Transformative TiAl alloys are in high demand for hot-end components such as aeroengine blades and for lightweighting advanced aerospace equipment. However, traditional TiAl alloys are brittle at room temperature and exhibit low long-term capability at high temperatures. To overcome these limitations, our team has proposed a new material-design paradigm based on “ordered structures of functional units.” By precisely regulating the intrinsic characteristics and ordered structures of the soft γ-TiAl phase, the hard α₂-Ti₃Al phase, and nano-twins, we considerably enhance the strength, plasticity, and high-temperature capability of polysynthetic twinned TiAl alloys. We also demonstrate the decisive roles of ordered-structure parameters, such as interface type, lamellar orientation, lamellar thickness, and phase proportion, on the mechanical properties of the alloy. The physical strengthening and toughening mechanisms include twinning-induced strengthening and plasticity in the γ phase, fatigue-strength-enhancing stacking faults in the α₂ phase, toughening via transformation of the α₂ phase, and γ/α₂ coherent interfaces, which improve fracture toughness. These insights illuminate promising directions for the development of TiAl alloys with ordered functional unit structures.

Single-crystal superalloys are widely used in aircraft engines owing to their excellent high-temperature performance. As key factors in controlling the formation of microstructures, solidification conditions directly influence the comprehensive properties of alloys. Therefore, it is of great significance to understand the mechanisms of alloy microstructure evolution under different solidification conditions and to optimize the solidification process to improve the performance of advanced aircraft engine hot-section components. To investigate the effects of the solidification conditions on the evolution of as-cast and heat-treated microstructures in the second-generation single-crystal superalloy DD6, single-crystal bars oriented along the [001] direction were prepared using high-rate solidification (HRS) and liquid metal cooling (LMC) processes. The results showed that in the as-cast state, the γ′ phases in the dendritic core of the HRS alloy exhibited a relatively regular cubic shape, whereas those in the LMC alloy were irregularly cubic; the γ′ phases in the interdendritic regions of the HRS and LMC alloys were irregularly cubic and larger in size than those in the dendritic core. As the pouring temperature increased, the size of γ′ phases in the HRS alloy first increased and then decreased, reaching a maximum at 1560 ^oC. At the same pouring temperature of 1590 ^oC, the size of the γ′ phases in the LMC alloy was smaller. After the heat treatment, the volume fraction of the γ + γ′ eutectic under all conditions decreased significantly. A trend was observed where the smaller the size of the γ + γ′ eutectic in the as-cast alloy, the greater the reduction in its content, regardless of the initial γ + γ′ eutectic content in the as-cast state. In the heat-treated HRS and LMC alloys, the γ′ phases in the dendritic core and interdendritic regions were uniformly distributed and regularly arranged, and they demonstrated good cubicity. Furthermore, the size uniformity of the γ′ particles in the LMC alloy was superior to that in the HRS alloy. The differences in the microstructures of the as-cast alloys were primarily caused by differences in the temperature gradient during directional solidification. Therefore, increasing the pouring temperature or employing LMC to enhance the temperature gradient helped to refine the dendritic structure of the alloy, reduce microsegregation, and this can simplify the alloy's heat treatment process.

Traditional austenitic stainless steel (ASS) faces challenges in operating safely under low-temperature sliding wear conditions because of its relatively low strength and hardness. To address this issue, this study focused on high-nitrogen 316LN ASSs. Through controlled rolling and annealing, three microstructures were designed: non-recrystallized, heterogeneous, and fully recrystallized microstructures (marked by NG, HS, and CG structures, respectively). The influence of environmental temperature and microstructure on the tribological behavior and wear mechanisms of high-nitrogen 316LN ASSs was investigated. The results demonstrate that the HS structure exhibits the lowest friction coefficient because the reduced number of abrasive particles limits the direct contact between the worn surface and the counterpart, outperforming the NG and CG structures. As the environmental temperature decreases, the wear rates of all the structures decrease, with the lowest wear rate observed at -120 ^oC. At this temperature, the CG structure exhibits the lowest wear rate—surpassing the NG and HS structures—attributed to its low stacking-fault energy, inducing martensitic transformation and forming a nano/submicron crystalline hardened layer. This layer effectively prevents crack propagation and enhances wear resistance. Although martensitic transformation and surface hardening also occur in the HS structure, the wear debris generated during sliding acts as a third-body abrasive, accelerating wear and degrading wear resistance. In contrast, the CG structure, which exhibits excellent low-temperature plastic deformation ability, shows only mild abrasion during the wear process.

