One of the most promising high-temperature structural materials in aerospace and civil industries is the lightweight and heat-resistant TiAl alloys. However, owing to their low ductility and fracture toughness, manufacturing TiAl parts is challenging. At present, additive manufacturing process is considered one of the most promising technologies for manufacturing TiAl parts. Based on the principles and characteristics of additive manufacturing technology, this paper summarizes the process-structure-property relation of laser metal deposition (LMD), selective laser melting (SLM), and electron beam melting (EBM) in the preparation of TiAl alloy. Furthermore, this paper discusses the future development trends of additive manufacturing technology.

Enhancing the strength of metallic materials has long been a primary goal for material scientists due to their significant potential for various industrial applications. However, the methods employed to increase the strength of metals often result in reduced deformation ability, leading to what is commonly termed as the strength-deformability trade-off dilemma. This paper offers a review of the advancements made in nanostructured multi-principal-element alloys (MPEAs) and discusses the challenges associated with simultaneously improving strength and deformability. This review summarizes the various common methods used to fabricate nanostructured MPEAs, including severe plastic deformation, physical vapor deposition, and mechanical alloying. In addition, this paper reviews the strengthening and deformation mechanisms intrinsic to these alloys. Finally, a brief outlook on potential future research directions for nanostructured MPEAs is provided.

GH4169 superalloys are used in gas turbine engines and power plants owing to their excellent mechanical properties and corrosion resistance at temperatures exceeding 600^oC. Because of their service condition, involving high temperature and complex stress, much attention has been attracted to the effect of temperature and loading mode on the mechanical properties and deformation mechanism. The effect of the loading mode, especially the multiaxial or coupled loading, on the mechanism of plastic deformation is still an outstanding open question despite numerous investigations on the effect of temperature on mechanical properties. In this study, the effect of tension-torsion coupled loading on deformation behavior was investigated, where the microstructures and underlying mechanism were revealed using SEM, TEM, EBSD, and neutron diffraction. It is found that the mechanical properties are dependent on the tension-torsion loading. For the tension specimens, the yield and ultimate strengths increase with the pretorsion angle; for instance, at the pretorsion angle of 720°, the increase rate is approximately 150% and 13%, respectively. At the pretension strain of 20%, the yield strength and elongation increase by approximately 31% and 16%, respectively. The density of dislocations increases in those samples after tensile and torsional deformations compared to the undeformed samples. Moreover, the density of dislocations for specimens deformed under the coupled loading is lower than those deformed under axial loading, indicating the dislocation annihilation effect. The yield strength is enhanced due to the strengthening effect of the initial dislocations produced during the preloading. The ultimate strength for the torsional specimens after pretension decreases because of the dislocation annihilation effect during the subsequent coupled loading. However, for the tensile specimens after pretorsion, such dislocation annihilation effect can be counteracted to some extent by the strengthening effect of the formed gradient structure by pretorsion on mechanical strength. These findings provide some insight into the regulation of the microdeformation mechanism process of materials through designing the coupled or multiaxial loading modes and the coordinated improvement of strength and toughness based on the achievement of gradient or hierarchical microstructure.

