Service reliability of deep-sea submersible pressure shells is critical for ensuring the safety of the submersibles. To manufacture pressure shells for deep-sea submersibles, titanium alloys have emerged as key materials owing to their exceptional service performances in the deep-sea environment. Herein, use of titanium alloys in deep submersibles is introduced. Then, the latest research on the primary failure modes of titanium alloys, including creep at room temperature, low cycle fatigue, and dwell fatigue, based on the types of titanium alloys used in deep-sea submersibles is reviewed. Additionally, the main factors that affect dwell fatigue, including the micromechanism of dwell fatigue damage and dwell fatigue model, are summarized. This work can serve as a reference for the development of new titanium alloys with high strength and low dwell effect. Finally, specific issues related to the service reliability evaluation of titanium alloy components used in the deep sea are outlined, and future research focuses are presented.

Owing to the multi-principal element and higher intrinsic configurational entropy, high-entropy alloys exhibit excellent mechanical and physicochemical performance, which has garnered extensive attention from researchers. By virtue of the excellent performances in terms of superior strength, ductility, toughness, impact resistance property, and adjustable phase stability, especially in cryogenic environments, high-entropy alloys have broad application prospects in fields such as deep-space exploration, low temperature superconducting, and the gas industry. In this paper, the deformation and strengthening-toughening mechanisms of high-entropy alloys are summarized by reviewing the cryogenic progress. Furthermore, the promising research directions of high-entropy alloys in cryogenic engineering application combined with the performance of traditional cryogenic materials are also presented.

Over the past few decades, electronic products have evolved towards miniaturization, intelligence, and multi-functionality. With the rapid development of new energy vehicles and 5G mobile communication technologies, solder, the most commonly used interconnecting material in the microelectronic industry, may continuously undergo alternating temperature excursions. As a result, researchers have focused on improving the thermomechanical reliability of solder joints. For several decades, researchers have widely studied Sn-based lead-free solder and have established that adding an alloying element or foreign reinforcement can overcome the limitations of traditional Sn-based binary/ternary solder, resulting in highly reliable solder joints. Recently, the interest in Sn-based alloys and composite solders has increased due to their improved mechanical performance. However, various concerns, such as high manufacturing costs, microstructural heterogeneity, and incomplete reliability data. This paper reviews the latest research progress on Sn-based lead-free solders for microelectronic interconnection over the past five years, in particular Sn-based multi-element alloys and composite solders. First, the advantages and disadvantages of typical solder preparation methods are compared and discussed. Second, the effects of an alloying element or foreign reinforcement additions on the solder's microstructure, properties, and thermomechanical reliability are summarized. Finally, this paper presents the main problems in preparing and investigating Sn-based lead-free solders and proposes tentative solutions. The aim is to provide an essential basis for understanding the current development and future research directions for fast-evolving future application scenarios.

Iron-based metal/ceramic nanocomposite materials have attracted increasing attention owing to their outstanding mechanical, electrical, and magnetic properties with potential applications in many industrial fields. However, several technical routes, such as mechanical alloying, sol-gel, and electrodeposition, have limitations, including lengthy synthesis processes, complex experimental equipment, and expensive raw materials. In view of the urgent demand for high-quality iron-based metal/ceramic magnetic nanocomposites, Fe-Y₂O₃ nanocomposite powders with different Y₂O₃ contents (mass fraction) have been prepared using a combustion-based route. The effects of the Y₂O₃ content on the microstructure, grain size, and magnetic and sintering properties of the nanocomposite powders were examined. The Fe-Y₂O₃ nanocomposite powders exhibited a connected network structure composed of nanoparticles regardless of the Y₂O₃ content, but the grain size decreased gradually with increasing Y₂O₃ content. The magnetic performance test showed that the iron nanopowder without Y₂O₃ had a saturation magnetic induction and coercivity (H_c) of 1.97 T and 6.4 kA/m, respectively. The saturation magnetic induction of the Fe-Y₂O₃ nanocomposite powders decreased gradually with increasing Y₂O₃ content, whereas the H_c increased. The saturation magnetic induction and H_c of the Fe-Y₂O₃ nanocomposite were 1.45 T and 58.9 kA/m, respectively, at a Y₂O₃ content of 2%. The as-synthesized Fe-Y₂O₃ nanocomposite powders were densified by pressureless sintering. When the Y₂O₃ content was low, the nanocomposites could reach a higher relative density at a lower sintering temperature of 700^oC. In contrast, densification was difficult to achieve when the Y₂O₃ content was increased to 1% or 2% even at a high sintering temperature of 1300^oC.

