Nanoporous metals have a porous structure with bicontinuous nanoscale voids and ligaments. Thus, nanoporous metals differ from their bulk counterparts in mechanical, physical, and chemical characteristics due to their unique ligament structure and high surface-to-volume ratio. The surface structure and chemistry of nanoporous metals play critical roles in their applications in catalysis, sensing, and other fields. The surfaces of nanoporous metals contain a substantial number of low-coordination sites, which are vital for improving their catalytic performance. Moreover, the addition of platinum to nanoporous gold has a massive impact on its catalytic and mechanical characteristics. High-resolution transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (STEM) are commonly used to study the atomic structure of crystals. However, since these techniques only provides two-dimensional projection images, it is usually hard or even impossible to directly and quantitatively resolve the three-dimensional (3D) structure of nanocrystals, especially their surface crystallography and coordination information. Compared to traditional TEM and STEM imaging technologies, electron tomography with atomic resolution provides a powerful means to resolve 3D atomic-resolution information of materials. In this work, the surface structure and chemical composition of nanoporous gold and nanoporous gold-platinum were analyzed using STEM, electron tomography, and three-dimensional reconstruction of energy dispersive spectroscopy (EDS) results. The atomic structure of the ligament surface was examined using electron tomography with atomic resolution. It was observed that, surface defects can be separated into two categories: kinks and steps on the {111} terrace, and dents and pits. Surface dents and pits introduce a greater number of low-coordination sites than kinks and steps. Furthermore, the segregation of Pt on the ligament surface was discovered by combining the atomic-resolution electron tomography with the 3D reconstruction of EDS results.

X80 pipeline steel, which is mainly composed of ferrite/bainite, is an important structural steel for pipeline transportation. The plastic deformation of X80 pipeline steel at different strain rates caused by geological and human factors deteriorates its strength. Microstructural transformation and strain localization during deformation are the fundamental factors that deteriorate the mechanical properties of steel. Therefore, in this study, the strain partitioning behavior and microstructure evolution mechanism of ferrite and bainite in X80 pipeline steel at different strain rates (10^-4 s^-1 to 10^-1 s^-1) under 5% deformation were revealed using representative volume element models and electron backscatter diffraction technology. The results show that when the strain rate is low (10^-4 s^-1 to 10^-3 s^-1), ferrite has sufficient time to complete the evolution of geometrically necessary dislocations (GNDs) to low-angle grain boundaries (LAGBs) and the transformation of LAGBs to high-angle grain boundaries (HAGBs). Ferrite can release strain distortion energy, which can weaken the strain localization behavior of X80 steel. As the strain rate increases, the strain response time decreases, hindering the transition from LAGBs to HAGBs. This results in the accumulation of high-density GNDs and LAGBs in ferrite, thereby intensifying strain localization. Additionally, when the strain rate is high (10^-2 s^-1 to 10^-1 s^-1), the strain partitioning coefficient between ferrite and bainite could be reduced, thereby producing the strain gradient in the vicinity of the interface and resulting in GNDs accumulation and back stress formation. Furthermore, ferrite and bainite could show compressive and tensile stresses, respectively, thus limiting the strain coordination between the two phases significantly, increasing the stress concentration near the interface, and reducing the strain hardening ability. The strain partitioning behavior between ferrite and bainite was further revealed to better understand the plastic deformation of X80 pipeline steel.

