A general mutual constraint exists between the strength-plasticity and the strength-electrical conductivity of metallic materials. This study proposes an independent space model of dislocation motion to show that the trade-off relation between the strength and plasticity of metallic materials is controlled by the independent spatial size of dislocation motion. Furthermore, the model is used to show that the metal type, alloy composition, deformation temperature, and strain rate are the key factors regulating the spatial size, whereas the microstructure or grain size distribution has a limited influence on it. This finding explains why it is difficult to simultaneously improve strength and plasticity through microstructure or grain size optimization when the metal type, composition, and deformation parameters are fixed. Further, based on the grain size dependence of dislocation piling-up in single-phase alloys, a quantitative trade-off model between tensile strength and uniform elongation is established and experimentally verified in a variety of metal alloy systems. Three high-strength and high-electrical conductivity principles for metal wires are proposed by analyzing the differentiated effects of grain boundaries, orientation, and nanoprecipitation relative dislocation piling-up and electron scattering: elongated grains, strong texture orientation, and nanoprecipitate regulation. Based on the three principles, a quantitative model describing the relationship between strength and electrical conductivity is constructed, and the underlying mechanisms of typical phenomena, such as the synergistic improvement and mutual constraint of strength and electrical conductivity in different metal wires, are systematically explained. Finally, high-performance conductors based on three principles, with performance that breaks through the existing strength-electrical conductivity constraint, are developed. Establishing quantitative models describing the relationships between strength and plasticity, as well as between the strength and electrical conductivity of metallic materials, can efficiently guide material selection, composition design, and microstructure process control, ensuring the service safety of components.
With the advancement of the global carbon neutrality strategy, proton exchange membrane fuel cells (PEMFCs), a typical type of low-temperature fuel cell, have been widely applied in transportation power systems, portable power equipment, and distributed energy systems because of their notable technical advantages, including high energy conversion efficiency (> 60%), low operating temperatures (60-80 oC), and near-zero carbon dioxide emissions. As a key fuel cell component, the bipolar plate serves several essential functions, including gas distribution, electron conduction, and management of reactant flow fields. The corrosion resistance and electrical conductivity of bipolar plates directly determine overall fuel cell performance, including energy conversion efficiency, durability, and manufacturing cost. Therefore, bipolar plates with high corrosion resistance and high conductivity for PEMFCs have recently been the focus of intensive research. This study reviews various metallic bipolar plates, their surface modification strategies, and the resulting performance, including Al/Ti alloys and austenitic/ferritic stainless steels. Both Al and Ti alloys require noble metal coatings, such as Au/Ni-P and CrN, to balance corrosion resistance and conductivity, leading to high costs and complex fabrication processes that hinder commercialization. Although the corrosion resistance of austenitic stainless steels can be enhanced through synergistic Cr/Mo alloying, the rapid thickening of the passivation layer leads to excessively high interfacial contact resistance during service. In contrast, ferritic stainless steels are prone to intergranular corrosion; however, this can be mitigated through ultralow carbon content and stabilization by Ti/Nb microalloying. Their application in bipolar plates remains constrained because of poor formability and corrosion current densities in the uncoated state that often exceed the target values set by the United States Department of Energy. Although surface coating technologies, such as CrN and conductive polymers, can improve corrosion resistance and conductivity, process complexity and coating durability remain major concerns. Furthermore, this paper introduces a novel alloying strategy for high-Cr ferritic stainless steels used as coating-free bipolar plates, which simultaneously achieves excellent ductility, high corrosion resistance, and high electrical conductivity under simulated PEMFC conditions. Finally, future development directions for PEMFC bipolar plate materials are discussed.
Due to their excellent performance, near α and α + β dual-phase titanium alloys are critical materials in aerospace engineering. Enhancing the performance stability of titanium alloy forgings has become a focal point of research in engineering applications. However, the microtextures within these forgings significantly affect key properties such as fatigue resistance, which limits the overall performance of titanium alloy forgings. Recent studies indicate that optimizing the hot working process to improve the crystallographic orientation and microstructure uniformity is an effective means of enhancing alloy performance. This study reviews the origins of microtextures in titanium alloy forgings, their potential negative effects, and the optimization processes involved. Finally, the study presents several research guidelines aimed at improving the microstructural uniformity of titanium alloy forgings.