Microbiologically induced corrosion (MIC), particularly those caused by sulfate-reducing bacteria (SRB), is considered as a destructive mechanism in buried pipeline steels, which has received considerable attention since its discovery a century ago. Research has shown that sessile cells that are attached to a metal surface embedded in biofilms with extracellular polymeric substances (EPS) are responsible for the occurrence of MIC. However, the commonly used MIC control strategy, i.e., biocides, cannot perform sterilization of sessile cells because of the protection of the biofilm. Cu-bearing pipeline steel is a newly developed steel that can be used for MIC control in buried pipes. The continuous release of cytotoxic Cu ions from the steel matrix leads to MIC resistance. However, the influence of Cu content on MIC resistance remains unclear. A lower Cu content in steel is not beneficial to its MIC resistance, whereas a higher Cu content can increase the cost and may cause “hot shortness” during thermal processing. Therefore, clarifying the influence of Cu content on the properties of Cu-bearing pipeline steel is important for the practical application of the steel. In the present study, the influence of Cu content (0, 0.7%, and 1.34%; mass fraction) on the corrosion behavior of X65 grade Cu-bearing pipeline steel was investigated through electrochemical measurements and surface analysis during 14 d immersion in sterile and SRB-inoculated NS4 solutions, respectively. Experimental results demonstrated that the antibacterial properties and corrosion resistance of the steel improved with the increase of Cu content. When Cu content was increased to 1.34%, the pitting corrosion of the steel in the SRB-inoculated medium was almost suppressed. The protective corrosion products formed in the sterile medium and antibacterial Cu ions continuously released from the steel in the SRB-inoculated medium resulted in the remarkable corrosion resistance of Cu-bearing pipeline steels.

M50 bearing steel is widely used in the manufacture of aeroengine spindle bearings. The voids generated by the thermal processing of bearing steel can easily initiate fatigue cracks and lead to fatigue failure of the bearings. Thus, it is essential to understand the steel production conditions, void distribution in the steel, and effect of the subsequent treatment on the healing process of voids to improve the thermal processing and mechanical properties of the steel. In this work, the thermal deformation of the M50 bearing steel was conducted using a thermal simulation machine. The effects of the strain rate (0.001-1 s^-1), deformation temperature (1000-1150^oC) and strain (10%-50%) on the formation of voids and void healing during the subsequent thermal treatment were systematically studied using OM, SEM, EBSD, and in situ scanning methods. The results show that the formation of voids between the carbide and matrix is attributed to the different hardness values between the matrix and primary M₂C and MC carbides. In addition, the carbide fractures can promote the formation of internal voids. The quantitative analysis of the voids indicated that most voids are generated under the following conditions: a high strain rate of 1 s^-1, low deformation temperature of 1000^oC, and medium deformation of 30%. Applying a heat treatment after deformation can significantly promote the void healing process, and the Cr element is enriched in the healing zone due to its rapid diffusion in γ-Fe.

The ever increasing demand for safe and lightweight steel has promoted the development of advanced high-strength steel (AHSS). Recently, many AHSSs have been developed through chemical heterogeneity, resulting in microstructure refinement and mechanical property optimization. Although many efforts emphasize the construction of Mn-heterogeneous high-temperature austenite (γ-Fe), the influence of Mn-heterogeneous distribution remains unclear. In this work, different austenitization times and temperatures are applied to Mn-partitioned pearlite, followed by the same quenching and partitioning process. The effect of Mn distribution in high-temperature austenite on the microstructural evolution and mechanical properties is systematically investigated. Results show that the Mn-heterogeneous high-temperature austenite can tailor the austenite-to-martensite transformation during quenching. The Mn-depleted austenite is then readily transformed into lath martensite, and the Mn-enriched austenite is mainly retained as film roughness (RA), both of which assemble the ghost pearlite. With an increase in austenitization time and temperature, the Mn atom diffusion from the Mn-enriched austenite (originated from cementite lamellae) to the Mn-depleted one (originated from ferrite lamellae) increases, leading to the decreased chemical heterogeneity in high-temperature austenite. Thus, the fraction of ghost pearlite decreases while the fraction and size of blocky RA and coarse lath martensite increase. A wider lath martensite lowers the strength of the yield or the elastic limit of steel. The increased fraction and size of blocky RA ensure an increased uniform elongation by transformation-induced plasticity effect, whereas the transformation product (i.e., fresh martensite) is detrimental to the post-uniform elongation. Meanwhile, because the fractions of RA and martensite hardly change with austenitization condition, the ultimate tensile strength (about 1700 MPa) and total elongation (about 20%) are relatively constant. Therefore, tuning the Mn distribution in high-temperature austenite provides an effective strategy to tailor yield strength and uniform elongation while maintaining large ultimate tensile strength and total elongation.