Marine-grade high strength steel (yield strength = 590 MPa) is a low-carbon, low-alloy steel characterized by high strength and toughness, excellent weldability, and seawater corrosion resistance. Thus, it is suitable for structural applications and widely used in the shipbuilding industry. Recently, wire arc additive manufacturing (WAAM) has attracted significant attention worldwide because of its high deposition rates and material utilization ratios, low material and equipment costs, and good structural integrity. However, the research on WAAM of 590 MPa marine-grade high strength steel is limited. In this work, 590 MPa marine-grade high strength steel components were produced by cold metal transfer and pulse-arc additive manufacturing (CMT + P-WAAM). The effect of the heat input on the microstructures and mechanical properties of the developed steel were investigated using several techniques, including OM, SEM, EBSD, and TEM. The results indicate that at a heat input of 5.6 kJ/cm, the microstructures of the WAAM deposited metals are mainly upper bainite and granular bainite, the area fraction of the martensite-austenite (M-A) constituents accounts for about 14.82%, the length ratio of the effective high-angle grain boundary (grain boundary angle α > 45°) is 36.3%, the tensile strength of the deposited metals are 843 and 858 MPa in the horizontal and vertical directions, respectively, and the average microhardness is about 286 HV. However, its impact absorbed energy at -50^oC is only 15 and 16 J in the horizontal and vertical directions, respectively. At a heat input of 13.5 kJ/cm, the low cooling rate and the high inclusion (inclusion size d > 0.4 μm) content promote the formation of a large quantity of acicular ferrites with lath bainites and a small amount of granular bainites. The area fraction of the M-A constituents is reduced to 4.21%, and the length ratio of the effective high-angle grain boundary is increased to 52.4%. The tensile strength of the deposited metals in the horizontal and vertical directions is reduced to 723 and 705 MPa, respectively. Similarly, the average microhardness is also reduced to 258 HV, but the low-temperature impact absorbed energy is greatly improved, reaching 109 and 127 J, respectively, which is 7-8 times that of the WAAM deposited metal at a low heat input. The impact fracture characteristics also changed from a quasi-cleavage fracture to a typical ductile fracture.

The use of advanced high strength steel (AHSS) sheets has been acknowledged as an important solution for vehicle weight reduction, and thus, carbon dioxide emission reduction. The development of third-generation AHSS has become one of the steel industry's most prominent concerns in recent years. However, the selective oxidation of alloy components such as silicon and manganese makes obtaining high-quality hot-dipped galvanized steel sheets extremely difficult. To determine an optimal process window for controlling the surface microstructure of AHSS, the effect of dew point on selective oxidation of silicon and manganese, and decarburization in a 0.2%C-1.5%Si-2.5%Mn (mass fraction) steel sheet was studied by performing continuous annealing simulation experiments. Glow discharge optical emission spectrometry (GD-OES) was used to determine the depth profiles of alloy elements, and SEM and OM were used to determine the depths of internal oxidation and decarburization zones in the subsurface. The surface and internal oxides' precise microstructures were studied using TEM on a FIB-prepared cross-sectional specimen. The increasing dew point of the atmosphere through the heating and soaking section portion of continuous annealing results in the transformation of external oxidation of silicon and manganese to internal oxidation. When the steel was annealed in an environment with a dew point of -40^oC, a continuous silicon, manganese external oxidation layer with an average thickness of 40-50 nm covered the surface. When the dew point was elevated to +10^oC, a subsurface oxidation layer approximately 5-μm thick formed. Due to the substantially lower oxygen pressure required for the Si/SiO₂ equilibrium, the internal oxides exhibited a core-shell structure consisting of a Si-rich oxide core and a surrounding Mn-Si mixed oxide shell. A higher dew point results in the formation of an obvious decarburization layer in the subsurface, which is visible as a layer of ferrite grains with significantly decreased microhardness. When the dew point was increased from -40^oC to +10^oC, the thickness of the decarburized zone increased from 0 μm to 45 μm, and the C content of the decarburized zone decreased from 0.18% to 0.01%. External oxidation can no longer be decreased further by increasing the dew point, yet the depth of internal oxidation and decarburization in the subsurface continues to increase. Therefore, maintaining an appropriate dew point range for the annealing atmosphere is necessary to manage external oxidation and decarburization. The optimal dew point should be adjusted between -20^oC and -10^oC when annealed at 870^oC for 120 s in 5%H₂-N₂ atmosphere.

