By virtue of their high thermal conductivity, low thermal expansion coefficient, and excellent high-temperature creep strength, high-Cr (mass fraction: 9%-12%) martensitic heat-resistant steels are the putative main constituents of the key equipment in ultra-supercritical (USC) power plants. However, the harsh environment caused by enhancing the steam parameters has recently challenged the high-temperature properties and the continually deteriorating creep strength during service has seriously threatened the safety and reliability of these steels. Previously, the creep strength of high-Cr martensitic heat-resistant steels was enhanced by optimizing the alloying compositions to promote the dispersed precipitation of strengthening phases, but the enhancement effect of reinforced single-precipitate strengthening is limited. In recent years, synergistic strengthening reinforcement of dislocation-precipitate-interface has emerged as a promising solution because the introduced dislocations promote various precipitations and the phase transformation can be controlled to tailor the lath structure, thus reinforcing the dislocation-precipitate-interface interactions and synergistically enhancing various strengthening effects. This paper overviews the synergistic strengthening of dislocation-precipitate-interface and microstructure control in high-Cr martensitic heat-resistant steels subjected to thermo-mechanical treatments. The review covers alloying optimization to improve the creep strength, the phase transformations during heating treatments, and the mechanism of microstructural degradation at high temperatures. It also compares the effects of single-precipitate and synergistic strengthening processes on creep strength and introduces microstructure control in welded joints by thermo-mechanical treatments in terms of creep failure behaviors. This research aims to guide the design and engineering applications of high-Cr martensitic heat-resistant steels and other precipitate-strengthening heat-resistant steels for USC power plants.

The thermal power generation industry in China is facing heavy pressure from environmental protection sectors as the “emission peak-carbon neutrality” goal has been proposed. Oxyfuel combustion and operating with high steam parameters are considered promising technologies that can effectively reduce CO₂ emissions from coal-fired power plants. However, using these two technologies, the currently used ferritic/martensitic heat-resistant steels and austenitic stainless steels in traditional boilers cannot meet the requirements of good creep strength and corrosion resistance to hot CO₂-rich gasses. Thus, nickel-based alloys must be considered. Given that flue gasses related to oxyfuel combustion are characterized by high CO₂ concentrations, this work reviews recent research progress on the high-temperature corrosion of Ni-Cr alloys in CO₂-rich gasses. Herein, the effect of CO₂ on protective Cr₂O₃ scale formation is introduced, and the related carburization mechanism caused by CO₂ ingress is clarified. Moreover, the impacts of H₂O(g), SO₂, temperature, and alloying elements on the high temperature resistance of Ni-Cr alloys in CO₂-rich gasses are summarized. Based on the current findings, future research on high-temperature corrosion of Ni-Cr alloys in CO₂-rich gases should focus on the following key points, such as analyzing the microstructure of Cr₂O₃ scales formed in different gasses; elucidating the interaction of CO₂, H₂O(g), and SO₂ molecules with rare earth elements at grain boundaries of oxide scales; and investigating the effect of HCl(g) impurities in CO₂-rich gasses on Cr₂O₃ scaling of Ni-Cr alloys.

Mg-Zn series alloys, an important alloy system among magnesium alloys, have garnered considerable attention due to their rich phase composition, outstanding deformability, and aging strengthening effects. These alloys demonstrate great potential for applications in the aerospace, automotive, and biomedical industries. However, the wide solidification temperature range and large shrinkage of these alloys render them largely susceptible to hot tearing, limiting their applications to a certain extent. Thus, investigating the hot tearing behavior of Mg-Zn series alloys is important. In this paper, a comprehensive summary of theories pertaining to hot tearing, the effects of alloying elements and casting process parameters on the susceptibility of Mg-Zn series alloys to hot tearing, and the current status of research on the numerical simulation of this phenomenon are presented. Furthermore, this article discusses the influence of microstructure and solidification parameters of Mg-Zn series alloys on their hot tearing susceptibility and proposes limitations and suggestions for the current research on the hot tearing behavior of magnesium alloys to guide the design and application of Mg-Zn series alloys.