Copper matrix composites are expected to be essential conductive materials for harsh service environments in the future, due to their remarkable properties, including high strength, excellent electrical and thermal conductivity, and their resistance to high temperatures, wear, and arc ablation. This paper summarizes the typical materials, primary preparation techniques, microstructures, and properties of conductive copper matrix composites. It further analyzes the basic principles and characteristics of in situ preparation methods. Finally, the relationship between microstructures and the overall properties of various high-strength and high-conductivity copper matrix composites is discussed, and the problems and challenges that future research may face are considered.
Freckles severely degrade the crystalline integrity and high-temperature mechanical properties of superalloy blades, posing a critical barrier to the fabrication of large-sized directionally solidified blades for heavy-duty gas turbines. This paper reviews the research progress on freckles over the past several decades, both domestically and internationally. It summarizes investigations of elemental segregation, thermo-solutal convection, and dendrite arm fragmentation during freckle formation using techniques such as atom probe tomography, synchrotron radiation, and numerical simulation. Freckle formation criteria based on temperature fields or mushy-zone density have been described, and the influences of blade geometry and solidification parameters on freckle distribution have been elucidated. Moreover, freckle control techniques and methods are comprehensively reviewed. At present, most studies on freckles focus on cases involving “small-sized specimens, low withdrawal rates (≤ 1.5 mm/min), and alloys with high W/Re contents”, which exhibit relatively significant differences from the freckle solidification conditions and occurrence patterns of “large blades, high withdrawal rates (≥ 2.0 mm/min), and alloys with low W/Re content” in the engineering practice of heavy-duty gas turbines. Therefore, developing rapid freckle prediction methods and effective control technologies specifically suited to the solidification conditions of large blades in heavy-duty gas turbines has become a critical need and a key research direction for the present and future.
Fatigue is an important failure mode arising during the service conditions of superalloys. The fatigue crack propagation behavior of superalloys is influenced by many factors, such as temperature, stress, and ambient O. Among these, ambient O plays an important role. Numerous studies showed that fatigue crack propagation in a vacuum proceeds mainly as a transgranular fracture, while in air, it occurs mainly as an intergranular fracture. This is because the grain boundary is oxidized, making it weak. There is a lack of studies on the influence of different degrees of O accumulation on oxygen-induced grain boundary damage and ways to characterize this influence. The degree of O accumulation at the grain boundary is usually difficult to evaluate quantitatively. Therefore, to investigate the influence of O accumulation on the damage to grain boundary and to devise a method to characterize this influence, this study designed a series of aging experiments with different parameters to introduce different O accumulation variables. The fatigue crack growth test of aged nickel-based superalloy GH4738 was performed at room temperature ((23 ± 3) oC). For higher aging temperatures and longer aging time, the fatigue crack growth rate was higher, and the fatigue life was shorter. The fatigue fracture morphologies were observed. The grain boundary damage caused by the O accumulation was evaluated by considering the stress intensity factor range (ΔK), corresponding to the complete transformation of fatigue fracture from the transgranular mode to the intergranular mode. The grain boundary separation work calculated based on the molecular dynamics theory was used to verify the law of change in the turning point of the intergranular fracture mode observed experimentally. The experimental and theoretical results taken together showed that with an increase in O accumulation, the fatigue crack growth rate increases, service life decreases, and the turning point of the complete intergranular mode of fatigue fracture approaches the crack initiation end. O caused a reduction in the grain boundary separation work. As the O atom concentration increased, the weakening of the grain boundary became more pronounced. Addition, the higher temperature resulted in faster reduction rate of the grain boundary separation work.