Hydrogen is clean energy that can replace traditional fossil fuels in the future because of its high energy density, easy recharging, and availability of current liquid fuel infrastructure. However, the polymer-electrolyte membrane fuel cell requires controlled storage and efficient hydrogen release. Recently, liquid-phase chemical hydrogen storage materials with high gravimetric hydrogen density have emerged as promising candidates to overcome such challenges. Among these materials of interest, hydrous hydrazine (N₂H₄·H₂O) is the best candidate; however, it has not been fully explored as an alternative for chemical hydrogen storage applications. A catalyst is essential to hydrogen production at a sufficient reaction rate for N₂H₄·H₂O-based hydrogen generation systems. In this study, a series of supported Ni_{100 - x} Ir _x /Al₂O₃ catalysts were prepared using simple impregnation, roasting, and reduction method. The effect of reaction conditions on the activity and selectivity was evaluated in decomposing N₂H₄·H₂O to hydrogen. The phase/structure of the catalysts was characterized using XRD, TEM, XPS, BET, and H₂-TPD to gain insight into the catalytic performance of the Ni_{100 - x} Ir _x /Al₂O₃ catalysts. It indicated that the Ni₆₀Ir₄₀/Al₂O₃ catalyst, comprising Ni-Ir alloy nanoparticles with an average size of 2-4 nm and crystalline γ-Al₂O₃, exhibited excellent catalytic activity (> 200 h^-1) and selectivity (> 99%) toward hydrogen generation from N₂H₄·H₂O at different temperatures, from 293 K to 353 K. The Ni₆₀Ir₄₀/Al₂O₃ catalyst is durable and stable; however, the catalytic activity decreased from 249.2 to 225.0 h^-1 (~9.7%) after five runs with 99% H₂ selectivity at 323 K toward the dehydrogenation of N₂H₄·H₂O. In addition, parameters, such as temperature, N₂H₄·H₂O and NaOH concentration, and catalyst mass on N₂H₄·H₂O decomposition were investigated over the Ni₆₀Ir₄₀/Al₂O₃ catalyst. The kinetic rate equation for catalytic decomposition of N₂H₄·H₂O could be represented using the following expression: r = -k[N₂H₄·H₂O]^0.346/0.054[NaOH]^0.307[Catalyst]^1.004, where k = 4.62 × 10⁹exp(-5088.49 / T). The results could provide a theoretical foundation for applying N₂H₄·H₂O as a promising hydrogen storage material.

Ni₃Al-based superalloys are widely used in aero-engine parts. In addition to having a good temperature bearing capacity, the oxidation resistance of the alloy is also high. In this work, a multiphase Ni₃Al-based superalloy was selected as the experimental material. Three micro-regions (γ' + γ dendrite, interdendritic β phase, and γ' envelope) containing different phases were obtained by heat treatment. The isothermal oxidation behavior of the micro-regions was studied under 1000^oC, where the three micro-regions exhibited different oxidation behaviors at the initial stage of oxidation. The γ' envelope has an obvious double-layer oxide scale showing a cellular bulge. The outer layer is a mixed layer (NiO, NiFe₂O₄, and Al₂O₃), and the inner layer is a single Al₂O₃ layer. However, the γ' + γ dendrite and the interdendritic β phase form a single layer of the Al₂O₃ film. With increasing isothermal oxidation time, the oxide scale composition of the three micro-regions gradually tends to be the same, forming a dense single Al₂O₃ layer.

Titanium alloys are used in the aerospace industries, chemical industries, biomedicine, marine ships and other fields because of their high strength-to-weight ratio, good corrosion resistance, and biocompatibility. However, titanium alloys have low hardness and poor wear resistance, which limit their applications, especially under sliding contact. Plasma nitriding (PN) is an effective method for improving titanium alloy's tribological properties. PN can produce a composite layer composed of TiN and Ti₂N to improve the friction and wear properties of titanium alloy. However, the high temperature in the nitriding treatment results in a brittle “α-stabilized layer” (a continuous layer of α phase titanium enriched with interstitial nitrogen atoms) and unfavorable phase transformations in the substrate that can impair the fracture toughness, ductility, and fatigue properties. In this study, a low-temperature plasma-composite treatment, consisting of plasma oxidizing and oxynitriding, has been developed to improve the hardness and wear resistance of Ti-6Al-4V alloy. The effect of the preoxidation process on the surface microstructure, phase composition, and wear performance of titanium alloy was studied. The microstructure and phase composition of the plasma-composite layer were observed using SEM, TEM, XRD, and other methods. The results showed that the composite layer of titanium alloy treated using plasma-composite-treatment is composed of compound and diffusion layers, and the phase is TiO₂ (rutile) and TiN_0.26. Microhardness and wear-resistance properties of the composite layer were characterized using microhardness tester, nanoindentation, and reciprocating-friction tester. The plasma-composite treatment can increase infiltration depth of the diffusion layer, surface hardness, elastic modulus, and wear resistance of the titanium alloy more than the traditional nitriding process.