The service environment of deep-sea oil and gas pipelines is becoming increasingly harsh, making it difficult for traditional single-metal pipeline steels to meet the unique demands of such environments. Bimetal composite materials leverage the advantages of bimetal components to achieve properties that are not possible with single metal materials. Hot rolling, a solid-phase bonding process that joins pipeline steel substrates and stainless steel at high temperatures, is an efficient method for creating strong interfaces. This technique is particularly suitable for the large-scale industrial production of bimetal clad plates, necessitating the establishment of an industrial production line. To overcome the challenge posed by deformation inconsistencies that adversely affect the properties and interface bonding strength of heterogeneous bimetal clad materials during hot rolling, the precise control of the hot rolling process for X65/Inconel 625 bimetal composite plates intended for deep-sea oil and gas transportation was investigated. Thermal compression tests on the X65 pipeline steel and Inconel 625 corrosion-resistant alloy were carried out using a Gleeble-3800 thermal simulation testing machine. The flow stress, constitutive relationships, thermal working diagram, interface bending resistance, and microstructure characteristics of the bimetallic materials were examined. The results indicate that the peak stress difference in the bimetallic materials decreases as the deformation temperature increases and the strain rate decreases. At high temperatures (≥ 950 ^oC), the primary softening mechanism for X65 steel is dynamic recovery, whereas that for Inconel 625 is dynamic recrystallization. Our findings suggest that the optimal hot rolling process for the bimetal clad plate should involve a final rolling temperature of 1000 ^oC and a reduction of 70%, based on theoretical analysis and experimental data. The interface is straight and well-combined, and demonstrates strong bending deformation ability. The bimetal clad plate achieves a rolling direction yield strength of 469 MPa, tensile strength of 606 MPa, elongation of 30%, and shear strength of 442 MPa.

To ensure the safety of ultra-deep wells for oil and gas exploitation, the pressure components of blowout preventers (BOPs) are designed with large and thick sections. Existing low-alloy steels, such as 35CrMo, 20CrMoV, and 25CrNiMo, fail to meet the requirements of heat resistance, corrosion resistance, strength, and toughness necessary for BOP pressure parts in ultra-deep wells. A novel type of F22M steel, developed through synchronous micro-alloying with V, B, and rare earth elements based on F22 steel—a heat-resistant steel for steam power plant pipes—has been successfully applied in a test ultra-deep well. However, further detailed study of this steel is necessary for optimization. In the present work, the effects of tempering temperature on the microstructure and mechanical properties of F22M steel were investigated by OM, SEM, TEM, and XRD. Additionally, the strengthening-toughening mechanisms of tempered F22M steel were analyzed. The results reveal that the microstructure of F22M steel tempered within the 610-670 ^oC range comprises predominantly bainite with a small amount of tempered sorbite. As the tempering temperature increases, the dislocation density decreases from 6.23 × 10¹⁵ m^-2 at 610 ^oC to 3.38 × 10¹⁵ m^-2 at 670 ^oC due to bainitic lath recovery. Moreover, M₃C carbides, which initially form as strips along bainitic lath boundaries, gradually evolve into spherical, dispersed granular M₇C₃ carbides within the matrix. Consequently, strength decreases smoothly, while impact toughness improves significantly. Notably, the impact toughness of F22M steel tempered at 650 ^oC reaches 277 J at -29 ^oC, 7.9 times higher than the 31 J observed at 630 ^oC. Quantitative analysis reveal that dislocation and precipitation strengthening are the primary contributors to the yield strength of tempered F22M steel. However, the softening effects resulting from bainitic lath recovery and carbide evolution during tempering significantly enhance the steel’s impact toughness.