Owing to its outstanding advantages, such as low specific gravity, high specific strength, and good machinability, 2024 aluminum alloy has been used as various load components in the aerospace field and has become an important lightweight material. The properties of the 2024 aluminum alloy are highly correlated with its microstructures. Accordingly, in this study, 2024 aluminum alloy deposited specimens were fabricated using wire arc additive manufacturing. Further, the microstructures and mechanical properties of the deposited specimens were investigated in different regions. The layered characteristics could be observed macroscopically in the deposited specimens, and a single deposition layer was divided into two regions: interlayer and innerlayer. The grain morphology changed from equiaxed grains in the innerlayer region to columnar grains in the interlayer region. The deposited specimens mainly included α-Al, θ-Al₂Cu, and S-Al₂CuMg phases. In the nonequilibrium solidification process of additive manufacturing, the deposited specimens presented element segregation. The distribution of Mg in the Al matrix was uniform for the innerlayer region. However, Cu was segregated as eutectics at the grain boundary in the interlayer region. The average tensile strength, yield strength, and elongation of deposited specimens were (323.5 ± 6.6) MPa, (178.7 ± 6.2) MPa, and (9.03 ± 0.67)%, respectively, which were higher than those of cast annealing 2024 aluminum alloy. Owing to the difference in the microstructure, the innerlayer and interlayer regions showed different crack propagation behavior. The cracks in the interlayer region propagated along the distribution path of eutectics, showing intergranular fracture, and the crack propagation mode in the innerlayer region changed to transgranular fracture.

Multielement and multiphase intermetallic alloys based on an ordered orthorhombic (O) phase Ti₂AlNb, where the presence of a long-range order superlattice structure effectively impedes the movement of dislocations and high-temperature diffusion, are a class of highly promising lightweight high-temperature structural materials for aerospace applications due to their high specific strength and superior fracture toughness. Thermal stability of microstructures in the hot rolled sheet of a low-density Ti₂AlNb-based alloy has been investigated in a temperature range from 600^oC to 1100^oC for 12 h via OM, SEM, XRD, and TEM/STEM. The results showed that the initial Ti₂AlNb-based alloy hot rolled sheet consisted of α₂, B2, and O phases. Furthermore, the Ti₂AlNb-based alloy hot rolled sheet at 600^oC for 12 h consisted of α₂, B2, and O phases, where the particle shaped α₂ phase was distributed in the B2 matrix, and lath-like O phase lay inbetween the α₂ particles. The spheroidization of the α₂ phase started to occur along with the coarsening and solutionizing of the lath O phase in the B2 matrix at a temperature between 800^oC and 900^oC for 12 h, while the hot rolled Ti₂AlNb-based alloy plate was still composed of α₂, B2, and O phases. When the temperature reached 950^oC, the O phase disappeared in the B2 matrix. Only α₂ + B2 two phases were present in the hot rolled Ti₂AlNb-based alloy at 950-1000^oC for 12 h, where the α₂ phase was spheroidized and tended to distribute surrounding B2 grain boundaries. When the temperature rose to 1100^oC, the alloy contained a B2 single phase with only some residual α₂ phase. Moreover, the Vickers microhardness contour vs temperature plot revealed that a peak hardness of as high as 509 HV appeared at 600^oC due to the presence of numerous fine O laths.

