TiAl alloy sheets are important strategic structural materials in aerospace and automotive industry because of their high strength-to-weight ratio and high service temperature. However, the preparation of TiAl alloy sheets is difficult, especially the rolling of large-size high-performance TiAl alloy sheets. The preparation process, size, microstructure, and mechanical properties of hot-rolling TiAl alloy sheets using ingot metallurgy, powder metallurgy, direct rolling, and roll bonding approaches in recent years were reviewed. The features and existing problems of the above processing routes were discussed. Meanwhile, some suggestions on rolling large-size TiAl alloy sheets and its future development were proposed.

Skutterudite thermoelectric materials are one of the most promising candidates for critical components in thermoelectric devices because of their excellent electrical transport properties. Thermoelectric devices require p- and n-type skutterudite materials with matching properties. However, the p-type skutterudite materials have considerably worse thermoelectric and mechanical properties than those of n-type. Thus, it is important to enhance the thermoelectric and mechanical properties of p-type skutterudite materials for the development of high-efficiency thermoelectric devices. This study summarizes the recent research progress on the nano-mesoscopic scale regulation of the microstructure for p-type skutterudite thermoelectric materials. The thermoelectric and mechanical properties of p-type skutterudite materials can be notably enhanced by adjusting the microstructure at the nano-mesoscopic scale; thus, providing scientific and technical supports for the thermoelectric device's application.

Powder metallurgy (PM) nickel-based superalloys are widely used as high-temperature, fatigue-resistant components in aircraft and gas turbines. However, many powder particle boundaries and carbides are found in alloys prepared via traditional methods, such as hot pressing (HP) and hot isostatic pressing, causing serious damage to the properties of PM superalloys. New methods and processes must be developed to improve the performance of PM superalloys. SEM, EBSD, and TEM were used in this work to investigate the microstructure, mechanical properties, and oxidation behavior of FGH4097 alloys prepared via spark plasma sintering (SPS) and HP. Owing to the advantages of uniform microstructure, fine grain, and less carbide precipitation, SPSed alloy has better tensile properties and oxidation resistance than HPed alloy. At 25^oC, SPSed alloy's yield, tensile strengths, and elongation are 998 MPa, 1401 MPa, and 17.1%, respectively; whereas those of HPed alloy are 951 MPa, 1262 MPa, and 14.4%, respectively. Although the two alloys exhibit similar yield and tensile strengths at 700^oC, SPSed alloy shows merely 80% higher ductility than HPed alloy. The oxidation mass gain of the two alloys oxidized at 900^oC for 100 h follows the parabolic law. A continuous and dense Al₂O₃ scale is formed on the surface of SPSed alloy, which effectively prevents the inward diffusion of O and outward diffusion of Cr and Ti. The mass gain is merely 0.19 mg/cm², and the oxidation rate is 1.03 × 10^-7 mg²/(cm⁴·s). Conversely, the oxidation rate of HPed alloy is approximately 2.6 times that of SPSed alloy. Severe internal oxidation occurs in HPed alloy, resulting in abundant less protective NiCr₂O₄ and TiO₂ formation on the surface, as well as large cracks and spalling.