Maintaining steel’s high mechanical properties after a long period of tempering (aging) is a worldwide challenge. This study investigates the microstructure and mechanical properties of a low-carbon, low-alloy steel weld metal consisting of acicular ferrite (AF) using OM, SEM, EBSD, TEM, and impact and tensile tests. The precipitation kinetics is used to analyze and discuss the experimental results to study the microstructure evolution and its effect on the mechanical properties of the low-carbon, low-alloy steel weld metal consisting of AF after tempering for a long time in a high-temperature environment. The results show that after tempering at 580-700 ^oC for 1-12 h, the dislocation density and size of the AF showed no noticeable change, indicating that the microstructure of the AF was very stable after high-temperature, long-time tempering. Precipitates increase along with the increase in tempering temperature, pin grain boundaries and dislocations, hinder the coarsening of AF, and thus increasing the precipitation strengthening effect. The tensile strength increased at 640-700 ^oC and peaked at about 700 ^oC. The kinetics curves and calculations indicate that the “nose” temperatures of both the nucleation rate-temperature (NrT) and precipitation-temperature-time (PPT) curves are about 700 ^oC; precipitation occurs in large amounts at about 640-730 ℃ and thus provides a strong precipitation strengthening effect. High-density dislocation and Mo addition increase the nucleation rate and refine the precipitates of TiC. Adding Mo increases the “nose” temperature of the NrT and PTT curves and brings forward the precipitation start time, thus improving the high-temperature tempering mechanical properties of AF.

Oil casing steel plays a critical role in the oil and natural gas industry, and its performance is significantly influenced by non-metallic inclusions. Rare earth (RE) elements can effectively modify these inclusions. In this study, industrial experiments were conducted to investigate the effects of rare earth metal alloying on inclusion characteristics during the refining, continuous casting, and hot-rolling processes. The evolution of inclusion morphology, quantity, and size was analyzed using SEM-EDS, FactSage 8.3 thermodynamic software, and an OTS One Bond inclusion analysis system. Results show that when the rare earth content was 4 × 10^-6, the total oxygen content decreased to 7 × 10^-6. In addition, rare earth microalloying transformed Ca-Al-O inclusions into Ca-RE-Al-O inclusions. Following alloying, the inclusion number density in continuous casting billet samples increased from 44 mm^-2 to 46 mm^-2, with the number density of 0-2 μm inclusions rising from 13 mm^-2 to 18 mm^-2, while the number density of 5-30 μm inclusions fell from 7 mm^-2 to 5 mm^-2. Overall, the average inclusion size decreased after rare earth metal addition. XRD and XRF analyses revealed the formation of rare earth phases in the refined slag after vacuum degassing rare earth alloying. Thermodynamic calculations indicate that at 1600 ^oC, the Gibbs formation energies of CaO and CeAlO₃ in steel were -357088.82 and -86892.89 J/mol, respectively, supporting the formation of these inclusions upon rare earth addition. In RE-free furnaces, both thermodynamic calculations and experimental results showed that CaS inclusions formed during solidification, with CaS precipitating around the edges of Ca-Al-O/Ca-RE-Al-O inclusions. In RE-containing furnaces, the addition of rare earth reduced the precipitation of calcium aluminate inclusions, leading instead to the formation of CaO·REAlO₃ inclusions, which likely serve as nucleation sites for CaS precipitation during solidification. During hot-rolling, long strip inclusions were observed in steel without rare earth; however, rare earth alloying improved the deformation ability of inclusions, preventing the formation of long strips due to inclusion crushing. Notably, the modifications induced by rare earth were independent of the inclusions’ Ca content. In hot rolled tube samples without rare earth, only inclusions with moderate Ca content exhibited good deformability. The low yield of rare earth metal was primarily attributed to reactions between the molten steel and refining slag, as well as the removal of rare earth inclusions from the molten steel to the slag.