Mg and Mg-based alloys are distinguished by their high specific strength-to-density ratios but demonstrate low strengths at ambient to elevated temperatures. Producing Mg-based composites offers an effective means of strengthening Mg. Nevertheless, the mechanical properties of Mg-based composites are primarily dependent on their architectures. Here, bioinspired Mg-based composites with fish-scale-like orthogonal plywood and double-twisted Bouligand-type (i.e., double-Bouligand) architectures were fabricated by the pressureless infiltration of an Mg melt into the woven contextures of stainless steel fibers. The phase constitution, microstructure, and tensile properties of the composites at room temperature and 200^oC were compared with a composite where stainless steel fibers were randomly oriented in-plane. The relationships between the microstructure and mechanical properties were also explored. The results showed that the stainless steel fibers played a notable role in strengthening the composites and were pulled out from the Mg matrix to promote plastic deformation and energy consumption. The mechanical properties of the composites were closely associated with their microstructures, with fish-scale-like architectures displaying higher strengths and larger plasticity than the randomly oriented ones. In particular, the double-Bouligand architecture allowed coordinated deformation between the fibers of different orientations and promoted crack deflection along the fibers, thereby alleviating the localization of deformation and damage in the composite. Therefore, it bestowed larger plasticity at room temperature and higher tensile strength at high temperature. By exploiting new bioinspired architectures, this study provides guidance for optimizing the architectural design of Mg-based composites to improve their mechanical properties.

Single-crystal superalloys have been developed to the 5th generation to improve their temperature capacity. Thus, a rare metal Ru is doped to the 4th and 5th generations based on the 6%Re (with the same mass fraction) contained in third-generation superalloys. Compared with Re addition in low-generation superalloys, improvement of temperature capacity decreases with Ru addition in high-generation superalloys; however, the cost of superalloys containing Ru has increased significantly. Therefore, considerable attention must be paid to the development of third-generation single-crystal superalloys because of their superior cost performance. Thus, considering the slight precipitation of the topologically close-packed (TCP) phase and low properties at the intermediate temperature of third-generation single-crystal superalloys, Al is considered as a significant element affecting microstructure stability, which is determined by calculating the number of electron vacancy (N_v). By reducing 0.4%Al, no TCP phase is precipitated in the superalloy after long-term thermal exposure at 1100^oC for 1000 h; therefore, good microstructure stability is obtained. The concentration of Re and Co is decreased slightly to increase the stacking fault energy of the superalloy and to enhance the properties at intermediate temperature. The stress rupture life at 760^oC and 800 MPa extends from 40 h to 150 h by reducing 0.4%Al followed with reduction of 0.25%Re and 1%Co. Moreover, the stress rupture properties at high temperature remain unchanged. Based on the abovementioned research, a third-generation single-crystal superalloy is developed, and the causes of stabilization of the microstructure and improvement to properties at intermediate temperature are also discussed.

Al alloy and Mg alloy are not only the lightest metal structural materials, but also have the advantages of high specific strength and damping performance, which are very attractive for automobile, high-speed rail and aerospace. To meet the requirements of structural lightweight and different service environments, it is usually necessary to join Al alloy and Mg alloy into a complete structure. As a new solid-state joining method, friction stir welding (FSW) has obvious advantages in the field of Al alloy/Mg alloy hybrid structure because its welding temperature is lower than the melting point of base metal. However, the formation temperature of Al alloy/Mg alloy intermetallic compounds (IMCs) is lower than the melting point of Al and Mg, if the thickness of base metal exceeds 10 mm, the Al alloy/Mg alloy FSW is still very difficult because of the formation of IMCs in the weld. To obtain some control methods of the formation of IMCs, 5A06-H112 Al alloy and AZ31B-O Mg alloy plates with a thickness of 15 mm were used for the Al alloy/Mg alloy FSW. Liquid nitrogen was sprayed near the rear of the stirring head to locally cool the upper surface of the weld. EBSD and TEM instrument were used to obtain the phase distribution and grain orientation spread at different positions of the welded joint. The precipitation behavior of IMCs at the interfaces on both sides of the stirring zone (SZ) of Al alloy/Mg alloy joints under ambient temperatures and liquid nitrogen cooling conditions was studied. The results indicate that Al₃Mg₂ mainly precipitates on the Al alloy side of SZ; the main precipitation on the Mg alloy side is Al₁₂Mg₁₇, and it was generated eutectic reactions with Mg at the upper interface; liquid nitrogen cooling on the surface can reduce the peak temperature and high-temperature residence time at various positions of the joint, and has a significant inhibitory effect on the precipitation of IMCs and the formation of low melting point eutectic. When surface cooling is not applied, almost only Al₃Mg₂ phase is precipitated in the SZ at the upper and middle parts of near the interface of Al alloy side, and at the bottom, only fine equiaxed Al grains are observed in the SZ. At the side of magnesium alloy, Al₁₂Mg₁₇ phase is mainly precipitated at the upper interface of SZ and it forms low melting point eutectic with Mg, and at the same time, a thin layer of Al₃Mg₂ is precipitated between the eutectic zone and the Al alloy in SZ. Two layers of Al₃Mg₂ and Al₁₂Mg₁₇, which are sticked close to each other and there is a distinct boundary layer between them, is precipitated between SZ and the Mg alloy at the middle and lower interfaces, and the total thickness of the IMCs layer at the middle interface is much greater than the IMCs layer thickness at the bottom interface, at here the peak temperature is lower, the thickness of the Al₃Mg₂ layer is decreased more significantly. When liquid nitrogen cooling is applied to the weld surface, in addition to Al₃Mg₂ phase precipitation, there are also a small number of Al₁₂Mg₁₇ and Al grains in the SZ at the upper part of the interface of Al alloy side, while equiaxed Al grains are present in the SZ at the middle and bottom parts of the interface. At interface close to the Mg alloy side, the precipitation behavior of IMCs in each parts is similar to that without surface cooling, but the total thickness of the Mg + Al₁₂Mg₁₇ eutectic layer and Al₃Mg₂ layer precipitated at the upper part of the interface, and the thickness of the IMCs layer at the middle and lower SZ interfaces are significantly decreased, and the thickness of the Al₃Mg₂ thin layer decreases more significantly. The strain rate has a significant impact on the precipitation of IMCs, and it is confirmed by that, the actual thickness of the interface layer is much greater than the theoretical thickness calculated by the diffusion law.