β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B has attracted considerable attention owing to its higher strength and excellent creep resistance at elevated temperature. Indeed, its application temperature is much higher than that of Ti-48Al-2Cr-2Nb. Because γ-TiAl alloys are exposed to air at elevated temperatures for a long time during application, an oxidation layer is formed in the surface. The oxidation layer, which is potentially harmful to the mechanical properties of the crack nucleation sites, was observed near the surface. Concerning the β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B, it has median Nb content and low Al content. Additionally, a considerable β₀-phase with lower Al content is retained. To better understand the influence of the composition and microstructure on the oxidation behavior of γ-TiAl alloys, it is necessary to investigate the oxidation behavior and microstructure evolution in the surface of β-solidifying γ-TiAl alloys during thermal exposure. In this study, samples of β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B were obtained by investment casting and thermal exposure at 700°C for different times, and the oxidation behavior and microstructure of different phases in the surface were compared. The results showed that the constituents of the oxidation layer on the surface varied with the exposure time. The volume fractions of TiO₂-R, α-Al₂O₃, Ti₂AlN, and Nb₂Al increased by increasing the exposure time. Metastable κ-Al₂O₃ was detected in the sample exposed for a short time, but it was transformed into α-Al₂O₃ after exposure for 200 h. Moreover, metastable Ti₄O₇ and TiAl₂O₅ were detected in samples exposed for 200 and 500 h. The microstructures, morphologies, and heights of oxidations in the surface of a specific phase are different, varying by increasing the exposure time. These variations are related to the different oxidation behaviors during thermal exposure, i.e., the γ-phase experienced selective oxidation after a short time exposure, α₂-phase changed from internal oxidation to selective oxidation when the exposure time reached 200 h, while the β₀-phase suffered internal oxidation during the entire exposure. The different oxidation behaviors of each specific phase contributed to the different Al contents. Dispersed TiO₂ was formed during internal oxidation, and it kept growing during thermal exposure, forming a continual layer at the end. The continual Al₂O₃ layer was formed during selective oxidation, in which the Ti element was rejected in the reaction interface. When the content produced during the internal oxidation of the Ti element reached a critical value, dispersed TiO₂ was formed and kept growing to form the continual layer. The alternating formation of continual Al₂O₃ and TiO₂ layers resulted in the layer structure observed in the surface.

C-HRA-5 steel is a new type of austenitic heat-resistant steel with excellent oxidation resistance and corrosion resistance, and high endurance strength at high temperatures. This steel can be used in superheaters and reheaters applied in 630-700^oC advanced ultrasupercritical fossil-fired power plants, which is of great strategic significance to realize national energy savings and emission reduction targets. Detwinning often occurs in austenitic stainless steel when high-temperature fatigue and creep set in. However, the detwinning mechanism in C-HRA-5 steel during high-temperature fatigue and its influence on fatigue crack initiation and propagation remain unclear. Therefore, in this work, the detwinning behavior of C-HRA-5 steel under strain-controlled low-cycle fatigue (LCF) was studied at 700^oC. The evolution of twin boundaries (TBs), detwinning mechanism, and influence of residual TBs on fatigue cracks were analyzed using SEM, EBSD, and TEM. After solution treatment at high temperatures, the fraction of low coincidence site lattice boundaries reached 69% in C-HRA-5 steel with dominant Σ3-type TBs. There was a remarkable detwinning effect under LCF loading. The degree of detwinning increased with increasing strain amplitude ranging from 0.3% to 0.7%. The detwinning mechanism was mainly related to the interactions at the dislocation TBs and precipitation of M₂₃C₆ at TBs. The plastic deformation of C-HRA-5 steel was dominated by planar slip under LCF loading, which resulted in dislocation gliding on particular planes. Therefore, numerous dislocation slip bands were formed. The collision of dislocation slip bands with TBs changed the coherent orientation of TBs and eventually led to detwinning. Meanwhile, the dislocation-TBs interaction induced the precipitation of M₂₃C₆ carbide at some TBs during LCF at 700^oC, which strengthened the pinning of TBs on dislocations, thus accelerating the detwinning process. The residual TBs had higher strength than random grain boundaries and could inhibit the initiation and propagation of fatigue cracks.