Designing high-performance aeroengine is important for development in the aviation industry. One of the key components is turbine disk material that can operate at 800^oC. Among various methods for strengthening alloys, increasing the alloying degree is important, and GH4151 is one of the typical alloys with a high alloying degree. It comprises a large number of refractory metal elements and γ'-forming elements. OM, SEM, and JMatPro software were used to study the sensitivity of GH4151 microstructure evolution during heat treatment processes. The results show that a high alloying degree produces a complex microstructure with low-melting phases, such as Laves, γ/γ′ eutectic, and η phases. Due to the difference in incipient melting temperature of each precipitated phase, a three-stage heat treatment was developed to effectively eliminate the harmful phases in the alloy. The contents of segregation elements Nb and Ti in the as-cast GH4151 alloy have an obvious influence on the incipient melting temperature, whereas the effect of Mo content is relatively slight, and that of W content is not obvious. Decreasing Ti content while increasing Nb and Mo contents could reduce the incipient melting temperature of the η phase. Furthermore, increasing Ti and Mo contents while decreasing Nb content could reduce the incipient melting temperature of Laves phase. A large amount of γ'-forming elements contributes to the cooling rate sensitivity of γ′ phase evolution. 15^oC/min is the critical value for the irregular growth of the γ' phase in the GH4151 alloy. When compared to alloys with low γ′-forming elements content, the γ′ phase in GH4151 alloy has a larger size when the cooling rate is > 15^oC/min, and exhibits an irregular shape when the cooling rate is < 15^oC/min. Thus, a high alloying degree contributes to the complex and sensitive microstructure evolution behavior of GH4151 alloy.

With the improvement of the steam parameters of thermal power units, the requirements put forward for the stress rupture strength and structural stability of heat-resistant materials for boiler superheater/reheater pipes become higher. SP2215, as a new heat-resistant alloy, is an excellent candidate for 620-650°C ultra-supercritical boiler superheater/reheater. In this study, the correlation between microstructure evolution and properties of the SP2215 heat-resistant alloy aging at different temperatures and time was studied via a series of creep and impact tests. The results show that the SP2215 alloy has excellent microstructure stability in high-temperature condition. Moreover, various nanoscale precipitations such as Cu-rich, MX, NbCrN, and M₂₃C₆ phases occur during aging. In the early period of aging, with the increase in aging temperature and aging time, the precipitations increase rapidly, improving the strength of the material; however, the impact toughness of the SP2215 alloy decreases considerably, with substantial intergranular fracture caused by the continuous precipitation and growth of M₂₃C₆ at the boundary, as shown with the quantitative calculation using the JMA model. In the late period of aging, the precipitations gradually stabilize, and the grain size remains in the range of 4.5-5 grade. As a result, the 1 × 10⁵ h stress rupture strength of the SP2215 alloy at 650 and 700^oC still remain more than 120 and 70 MPa, respectively. Hence, the alloy can be used as a domestic replacement for foreign HR3C, Super304H, and other similar heat-resistant alloys.

Mg-Sn alloy is a high temperature-creep resistant magnesium alloy that has potential applications in lightweight automobiles. The addition of Sn to Mg can reduce the overall cost of the alloy as Sn is cheaper than rare earth elements. Sn and Mg form Mg₂Sn phase on the grain boundary, and this Mg₂Sn phase has an excellent precipitation hardening effect. However, coarsened Mg₂Sn phase can reduce the age hardening effect of the alloy. Previous experimental studies have showed that the addition of Al element can considerably improve the age hardening effect of Mg-Sn alloy as it segregated at the interface between the Mg matrix and Mg₂Sn phase. However, there is a lack of research on the different orientations of Al-doped Mg matrix and Mg₂Sn phase and the distribution position of Al element at the interface. Therefore, in this study, the interface adhesion energy, interface energy, and interface doping energy of Al-doped Mg/Mg₂Sn with different index surfaces were calculated based on density functional theory to determine more stable doping positions. The effects of Al doping on the electronic structure of Mg(0001)/Mg₂Sn(022) interface were analyzed using the density of states and crystal orbital Hamilton population. The results demonstrate that only a part of the Al-doping positions is beneficial in strengthening the stability of Mg/Mg₂Sn interface. After the addition of Al, the adhesion energy of Sn termination at Mg(0001)/Mg₂Sn(001) interface is higher than that of Mg termination, but the adhesion energy of Sn termination at Mg(0001)/Mg₂Sn(111) interface is lower than that of Mg termination. In addition, the interface energy of Mg(0001)/Mg₂Sn(022) interface doped with Al decreased by 0.07 eV/nm compared to that of Mg(0001)/Mg₂Sn(022) interface. The addition of Al element to Mg(0001)/Mg₂Sn(022) facilitates the doping of a special position, which shows an obvious interaction between the s orbital of Al and the p orbital of Sn after Al doping. Moreover, the Al—Sn bonding is found to be dominant at the interface.