690 alloy, an austenitic nickel-based corrosion-resistant alloy, is widely regarded as an ideal material for steam generator tubing in nuclear power plants. Its superior resistance to stress corrosion cracking, excellent workability, and simple composition make it highly suitable for this application. In China, the domestic production of Alloy 690 tubes involves creating master alloy ingots through a combined melting process, utilizing vacuum induction melting and electro-slag remelting (ESR). To meet the requirements for product quantity and quality stability, typically, the master alloy ingots weigh at least 3 t. During the ESR process, the ingot solidifies progressively in a water-cooled copper mold, resulting in a coarse columnar-grain structure. This solidification structure optimizes the ingot's microstructure and minimizes element segregation. However, it also produces numerous straight grain boundaries, complicating the subsequent cogging of the ingot. This study aims to understand the deformation characteristics of 690 alloy ESR ingots and enhance their forging quality. The high-temperature plasticity of columnar-grain 690 alloy was investigated using Gleeble and conventional tensile testing machines. Results indicate that the plasticity of 690 alloy columnar-grain samples decreases considerably around 1050-1100 oC. Within this low plasticity range, extensive cracking along grain boundaries occurs, leading to intergranular brittle fracture. Above 1100 oC, M23C6 carbides along the grain boundaries dissolve extensively, reducing the stability of the grain boundaries and their resistance to sliding. For 690 alloy columnar-grain samples, poor deformation coordination among grains result in severe local plastic deformation along the grain boundaries. This promote the formation of cavities and cracks, thereby deteriorating the alloy's high-temperature tensile properties. At 1150 oC and higher temperatures, dynamic recrystallization in 690 alloy is significantly enhanced, which effectively restricts the propagation of intergranular cracks and improves the material's uniform deformation capability.
To improve the oxidation resistance, which is compromised by high concentrations of W and Mo in control alloys, developing a new fourth-generation Ni-based single crystal superalloy is essential. In this study, we investigated the oxidation behavior and γ' phase degradation of a new fourth-generation single crystal superalloy containing rare earth (RE) elements at 1100 oC. After an initial 3 h of oxidation, a NiO layer and a discontinuous, needle-like Al2O3 layer rapidly formed on the sample surface, accompanied by the formation of a RE oxide film beneath the NiO layer. The RE oxide film effectively inhibited the internal diffusion of O element and its reaction with refractory metal elements, preventing the formation of spinel oxides and reducing the thickening rate of the spinel oxide layer. As a result, the discontinuous needle-like Al2O3 layer transforms into a partially continuous Al2O3 layer during the second stage (3-25 h) and delayed oxidation-induced mass loss in the third stage (25-60 h). After 100 h of oxidation, continuous Al2O3 and NiAl2O4 spinel layers were formed on the alloy surface, effectively hindering both the outward diffusion of alloy elements and the inward diffusion of O element. Moreover, the γ'-free layer exhibited a notable increase in thickness with the oxidation time. No topologically close-packed phase was detected in the γ'-free layer or in the interior of the sample, indicating the superior high-temperature stability of the alloy.
Rare-earth elements significantly enhance key service performances of nickel-based superalloys. However, due to burning loss, the actual yield of rare-earth elements within the alloy is challenging to control precisely in practice. This study investigates the burning loss pathways of Sc during the melting and casting of a nickel-based superalloy BYG36. Samples were collected from six typical locations in the vacuum induction melting system where burning or volatilization products might be present. The samples were thoroughly characterized regarding Sc content, phases, morphology, and atomic-scale structures. While no Sc residue was found in the furnace ash, observation window, or entry nozzle—indicating minimal volatilization of Sc—a substantial amount of Sc was found adhering to the inner side surface of the crucible, the crucible rim, and the inner side surface of the sprue. Atomic-scale characterizations revealed that the Sc-rich phase on the inner surface of the crucible was a cubic-structured Al1.3Sc0.7O3, in contrast to the previously assumed orthogonal-structured ScAlO3. Intermingling with Al2O3 particles in the refractory materials, this cubic-structured phase likely formed through reactions of Sc with Al2O3 during the melting process. In contrast, Sc residue on the crucible rim and the inner side surface of the sprue was identified as cubic-structured Sc2O3 particles deposited directly on the refractory material surfaces. These inclusions originated from reactions of Sc with O in the alloy melt and adhered to the refractory surfaces as the melt slowly flowed over the crucible rim and sprue during casting.