Bearing is one of the most technologically important engineering components in machines. With the development of several advanced steel-refining technologies to suppress the detrimental effect of nonmetallic inclusions on the mechanical properties of materials, the impact of carbides on the service life of bearings has gradually highlighted. The carbides have become a key factor in determining the performance of a bearing, particularly for primary carbides formed during the solidification of high carbon-chromium bearing steel. Therefore, exploring the formation mechanism of primary carbides and their control strategies is vital to improve the manufacturing process of bearing steel as well as the service life and reliability of bearings. To clarify the formation mechanism of primary carbides and the effects of the processing technique, as well as the addition of rare earth elements, a modified type of GCr15 high carbon-chromium bearing steel with and without rare earth elements was remelted and solidified at different cooling rates. After solidification, the quantity, area, average size, and chemical composition of the primary carbide in the as-cast bearing steel were characterized and analyzed via OM, EPMA, SEM, and XRD. The results show that the type of carbide in GCr15 series bearing steel is M₃C cementite with high Cr content (more than 15%, mass fraction). The nucleation rate of M₃C cementite increased with the increase in the cooling rate; thus, the number of carbides increased considerably. However, at very high cooling rates, the primary austenite was refined and the diffusion time of C and Cr elements required to form carbides declined; therefore, the size of carbides was reduced significantly, resulting in more uniform dispersion of the carbides. Moreover, the addition of rare earth elements could refine the primary austenite, and subsequently, refine the carbide to some extent. Considering the properties of the primary carbides at different cooling rates, the kinetic formation mechanism for the primary carbide in high carbon-chromium bearing steel during solidification is proposed.

The power industry is changing from rapid growth to high-quality development, and there are urgent demands for the high-quality and high-service reliability of overhead lines. Al-Mg-Si alloys are widely used in the production of long-distance overhead lines owing to their high strength-to-density ratio, good conductivity, and corrosion resistance. In the overhead line service, surface defects reduce their mechanical properties, and surface roughness greatly affects its fatigue properties. A single-strand conductor of 6101 aluminum alloy was employed to investigate the fatigue properties of the conductors with different roughness. The fatigue strength of the alloy wires decreased gradually with an increase in the surface roughness (maximum height of profile, R_z). As R_zincreased from 57.9 to 161.7 μm, the fatigue limit decreased by ~36.4%. The result indicates that an increase of R_z increases the theoretical stress concentration factor K_{\mathrm{t}}, which facilitates the initiation of fatigue cracks, and the fatigue strength decreases accordingly. Furthermore, the surface roughness is equivalent to the size of the initial crack a_{\mathrm{0}} = \frac{\sqrt[]{\mathrm{\pi }L{R}_{\mathrm{z}}}}{\mathrm{2}} (L is the average value of the arerage width of profile element). A model suitable for predicting the fatigue life of conductors with different surface roughness was obtained.

Selective laser melting (SLM) has been widely used in many fields owing to its high manufacturing accuracy and excellent performance. A rapid solidification rate of 10³-10⁶ K/s is achieved during the SLM process, resulting in unique microstructures and highly supersaturated solid solutions beyond the normal solubility limits of alloying elements, thereby providing new opportunities for the development of microstructures and optimization of their properties. To date, SLM has been used for manufacturing a wide range of metallic materials, such as Ti-based alloys, superalloys, and stainless steel. However, specific difficulties are associated with melting aluminum powder using a laser owing to the high laser reflectivity, tenacious surface oxide, poor spreadability (particularly of low-density aluminum powder), high thermal conductivity, and large freezing ranges of many aluminum alloys. Consequently, high-strength aluminum alloys, such as the 2xxx, 6xxx, and 7xxx series, exhibit poor SLM formability. The SLM-formed aluminum alloys that are practically applied in industries at present are limited to the Al-Si base and Al-Mg-(Sc, Zr) alloys. Al-Mg-(Sc, Zr) alloys achieve high strength and ductility with low mechanical anisotropy, thus showing considerable advantages over conventional and SLM-formed Al-Si-Mg alloys. However, the strength of the present SLM-formed aluminum alloys is still lower than that of conventional high-performance ones. Based on the technical characteristics of liquid quenching in SLM, this study focuses on designing high-strength Al-(Mn, Mg)-(Sc, Zr) aluminum alloys specifically for SLM by simultaneously increasing the (Mn + Mg) and (Sc + Zr) contents. The effect of aging treatment on the microstructure and mechanical properties of the SLM-formed alloy was systematically studied. Results show that the alloy exhibits good SLM formability with a relative density of more than 99.0%. A typical multilayer distribution of laser tracks generated in the SLM process can be observed. Few columnar grains are observed in the center of the molten pool, and numerous equiaxed grains are present in molten pool boundaries with an average grain size of 4 μm. The mechanical properties of the alloys are considerably improved after aging treatment at a low temperature (≤ 350°C) owing to the precipitation of Al₃Sc nanoparticles. The maximum Vickers hardness, maximum compressive yield strength, and maximum compressive strength of the aged alloy are (218 ± 5) HV, (653 ± 3) MPa, and (752 ± 7) MPa, respectively, with a compressive elongation of greater than 60% for all samples, higher than that of most SLM-formed aluminum alloys and T6-treated AA7075 alloys. Combining various strengthening mechanisms facilitates SLM-formed Al-(Mn, Mg)-(Sc, Zr) alloys with high strength.

