described using the structural unit model. GBs with structures consisting of similar types of structural units exhibit comparable temperature dependencies in their mobility. The complex temperature dependencies of migration behavior shown by some GBs appear to be related to structural changes featured by the transformation between variants belonging to the same type of structural units.