Magnesium alloys are characterized by low density, high specific strength, high specific modulus, and excellent heat dissipation, which have garnered increasing attention from both academia and industry. Brazing, as a metal and alloy joining technique, is highly valued for its stability in forming and its ability to preserve the microstructure and properties of the base metal. However, due to magnesium's high chemical activity, low melting point, high thermal and electrical conductivity, and tendency to oxidize easily, selecting and designing suitable solder for the brazing process present significant challenges. The Mg-Zn-Al alloy, with its low melting point and composition matching the base metal, is considered a suitable filler metal for brazing magnesium alloys. To study the interfacial diffusion behavior of the molten Mg-Zn-Al alloy during the brazing of Mg-based metals and the influence of the ultrasonic-assisted vibration parameters on the brazing process, a Mg/Mg-Zn-Al interface molecular model was constructed to study the microscopic mechanisms involved. The diffusion behavior at the Mg/Mg-Zn-Al interface was simulated using molecular dynamics on the nanoscale, exploring how ultrasonic assistance on the mechanical properties of the interface, including an analysis of the temperature and strain rate response at the brazing interface under ultrasonic assistance. The results show that ultrasonic assistance increases the diffusion coefficient of the system by 5-10 times and the thickness of the bonding layer by 2.5-7.6 times, significantly lowering the required brazing temperature. Furthermore, while both ultrasonic frequency and amplitude enhance interface equilibrium, increasing the ultrasonic frequency leads to a thicker and more uniformly distributed bonding layer. In terms of mechanical properties, the tensile strength of the ultrasonic-assisted brazing interface, with an amplitude of 0.2 nm and a frequency of 1000 GHz, is 11.5% higher than that of a high-temperature brazing interface. Meanwhile, compared with the mechanical behavior of the interface under various loading conditions, the mechanical properties of the brazing interface are more temperature-sensitive than strain-rate-sensitive, with high-temperature conditions amplifying strain-rate sensitivity for maximum tensile stress.
Al-Zn-Mg-Cu alloys are widely used to prepare aerospace lightweight structures owing to their excellent specific strength and damage tolerance. Their performance depends mainly on the high-density nanoprecipitates formed during artificial aging. However, natural aging after solution quenching changes the evolution path of the precipitates, affecting the subsequent artificial aging process. Currently, there is still considerable controversy regarding the impact of natural aging on the peak strength of these alloys after artificial aging. Therefore, this study investigated the contradictory effects of natural aging on the peak-aged strength of Al-Zn-Mg-Cu alloys after artificial aging. Using a newly developed ultrahigh-strength aluminum alloy, namely Al-9.8Zn-2.23Mg-1.38Cu-0.11Cr-0.1Zr (mass fraction, %), the effects of natural aging on precipitate evolution, solute element distribution, and mechanical properties in the peak-aged state were systematically explored. The results showed that natural aging accelerated the response of the material to subsequent artificial aging. The peak strengths after 0, 1, 7, and 30 d of natural aging and artificial aging were (708 ± 4), (685 ± 3), (712 ± 1), and (722 ± 1) MPa, respectively, exhibiting a trend of initial decrease followed by an increase. This behavior was attributed to the formation of Guinier-Preston I (GPI) zones (1.1-1.7 nm in diameter) during short-term natural aging (1 d), which partially dissolved during artificial aging. This reduced the number density of subsequently formed GPI zones, Guinier-Preston II (GPII) zones, and η′ phases and promoted their coarsening. In contrast, prolonged natural aging time increased the proportion of GPI zones, with sizes exceeding the critical nucleation threshold at artificial aging temperatures, facilitating the formation of finer and more dispersed precipitates during subsequent peak aging. Further, this led to a gradual increase in the proportion of GPII zones and decrease in the proportion of the η′ phase. Compared to the peak-aged sample naturally aged for 1 d, the sample naturally aged for 30 d exhibited an approximately 20% increase in the precipitate number density. In addition, the compositional gradient within precipitates of similar size became less pronounced, with significantly reduced maximum concentrations of Zn, Mg, and Cu.