The recrystallization texture plays a crucial role in determining the magnetic properties of non-oriented silicon steel. Texture evolution during grain growth depends on orientation-related grain size, grain boundary characteristic distribution, and the spatial distribution of texture components. Grain boundary segregation elements can hinder nucleation and growth of recrystallization grains by reducing grain boundary mobility, and thus alter the orientation-related grain size and spatial distribution of various texture components. However, the effects of these grain boundary segregation elements on the microstructure at the completion of primary recrystallization and on subsequent grain growth behavior remain unclear. In this study, the mechanisms by which segregation elements influence texture competition in Sb-containing non-oriented silicon steel during grain growth were elucidated using EBSD. The orientation pinning effect within Goss grain clusters suppresses the growth of Goss ({110}<001>) grains, allowing adjacent grains to grow rapidly by consuming Goss grains in these clusters. The grain boundary segregation element Sb reduces {111}<112> grains around Goss clusters and impedes the formation of large-size {111}<112> grains, leading to a weakened {111}<112> texture and enhanced λ texture components. These findings demonstrate that segregation element Sb can modify texture competition during grain growth by regulating the spatial distribution of various texture components, offering a novel approach for controlling recrystallization texture.

Homogeneous titanium matrix composites (TMCs), despite their unique microstructure and properties, often exhibit inadequate impact resistance. To overcome this limitation, this study presents a high-energy-absorbing and impact-resistant TMC by incorporating a dual design strategy: a porous surface layer and a functionally graded internal structure. A Ti6Al4V/TiC graded material, consisting of one porous titanium surface layer and five gradient layers, was fabricated using the ZrO₂ hollow sphere placeholder method combined with hot pressing sintering. The porous titanium layer exhibited a porosity of 40%, while each of the five gradient layers achieved a relative density of 98.8%. Moving from the bottom layer towards the porous surface, the TiC content was systematically increased from 0 to 8%, and the TiC particle size was enlarged from 3-5 μm to 15-53 μm. The microstructure and properties of the fabricated material were comprehensively characterized. The gradient layers were metallurgically bonded, ensuring structural integrity. The matrix microstructure comprised lamellar α-Ti and β-Ti phases, with TiC particles predominantly distributed along prior β-Ti grain boundaries. The identified strengthening mechanisms included grain refinement, dislocation strengthening, and dispersion strengthening. Compared with the monolithic Ti6Al4V layer, the incorporation of TiC particles effectively refined the prior β-Ti grains along with the α-Ti and β-Ti phases. The gradient layer containing 8%TiC particles (15-53 μm) achieved the highest hardness. In contrast, the layers containing 4% and 6%TiC particles (3-5 μm) demonstrated enhanced tensile strength and toughness. Notably, the layer with 4%TiC showed superior toughness, while the layer with 6%TiC achieved the maximum tensile strength. Compression tests revealed that the porous titanium surface layer possessed remarkable energy absorption capacity, undergoing progressive collapse deformation under relatively low compressive loads before reaching densification. The Ti6Al4V/TiC functionally graded material with a porous surface effectively enhanced protective performance through a synergistic mechanism: the highly energy-absorbing porous surface mitigated initial impact forces, the high-hardness impact-facing layers provided superior penetration resistance, and the high-strength, high-toughness backing layers contributed to excellent residual load-bearing capacity and overall impact resistance.