Martensite is an attractive crystalline structure to fabricate ultrafine grain steels by cold rolling and annealing because of its low equivalent strain. However, the deformation resistance of martensite increases inevitably with the increase in the carbon content of the steel. Accordingly, cracks are easily initiated in martensite before it reaches the desired strain, restricting the application of cold rolling and annealing to ultra-low and low-carbon steels. Thus, to extend the application of these methods from low to medium-carbon steel, compositional gradient steel was prepared by decarburizing medium-carbon steel. The carbon content increased from the surface layer to core layer in the gradient steel. The decarburized medium-carbon martensite was successfully cold rolled under large deformation with an equivalent strain of 1.5 with no microcracks on the sample surface. The microstructure and mechanical properties of the quenched and cold rolled gradient component steel were characterized and studied via OM, SEM, and tensile test. The experimental results revealed the gradient size of martensite along with the gradient carbon content in the microstructure. Further, different diffusion rates of carbon atoms during decarburization and austenitization resulted in the gradient austenite grain, which restrained the size of martensite. Compared with homogenous martensite of the experimental medium-carbon steel, the steel with gradient distribution of carbon exhibited low tensile strength, which decreased from 1700 MPa to 1525 MPa, but high tensile uniform elongation, which is increased by 40%; moreover, the gradient steel showed higher product of strength and elongation than homogeneous martensite steel with similar average carbon content without decarburization. The good combination of strength and plasticity in the compositionally gradient steel was attributed to the high strength and good plasticity provided by the core layer and decarburized layer, respectively. Additionally, the heterogeneity in the strain distribution led to an extra strain-hardening; thus, the surface layer restrains further propagation of micro-shear bands from the core layer.

As a structural steel material, carbon steel bears a certain extent of elastic tensile stress in actual service. Elastic tensile stress on steel is supposed to impact the electrochemical process and corrosion behavior, which may further influence the rusting behavior and the phase composition and structure of the formed rust layer. However, stresses on the steel substrate slightly influence the rust layer of carbon steel because no intrinsic change exists in the corrosion mechanism. Here, a remarkable effect of elastic tensile stress on Q235 carbon steel was found on the phase composition and structure of the rust layer formed in 5%NaCl salt spray. The effect on the rust layer was studied using SEM, XRD, and electrochemical impedance spectroscopy. The neutral salt spray test with four-point bending was used to preform the rust layer of Q235 steel under various stress levels. The results show that the elastic tensile stress accelerates the anodic dissolution, thereby promoting the generation of γ-FeOOH, which occurs faster in the electrolyte than the transformation of γ-FeOOH to α-FeOOH and Fe₃O₄/γ-Fe₂O₃ in the solid-liquid interface. Consequently, the mass fraction of γ-FeOOH in the rust layer increases as the stress level increases, whereas the mass fraction of α-FeOOH and Fe₃O₄/γ-Fe₂O₃ decreases accordingly. As the stress increases from 0 to 0.95σ_s (σ_s is yield strength), the mass fraction of Fe₃O₄/γ-Fe₂O₃ decreases from 53% to ~46%, the mass fraction of α-FeOOH decreases from ~30% to ~23%, and the mass fraction of γ-FeOOH increases from less than 17% to ~31%. Meanwhile, the phase composition change decreases the density and increases the thickness of the rust layer. Additionally, the acceleration of the anodic dissolution induced by the elastic tensile stress promotes the growth of the rust layer, which further increases the thickness of the rust layer. The increase in thickness and decrease in compactness of the rust layer jointly enhance the protective capability of the rust layer. The former increases the resistance to the electromigration of ions through the rust layer, and the latter mitigates the occlusion effect under the rust layer.