A matrix structure with high strength, such as lath martensite/bainite is created via quenching to achieve conventional high-strength low-alloy ultra-heavy plates. Subsequently, this structure is tempered to improve its toughness. However, it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling, causing inhomogeneous microstructure and mechanical properties along the normal direction. Therefore, it is necessary to enhance the hardenability of the alloy. At lower cooling rates, granular bainite/ferrites are formed in the center of the plates with low hardenability. While this leads to the incompletely transformed martensite/austenite islands (M/A islands), which often cause cracks, fewer high angle grain boundaries (HAGBs) are also formed, which can effectively impede crack propagation. Therefore, improving the strength, toughness, and hardenability is crucial for the development of high-strength low-alloy steel. The addition of nickel can improve the hardenability as well as the toughness of the heavy plates. In this study, two high-strength low-alloy steels with different nickel contents are designed. In addition, the effect of nickel content on hardenability and phase transition temperature is tested using end quenching and thermal mechanical simulation testing. The effects of nickel content on the microstructure and crystallographic characteristics of coherent phase-transformed products are characterized using SEM and EBSD. The results reveal that the increased nickel content greatly improves the hardenability and significantly reduces the phase transition temperature. At a low cooling rate of 0.5^oC/s, the microstructure of 2.94Ni steel is lath bainite, and the M/A islands are dispersed on a thin film, forming a phase transformation mode with higher HAGB density, block boundary density and V1/V2 variant pair content, and high hardness. This mode is dominated by the close-packed plane group. While the microstructure of 0.92Ni steel is granular bainite and the M/A islands are distributed in coarse blocks, forming a phase transformation mode with lower HAGB density, block boundary density and V1/V2 variant pair content, and significantly low hardness. Moreover, this mode is dominated by the Bain group. Additionally, the results demonstrate that at the cooling rate of 0.5^oC/s, as nickel content increases, the driving force of phase transformation is greatly improved to obtain a higher transformation rate than the steel with low nickel content. The maximum carbon content of untransformed austenite is higher, which promotes the complete transformation of bainite and produces fewer M/A islands. Therefore, this research possesses great potential for the composition design and process control of high-strength low-alloy steel.