The {111}/{111} near singular boundary is more resistant to intergranular corrosion than random boundary. At present, enhancing the fraction of such boundary to improve the performance against intergranular corrosion has been the latest issue in microstructure design and control for aluminum and its alloys. In the current work, high-purity aluminum was selected as an experimental material, and the effects of grain growth on {111}/{111} near singular boundary were investigated. First, the sample was given multi-directional forging at room temperature followed by recrystallization annealing at 370^oC. The recrystallized samples were heated at 500^oC for varied time to promote grain growth and to obtain microstructures with various grain sizes. Then, the {111}/{111} near singular boundary in the samples was measured by grain boundary inter-connection characterization, which was established on the basis of EBSD and five-parameter analysis. Results show that the length fraction of {111}/{111} near singular boundary increases with the increase of grain size. For example, the fraction of {111}/{111} near singular boundaries is 3.91% when the averaged grain size is 38 μm, whereas it increases to 6.56% as the averaged grain size reaches 77 μm. Off-line in situ EBSD coupled with grain boundary trace analysis indicates that the {111}/{111} near singular boundary is primarily formed via the encounter of two growing grains with <111>/θ misorientation relationships (θ is the rotation angle). Meanwhile, the {111}/{111} near singular boundary is also formed via the re-orientation of grain boundaries with <111>/θ misorientation. HRTEM observation reveals that the {111}/{111} near singular boundary has disclination, and the degree of atomic ordering of such a boundary is higher than that of random boundaries. Therefore, such a boundary is more resistant to intergranular corrosion compared with random boundary.

TiAl alloys are highly promising for high-temperature structural applications in the aerospace and automotive industries because of their low density, excellent high-temperature strength, and resistance to creep and oxidation. Nevertheless, low-temperature brittleness and poor deformability are the main factors severely restricting the widespread application of TiAl alloys. The process of β-solidifying γ-TiAl alloys results in alloys that consist primarily of α₂, γ, and B2 phases, and have superior hot workability. Further thermomechanical treatments are applied to achieve a fine microstructure and enhance the inherent ductility of γ-TiAl alloys. In this work, Ti-44Al-5Nb-1Mo-2V-0.2B alloy sheet with ultrahigh plasticity at 800^oC was achieved by cross hot-pack rolling (CHPR) and one-step annealing processes. SEM, EBSD, TEM, and tensile methods were used to investigate the hot deformation behavior, and the effects of different rolling processes and heat treatments on the microstructural evolution and mechanical properties of the alloy. The results show that the CHPR sheet had a more highly uniform deformation microstructure along the thickness direction and sheet plane compared with that of a unidirectional hot-pack rolled (UHPR) sheet, which consisted of residual lamellar colonies and equiaxed γ, α₂, and B2 grains at colony boundaries. The size of the residual lamellar colonies was significantly smaller and the content was lower in the CHPR sheet compared with the UHPR sheet. This was due to a large number of broken residual lamellae and complete recrystallization under the combined action of a bidirectional shear force and compressive stress during the CHPR process. The high-temperature flow-softening mechanisms of TiAl alloy in the CHPR process mainly included bending and kinked lamellae, β/B2 coordinated deformation, phase-transformation decomposition of α₂/γ lamellar, and dynamic recrystallization induced by primary and secondary twinning. To achieve further grain refinement, subsequent annealing of the CHPR-processed TiAl alloy was performed at 1200-1340^oC. A multiphase equiaxed microstructure with fine lamellar colonies was obtained at 1200^oC and a nearly complete lamellar microstructure was obtained at 1340^oC. Moreover, the room-temperature and high-temperature tensile properties of UHPR and CHPR sheets in the horizontal and vertical directions were compared with samples annealed at 1200^oC. The tensile properties of the CHPR sheets were more uniform in both directions. The multiphase equiaxed microstructure obtained in the CHPR alloy annealed at 1200^oC had the best strength-plasticity balance with a tensile strength of 624 MPa (515 MPa) and elongation of 1.32% (107.0%) at room temperature (800^oC). According to the fracture behavior, the fracture mode of these alloy sheets was translamellar or cleavage fracture at room temperature. Conversely, the fracture mode changed to ductile fracture at 800^oC, and the failure mechanism was mainly via microhole coupling. The fractures in the annealed sheets (1200^oC) had small and deep dimples, indicating optimal tensile elongation. The uniform and fine lamellar structure and equiaxed microstructure can hinder crack propagation and achieve enhanced mechanical properties.