This study verifies the body-centered cubic (bcc) formability of CrMoTi medium-entropy alloy (MEA) as a potential mold material via theoretical calculations based on the concepts of multiprincipal element alloys and practical experiments employing arc melting and additive manufacturing (AM) techniques. The hardness and thermal properties of arc-melted CrMoTi MEA were tested at room and elevated temperatures. At room temperature, the alloy possesses a hardness of 520.6 HV_0.3, thermal capacity of 371 J/(kg·K), and heat conductivity of 14.0 W/(m·K). Its hardness drops to 356.0 HV_0.3 at 600^oC, and its thermal capacity and heat conductivity increase to 446 J/(kg·K) and 28.4 W/(m·K), respectively, at 709^oC, exhibiting the characteristic of semimetals. AM techniques are efficient for fabricating highly customized molds and have been widely used. Moreover, in situ alloying can further improve the compositional flexibility in the AM process. The in situ alloying printability of two AM techniques, i.e., direct laser deposition (DLD) and selective laser melting (SLM), was investigated using a blend of elemental powders. The best densification within the AM approaches (7.46 g/cm³) is achieved using DLD, and the microhardness of DLDed samples reaches 634.6 HV_0.3. Conversely, the printability of SLM is relatively restricted. The optimal density and microhardness of the SLMed sample are 7.27 g/cm³ and 605.9 HV_0.3, respectively, which are lower than those of the DLDed samples. In the DLDed samples, the large melt pool can homogenize most elements but with a Cr burning loss. Mo melts insufficiently during the SLM process and remains a partially melted powder in as-built samples. Moreover, cracking is already inevitable in SLMed samples, indicating that homogenization can hardly be improved by applying excessive energy input. As a brittle bcc alloy, its matrix tends to fail under the thermal stress of the heat accumulation in the AM process. Furthermore, the phase transformation in a small melt pool also intrinsically harms printability for in situ alloying studies through AM. Results from this study reveal that DLD possesses advantages over SLM for the in situ alloying of brittle materials like CrMoTi MEA. Combining elements with adequate overlapping of the liquid zone could be essential for superior printability of AM in situ alloying, especially with a high ratio of introduced elements.

Cu-Al-Ni alloys are not yet widely used due to issues related to their coarse grains, unacceptable plasticity, and poor thermal stability. Here, the physical and mechanical properties, as well as the corrosion behavior, of Cu-Al-Ni alloys doped with Y element (Cu-13Al-4Ni-xY (x = 0.2, 0.5, mass fraction, %)) were studied. XRD, OM, SEM, TEM, electronic universal testing machine, and electrochemical workstation were used to characterize the microstructures and measure the properties of the Cu-13Al-4Ni-xY alloys. The results showed that at room temperature, the microstructure of Cu-13Al-4Ni-xY alloys was mainly an 18R martensite matrix. The (Cu, Al, Ni)₄Y second phase was characterized to have a hexagonal structure. The mechanical properties of the Cu-13Al-4Ni-xY alloys improved as the Y element content increased. For example, when the Y content was increased from 0% to 0.5%, the compressive fracture strain increased from 10.5% to 19.3% and the fracture strength increased from 580 to 1185 MPa. Additionally, the fracture type of the alloy changed from intergranular to transgranular with the addition of Y. Finally, the results from electrochemical experiments showed that the corrosion behavior of the alloys decreased slightly with the addition of Y.