Fossil-fired thermal power generation has dominated China's electricity production for a long time, contributing to around 70% of the total capacity. Developing long-life ultra-supercritical thermal power units is essential for improving coal-fired power generation efficiency, reducing harmful gas emissions, and achieving national energy conservation and emission reduction targets. The assembly and manufacture of advanced heat-resistant steel grades are required to address the above demands, serving as crucial components driving the technological advancement of thermal power units. Heat-resistant steel grades P11, P22, and P91, which are Cr-Mo based ferritic, possess a range of highly attractive properties, such as excellent mechanical properties, excellent corrosion resistance, and relatively low construction costs. These steel grades are widely used in pressure vessels and pipelines focused on high-temperature applications. Fusion welding techniques are invariably necessary to weld such heat-resistant-grade steels before they are positioned in high-temperature service. However, it is worth noting that drastic solid-state phase transformations in the heat-affected zones (HAZs) during thermal welding cycles can profoundly influence the heterogeneous microstructures of welded joints, determining their final mechanical properties to a large extent. Furthermore, it seriously threatens the safe and stable operation of thermal power plants. High-temperature confocal scanning laser microscopy (CSLM) revolutionized traditional metallographic experiments, enabling real-time morphology and quantitative analysis tracking. This innovation has facilitated investigations into the kinetic phase transformation process and microstructure evolution in steels at high temperatures. In this work, the kinetics of phase transformation and microstructural evolution in the HAZs of P11, P22, and P91 ferritic heat-resistant steels during continuous cooling processes were systematically investigated using CSLM. The results revealed that bainite laths preferentially nucleate in the order of increasing difficulty in the energy barrier on austenite grain boundaries, inclusions, internal grain distortion areas, previous bainite laths, and grain interiors. Meanwhile, the growth characteristics of bainite/martensite laths were documented as the phase transformation progressed. It is revealed that bainite laths attach to prior austenite grain boundaries and the previous bainite, while martensite laths grow radially inside the prior austenite grains. Both bainite and martensite laths cease growing when they encounter grain boundaries or other laths, eventually forming an interlocking microstructure. Additionally, the growth rates of bainite/martensite laths in the HAZs of P11, P22, and P91 ferritic heat-resistant steels exhibited considerable variations as the temperature decreased. The analysis revealed that as the temperature decreased, the growth rate of laths in the coarse-grained heat-affected zone was considerably higher than that in the fine-grained heat-affected zone, which can be attributed to the increase in the degree of supercooling and prior austenite grain size.

Recently, medium Mn steels (MMnS) have been extensively investigated because of the excellent mechanical combination of strength and ductility achieved at the relatively low alloying cost. Intercritical annealing (IA) is a key process of MMnS to form intercritical austenite that can be retained fully or partially at room temperature, which can trigger transformation-induced plasticity and then improve work hardening during deformation. However, this process leads to a relatively low yield strength because the recovery, recrystallization, grain growth, coarsening, and dissolution of precipitates could occur during IA. In this study, the microstructural evolution and resultant mechanical properties of Cu-V dual alloyed 3Mn steel were examined during two manufacturing processes: hot rolling → warm rolling at 550-650°C → IA at 690°C for 10 min (termed as WR-IA) and hot rolling → aging at 550-650°C for 70 min → IA at 690°C for 10 min (termed as Aging-IA). That is,the two processes differentiate in either the warm rolling or the aging process used as the intermediate process. WR-IA specimens exhibit significantly higher ductility than Aging-IA ones, but they both have the same yield strength. The former is attributed to a large quantity of defects introduced during warm rolling, which promoted austenite reverse transformation during IA and led to a large fraction of retained austenite. The resultant tensile properties include yield strength of 1230-1320 MPa and ductility of 23%-29%, which is superior to those of either V- or Cu-alloyed MMnS published in references. In particular, higher yield strength was achieved because the dual alloying of Cu-V and the two-stage thermomechanical process, that is,warm rolling plus IA, are adopted. The first warm rolling promoted Cu-rich precipitates dispersed for strengthening, and the precipitation of VC during subsequent IA could compensate for the softening caused by IA. Consequently, a high yield strength was achieved. Meanwhile, 25%-30% fraction of austenite was retained, thereby providing transformation-induced plasticity during deformation, leading to high ductility.

Alloying is often used to improve resistance to hydrogen-induced pulverization and cracking of hydrogen storage materials such as titanium and zirconium. However, it often affects the hydrogen storage performance of the material itself. Ti-Y alloys exhibit good mechanical properties, and they can effectively suppress hydrogen embrittlement. The properties of hydrogen absorption and desorption were investigated experimentally and theoretically in the present work. Y was uniformly doped into Ti films as a substitution atom using direct current magnetron sputtering. In addition, a Ni film of about 5 nm was subsequently deposited onto all sample surfaces to reduce surface contamination. Deuterium gas (D₂) was used for hydrogen absorption experiment. Hydrogen absorption results show that the deuterium concentration in Ti-Y films increases with the increase of Y concentration. Combined with the density functional theory (DFT) calculation, the effects of Y doping on the hydrogen absorption properties of Ti could be summarized as follows: (1) the binding energy of Y to H calculated by DFT is stronger than that of Ti, thereby increasing the concentration of D absorbed; (2) Y has a strong affinity for O to form Y₂O₃, which reduces O impurity concentration in Ti film and facilitates more D atoms to enter the Ti lattice to increase the amount of D absorption; (3) Y substitutes for the Ti atom to increase the binding energy of Ti—H adjacent to Y, making the D atom less likely to escape, and to reduce the diffusion barrier of D around Ti, which is distant from Y, making it easy for D to diffuse deeper into the sample. Therefore, the concentration of D absorbed in Ti samples increases with the increase of Y concentration. With regard to the properties of D desorbed in Ti-Y samples, the results show that the D desorption activation energy of the deuteride Ti-Y film could be increased by doping Y. The D desorption temperature is determined by the D thermal desorption kinetics of the Ni/Ti-Y film system, and Y doping may increase the apparent binding energy and diffusion activation energy for D of the overall Ti lattice. The surface potential barrier has an important effect on D desorbed from Ti. Furthermore, Y doping has a certain degree of influence on the hydrogen absorption and desorption performance of Ti thin films.