The rapid exploitation of marine resources in China has heightened the need for advanced marine engineering equipment and imposed more stringent requirements on the surface performance of its key components. CrN/NbN coatings, with their excellent corrosion and wear resistances, demonstrate potential for applications in marine service environments. In this study, CrN/NbN coatings were deposited on 45# steel substrates using arc ion plating technology. A multilayer/nanolayer design and ion etching process were implemented to reduce coating defect densities, thereby enhancing overall coating performance. SEM analysis revealed that S2-S6 coatings exhibited fine columnar structures, with well-defined and cohesive sublayer interfaces in S2 and S3 multilayer coatings. XRD and TEM analyses confirmed that the primary phases of the coatings were CrN and NbN. HRTEM analyses demonstrated that S6 coating present nanolayer structure with a modulation period of 8.9 nm, where CrN and NbN sublayer thicknesses were approximately 2.7 and 6.2 nm, respectively. A coherent interface was observed in the S6 coating, accompanied by the interdiffusion of Nb and Cr elements between the CrN and NbN sublayers. The fast Fourier transform (FFT) image displayed streak-like features characteristic of stacking faults, as well as two sets of diffraction patterns indicative of coherent sublayer interfaces. Nanoindentation tests revealed that among the fabricated coatings, the S1 monolayer coating exhibited the lowest hardness of (21.8 ± 0.7) GPa, while the S4 coating demonstrated the highest hardness of (30.1 ± 1.4) GPa, attributed to its coherent interfaces and stacking faults. Ion etching had minimal impact on coating phases and mechanical properties. However, ion bombardment effectively interrupted the continuous growth of large particles, resulting in smoother surfaces and interfaces and thereby reducing surface defect proportions. The defect percentages for S3 and S5 coatings were (2.7 ± 0.19)% and (2.43 ± 0.49)%, respectively. These lower defect densities contributed to higher pore resistance (R_po) and charge transfer resistance (R_ct). As sublayer thickness decreased, the electrochemical and tribocorrosion performance of CrN/NbN coatings improved progressively, with the S6 sample achieving the lowest corrosive wear rate of 2.42 × 10^-6 mm³/(N·m). The tribocorrosion failure mechanism was preliminarily explored, identifying layer-by-layer peeling as the dominant failure mode. Compared to NbN monolayer coatings, CrN/NbN multilayer/nanolayer coatings exhibited superior mechanical properties and corrosion resistance due to interface blocking and reinforcing effects. Furthermore, the application of ion etching to CrN/NbN multilayer/nanolayer coatings enhanced their electrochemical corrosion and tribocorrosion properties by disrupting the growth of large defects.

Infected bone defect is one of the most common and serious diseases in orthopedic surgery. Hence, it is important to design bone tissue engineering biomaterials with both antibacterial and osteogenic properties for the repair of infected bone defects. Accordingly, in this study, BA + Alg@Ca composite coatings (BA represents H₃BO₃ hydrothermal treatment, Alg@Ca represents CaG-incorporated alginate) composed of an inner layer (Mg(OH)₂), a middle layer (MgB₂O(OH)₆), and an outer layer (Alg@Ca) were constructed on pure Mg by hydrothermal treatment and dip coating, and the formation mechanism and in vitro biocompatibility were systematically investigated. BA + Alg@Ca coatings dramatically improved the degradation resistance of pure Mg, revealing a lower pH value and released Mg²⁺ concentration in the immersion test as well as nobler open circuit potential, larger impedance modulus, and lower corrosion current density in the electrochemical evaluation. In vitro antibacterial examination showed that BA + Alg@Ca coatings effectively inhibited the growth of gram-positive and gram-negative bacteria due to the synergetic effects of [B(OH)₄]^- and alginate. BA + Alg@Ca coatings also demonstrated good cytocompatibility with normal cell viability and proliferation. Moreover, these coatings could induce macrophage polarization to the M2 phenotype (anti-inflammatory), suggesting that they resulted in favorably selective cell adhesion and antibacterial performance. Overall, the BA + Alg@Ca coating-modified pure Mg can potentially be used for the repair of infected bone defects in the clinic.