Nickel-based superalloys have gathered much attention recently due to their excellent performance at high temperatures and corrosion resistance. Furthermore, their advancement is a crucial indicator to determine the development level of metallic materials. Thus, rapid development in microsystem technology which focuses on development of lightweight and miniaturized materials is required. Moreover, the demand for micromaterials such as ultra-thin strips is also growing, leading to higher demand and better performance requirements. China's production of ultra-thin strips started relatively late and nickel-based ultra-thin strips rely on imports. Strengthening the production research of nickel-based ultra-thin strips, meeting the needs of aerospace and emerging microsystems, and removing import dependence are key issues that need to be addressed in future. The accurate acquisition of microstructure variations and strip properties after thinning and heat treatment is crucial in controlling the forming accuracy and preventing defects. Remarkably, the GH3600 nickel-based superalloy with thicknesses from 0.5 mm with polycrystalline layer to 0.07 mm with local single crystal layer was obtained by cold rolling and annealing. The effects of cold rolling reduction and annealing temperature on the microstructure and mechanical properties of the superalloy were investigated, and changes in the microstructure and mechanical properties caused by thickness variation were analyzed. The results reveal that with the increase of cold rolling reduction, the austenite grains in the alloy are elongated along the rolling direction, and the annealing twins gradually disappear. A complete recrystallization microstructure was obtained after further annealing, and grain size increased with the annealing temperature while the strength and hardness decreased. After annealing at the same temperature, the material's yield strength increases with the reduction and refinement of the recrystallized grain. However, as the strip thickness decreases, the grain layer decreases along the direction of the strip thickness. After annealing at 1000 and 1050^oC, abnormal coarse grains appear in 0.07 mm thick strips, leading to the appearance of single grains in some areas along the thickness direction of the ultra-thin strips. The tensile strength and elongation of the strip are “smaller and weaker” with the decrease of strip thickness/average grain size ratio due to the size effect. The comparative analysis demonstrated that the average grain size of GH3600 ultra-thin strips annealed at 800-900^oC is approximately 7 μm, the local orientation difference is approximately 0.5, the yield and tensile strengths could reach up to 400 and 600 MPa, and the elongation is approximately 13%, respectively.

In the development of advanced steel, accurate and detailed knowledge about the kinetics of phase transformations and microstructure formation is critical. The critical issue in pearlite-austenite transformation is the consideration of diffusional paths of the alloy element. Simulation has been an available method to study the diffusion of alloy elements and the migration rate of the phase boundary in the complex morphological evolution of austenite growth. The isothermal pearlite-austenite transformations at 720 and 740^oC in Fe-0.6%C-2%Mn (mass fraction) alloy were studied by phase-field methods based on MICRESS. At different temperatures, the effects of diffusional paths on the austenite transformation were discussed. To achieve a semiquantitative verification of the simulated results, the migration rates of the austenite/pearlite boundary at 720 and 740^oC were estimated from the experimental kinetics curves by fitting the JMA equation. By measuring the Mn profile in austenite, the modes of the austenization at 720 and 740^oC can be verified as partitioned local equilibrium (PLE) and non-partitioned local equilibrium (NPLE) modes. The heterogeneous distribution of Mn in austenite at 740^oC can be observed with STEM-EDS. However, the homogenous distribution of Mn can be found near the pearlite/austenite boundary in austenite at 720^oC. The cases considering the γ, α, and interface-diffusional paths were simulated by phase-field methods to compare with the migration rates of the austenite/pearlite boundary. Because carbon is an interstitial element in steel and has an interstitial diffusional mechanism, it can be speculated reasonably that the diffusion of C mainly proceeded in austenite and ferrite through the considerations of the atom-size of carbon and the experimental results. Phase-field methods were used to study Mn diffusion in the lamellar pearlite-austenite transformation. With the analysis of the experimental estimations, the interface-diffusional path was observed as the dominant path for the Mn diffusion. It is because the Mn atoms have greater diffusivity in interfaces than in γ or α-diffusional paths. Furthermore, the diffusional activation energy is closely related to the diffusivity of Mn at the interface. Moreover, compared with the γ-diffusional path, the diffusional flux of Mn in ferrite is much larger than that in austenite. Thus, it can be concluded that the contribution of the α-diffusional path to the migration rate of the pearlite/austenite boundary is larger than that of the γ-diffusional path. As a result, considering the α-diffusional path in the thermodynamics analysis under NPLE mode makes more sense. However, ignoring the interface- and α-diffusional path, which is different from the traditional cognition in PLE mode, will result in a magnitude error for the thermodynamics analysis.