With the increase of global energy consumption and related environment pollution, new types of renewable clean energy resources and carriers are desirable. Given its high gravimetric energy density and combustion product (i.e., water), molecular hydrogen has attracted considerable attention. Obtaining molecular hydrogen from water splitting is the ideal strategy because inputs and outputs are carbon-free clean matter. In achieving this process, a suitable and highly efficient catalyst is a crucial parameter. Novel metal Pt is an excellent catalyst with high efficiency and chemical stability. However, owing to its high cost and insufficient reserves on Earth, the wide application of Pt in catalysis is strongly limited. Correspondingly, the design of a highly efficient hydrogen evolution reaction (HER) catalyst with low Pt loading is an important task for electrochemical water splitting in the field of renewable energy resources. Understanding the hidden mechanism is essential for the guiding principle of such a design. In this study, an excellent HER catalyst in cubic Pt₇Sb is proposed, in which Gibbs free energy for hydrogen adsorption (\mathrm{\Delta }{G}_{{\mathrm{H}}^{\mathrm{*}}}) is smaller than that from Pt. Thus, together with its good chemical stability, a better HER catalytic activity with reduced Pt loading can be obtained. Based on the analysis of electronic structures, a good agreement between the two descriptors of \mathrm{\Delta }{G}_{{\mathrm{H}}^{*}} and the projected Berry phase (PBP) is revealed. Considering that the PBP is purely decided by the bulk state, such an agreement indicates a strong relationship between the good catalytic performance and the topological nature of the intrinsic electronic structure. This work provides an excellent HER catalytic candidate with reduced Pt loading and a good example to show the role of the intrinsic topological nature in catalysts.

The microstructure and deformation mechanisms determine the strength and plasticity of structural materials. Specifically, the nucleation and movement of dislocations play a crucial role in the plastic deformation processing of crystalline materials. Just like the phase transformation, the plastic deformation and evolution of dislocations can be described as a kinetic behavior resulting from a thermodynamic driving force. In recent years, this research group has introduced the concept of thermo-dynamic correlation,which reflects the correlation between thermodynamics and kinetics as a trade-off relationship between thermodynamic driving forces (ΔG) and kinetic energy barriers (Q). It was found that an increase in ΔG is always accompanied by a decrease in Q in the processes of phase transformations and plastic deformations, and vice versa. Based on the so-called synergy rule of thermodynamics and kinetics, a new idea for the design and optimization of mechanical properties of materials has been proposed, that is the generalized stability criteria for phase transformation and deformation. The ΔG and Q are correlated by the concept of generalized stability, and it was suggested by many cases of metallic materials design that high driving forces and high generalized stability correspond to high strength and high ductility. To understand the thermo-dynamic correlation in the material deformation process and apply the generalized stability criteria for materials design, it is essential to comprehend the law of dislocation movement and evolution, and quantitatively describe the relationship between the driving force and the energy barrier of dislocation movement. The molecular dynamics (MD) simulation method has become an important means of studying the deformation mechanism of materials, dispite some limitations such as high deformation rate and small size of the description object. In this work, the thermo-kinetic behaviors of α-Fe deformation along [010], [111], [1\overline{\mathrm{1}}0], and [11\overline{\mathrm{2}}] crystal directions under uniaxial tension were studied by using MD simulation. The dislocation generation and evolution of α-Fe during the tensile process were analyzed. The results indicate that the yield strength of the material varies along the grain direction, with the order from high to low being [111], [110], [11\overline{\mathrm{2}}], and [010]. When stretching along different crystal directions, the trend of dislocation density, dislocation type, and dislocation initiation time differs. The earlier the dislocation initiation time, the lower the yield strength. Generally, the dislocation initiation time of single crystal iron advances with an increase in temperature and a decrease in elastic modulus and strength. The results of thermo-kinetic and generalized stability analysis of dislocation evolution show that the thermo-kinetic driving force is opposite to the kinetic energy barrier, and the generalized stability value depends on crystal orientation and temperature.