Pb is widely used as grid material for lead-acid batteries, an electrowinning electrode and a nuclear radiation shield. To improve the performance of these materials, alloying elements such as Ag, Sb, and Ca are commonly added. Pb's conductivity and strength can be improved using Al as an alloying element. However, the phase diagram of the Pb-Al alloy is characterized by the large liquid-liquid and liquid-solid miscibility gaps. When a homogeneous single-phase Pb-Al liquid is cooled into the miscibility gaps, Al-rich droplets/particles precipitate first from the melt, causing the Pb-Al alloy to form a microstructure with coarse Al-rich particles or serious phase segregation. Understanding the evolution of microstructure in the liquid-solid phase separation has remained a scientific challenge thus far. The solidification of the Pb-Al alloy is investigated using directional solidification experiments in this work. A numerical model is developed to describe the microstructure formation in a directionally solidified liquid-solid phase separation alloy using the population dynamics method. The evolution of the microstructure is simulated. The simulation results agree well with the experimental results. They show that a supercooling zone appears in front of the solidification interface, where the liquid-solid phase separation of the Pb-Al alloy occurs. In this zone, Al-rich particles (dispersed phase) form and grow by solute diffusing as they move toward the solidification interface. The nucleation rate and the number density of Al-rich particles increase as the solidification rate increases, whereas the average radius of the particles decreases. The Al-rich particles' Stokes movement velocity has the same direction as the melt's solidification velocity, resulting in an enrichment of Al-rich particles in front of the solidification interface. Because of the convective flow of the melt in front of the solidification interface, the cooling rate of the melt is unevenly distributed along the radial direction, resulting in an uneven distribution of nucleation rate, number density, and average radius of Al-rich particles. The formation of a solidification microstructure with the dispersive distribution of Al-rich particles is dependent on the solidification rate being fast enough to ensure that all size particles in the liquid-solid phase separation region move toward the solidification interface under the effect of the Stokes movement of Al-rich particles and the convective flow of melt.

CB2 steel (ZG12Cr9Mo1Co1NiVNbNB) is a ferritic stainless steel with excellent creep properties at high temperature (550-700^oC) and is mainly used in 600^oC ultrasupercritical units. The poor oxidation resistance of the material limits its practical applications in harsh, high-temperature environments, in which the steam unit faces high-temperature water vapor for a long period of time. Therefore, surface modification or coating has become an important means to improve the high-temperature oxidation resistance of the material. An inorganic silicate coating has the advantages of high thermal-chemical stability, similar thermal expansion coefficient, and simple preparation process, which can significantly improve the oxidation and corrosion resistance of the material. In this study, a new type of inorganic silicate composite coating was designed, based on the CB2 steel. The oxidation behavior of CB2 steel and coated specimens at 650^oC high-temperature steam atmosphere for 1000 h was studied by using a high-temperature water vapor simulation device. The results showed that the oxidation rate of the coated CB2 steel was 30 times slower than that of uncoated CB2 steel; thus, the coating exhibited a good protective effect. After 1000 h of oxidation, the oxide scale on the CB2 steel was loose and cracked with obvious voids and the oxidation product was mainly composed of Fe₂O₃. The inorganic silicate composite coating significantly improved the oxidation resistance of CB2 steel; after 1000 h of oxidation, no spallation areas or cracks were found.