Mold heat transfer has an essential influence on the initial formation of surface longitudinal cracks in slabs, and control over various process parameters in continuous casting is very important for achieving the desired qualified product. A study on the influence of parameters on mold heat transfer helps determine the law of mold heat transfer and realize fine control of the parameters. However, the cross-correlation between continuous casting parameters makes it difficult to identify the independent influence of each parameter on the mold heat flux. Based on the parameters in the production process of continuous casting using big data mining and analytics, a new method for obtaining the independent effect of each parameter on the mold heat transfer is proposed, namely the new method for independent process influence (IPI). The five links included in the IPI method can be calculated in turn: data preprocessing, cross-correlation level calculation, main related parameters testing, data filtering, and independent influence analysis. The results showed that, in addition to the conventional influence of the casting speed, superheat, slab width, mold taper, and total water flow on mold heat flux, the oscillation frequency, mold level, immersion depth of nozzle, stopper position, and argon blowing flow rate at different positions affected mold heat flux to a certain extent. The argon blowing flow rate of the stopper, stopper position, and immersion depth of the nozzle positively influence the mold heat flux. Conversely, the argon blowing flow rate of the nozzle, oscillation frequency, and total water flow rate have a negative influence. In addition, for the argon blowing flow rate in the nozzle and total water flow rate, an inflection point is reached after achieving the maximum values of 3.5 L/min and 8250-8750 L/min, respectively. This research can provide a new reference and basis for the systematic mechanism analysis of the heat transfer process and formation of longitudinal surface cracks in continuous casting and services for the fine control of high-quality steel production on site.

Advanced intermetallic β-solidifying γ-TiAl-based alloys have various potential applications in the aerospace and automobile industries due to their low density, functionality at higher temperatures, and high specific strength/modulus. The crucial aspect that needs to be considered when developing a new β-solidifying γ-TiAl alloy is to clarify the influence law of β-stabilizer elements on the phase transformation behavior of γ-TiAl alloys. In this work, the impact of W contents (0.5%-1.0%, atomic fraction) on the phase transformation behavior and microstructure characteristics of Ti-42Al-5Mn-xW (atomic fraction) alloy with low cost and superior temperature workability was systematically investigated. The findings demonstrate that there were minor changes in the β-phase single region temperature (T_β ) and γ phase solvus temperature (T_γ-solv); furthermore, the eutectoid reaction temperature (T_eut) increases with the W content from 0.5% to 1.0%. Addition of W influences the solid phase transformation pathway to a certain extent. When the concentration of W increases to 0.5%, the equilibrium phase of the alloy at near service temperature gradually changes from α₂ + γ + Laves to β_o + α₂ + γ + Laves. Additionally, W addition will also have a substantial effect on the lamellar microstructure. The volume fraction of lamellar microstructure considerably decreased after alloying with (0.5%-1.0%)W for Ti-42Al-5Mn alloy when being treated in the (γ + α + β) triple-phase region followed by furnace cooling. Increasing the W content to 0.8% and 1.0% results in the development of γ and β_o grain phases with almost complete removal of α₂/γ lamellar structures. However, the W-free and W-bearing Ti-42Al-5Mn alloys show near complete lamellar structures when treated in (α + β) two-phase region followed by furnace cooling. Furthermore, when the content of W increased from 0.5% to 1.0%, an equiaxed grain structure with refined lamellar colonies is typically obtained.

TC4 alloy components with complicated geometries can be directly fabricated using selective laser melting (SLM) at a low cost. These components are often used under complex service conditions. Thus, it is important to investigate the effects of the stress ratio (R) on the fatigue crack growth (FCG) rate (da / dN) in SLM TC4 alloys with defects at the steady state to develop guidelines for damage-tolerance design and fatigue life assessment. In this work, SLM TC4 alloys containing two different microdefects were used to qualitatively examine the effect of the defect size on the da / dN at the steady state. In addition, comparative studies using an alloy with smaller defect sizes were performed at the steady state and at R = 0.1, 0.3, and 0.5. The relationship between the da / dN and stress intensity factor range(ΔK)was plotted and analyzed by fitting the Paris formula. The results show that the Paris formula parameter, m, is constant and the parameter, C, increases, which means that the increase in the defect size increases the da / dN. The da / dN increases with an increase in R, and the da / dN curves converge at low ΔK, which are reflected in the increase in the parameter, m, and the decrease in the parameter C. Additionally, there is a linear relationship between m and lgC (the common logarithm of C), which is not affected by R. Finally, the change patterns in the da / dN caused by the microdefects and R were analyzed along with the fatigue damage mechanisms.