The 6xxx series aluminum alloys Al-Mg-Si(-Cu) are widely used in the automotive industry owing to their high strength-to-weight ratio, good formability, and corrosion resistance. Micro-alloying is an effective technique for enhancing the properties and microstructure of Al alloys. The effects of varying La additions on the microstructure and properties of Al-0.75Mg-0.75Si (mass fraction, %) alloy have been investigated using hardness, electrical conductivity, and tensile tests, as well as SEM and TEM. As the La content increases, the following observations are made: (1) the ductility and electrical conductivity of the alloy gradually increase due to the increase in the fraction of the AlSiLa secondary phases induced by the La addition and the amount of Si solute atoms consumed by these phases; (2) the strength of the alloy first increases due to the increase in the secondary-phase strengthening contribution of the AlSiLa phases and the grain refinement contribution, and then decreases owing to the decrease in the solid-solution strengthening contribution; and (3) the types of precipitates formed during aging gradually change; besides the β″ phase, polycrystalline β″ precipitates are also precipitated in the peak-aged alloys with La addition, while β″/U2, β′/U2, and β′/U2/β″ composite precipitates are precipitated in the over-aging condition of the La-added alloys.

The production of metallic composites is an effective way to combine the advantages of different metals. Specifically, the combination of titanium and CrMo steel in a composite pipe can yield good corrosion resistance and excellent abrasion performance, rendering it highly promising for use in the petroleum industry. However, the interfacial reactions between the two metals during the manufacturing process can lead to the precipitation of brittle carbides or intermetallic compounds, resulting in substantial weakening of the combining strength. This is particularly problematic for the TC11 titanium and CrMo steel matrix due to their higher alloy content. Thus, to improve the bonding properties of titanium-steel composites, an interlayer is added between the matrixes. In this study, the effects of interlayer materials (Fe and Nb), extruding temperatures (920 and 970^oC), and heat treatment on the bonding strength of high-strength CrMo steel and TC11 titanium matrixes were investigated. The results revealed that for the titanium-steel composite pipe, the bonding strength of the Fe-titanium interface dominates the shear stress (185 MPa) due to the locking effects of unevenly deformed grains. However, after heat treatment, M₂₃C₆ heavily precipitates in the Fe interlayer causing it to become hard and brittle, weakening the locking effects, and resulting in a significant decrease in shear stress (70 MPa). Conversely, the Nb-interlayer samples extruded at 920^oC mainly cracked along the steel-Nb interface, while those extruded at 970^oC mainly cracked along the Nb-titanium interface. Thus, the two interfaces respectively dominated the shear stress of the two Nb-interlayer samples, and this feature persisted after heat treatment. Moreover, the different cracking routes were found to be caused by the formation of a new NbC layer and a β-titanium layer, respectively. As the fractured NbC layer recovered during the heat treatment, the shear stress of the 920^oC extruded sample increased to 170 MPa and that of the 970^oC extruded sample decreased due to the solution of Ti_0.86Al_0.11Nb_0.03 particles and the discontinuous β-titanium layer induced by it. Thus, the comparative study of interlayer materials and different processing parameters on the interfacial shear stress can effectively improve the production of hot-extruded high-strength titanium-steel pipe.