Metallic glasses (MGs) have disordered microstructure and no defects like in crystalline materials and possess a suite of outstanding mechanical and functional properties, showing thus promising potential for wide applications. Due to the lack of long range structural order, it is fraught with difficulties to construct the structure-property relationship in amorphous materials. The study of relaxation dynamics provides a very important approach to understand MGs, and is vital to understand their stability and deformation behavior and remains a core issue in the field of condensed matter physics and materials science. In recent years, with the use of more advanced research methods and the deepening of research, it was found that there exists rich dynamics covered by the extremely wide time scale and the different length scales of glassy state. Different dynamic modes not only correlate with each other but also show distinction. This article reviews recent progress in the study of relaxation dynamics in MGs, and its role in understanding and modifying material properties and optimizing material preparation.
This paper briefly reviews the development and research history of strutures of the solid matters, and highlight two new strcutures of precipitates in Mg alloys found by our group recently. (1) The isothermally aged (Mg, In)2Ca "Laves phase" contains two separate unit cells promoting the formation of five tiling patterns. The bonding of these patterns leads to the generation of the present phase but without any six-fold rotational symmetry in a long-range on the (0001)L basal plane, constrainted by the Penrose geometrical rule, completely different from the known Laves phases. (2) The MgZn five-fold nanodomain structure is self-assembled by two separate unit cells (72° rhombus structure: MgZn2, and 72° equilateral hexagon structure: MgZn) under the Penrose geomotrical constraints, containing 2D five-fold symmetry locally and short-range ordered C14 and C15 Laves structures. These two special structures without any translational symmetry on the normal plane while periodical arrangement along the normal direction, are a new class of intermediate structures between crystals and quasicrystals. And thus, they does not belong to any crystals or 2D ordered structures in quasicrystals or quasicrystal approximants.
Microelement B is widely added into almost all commercial superalloys because B contributes to strengthening grain boundaries at high temperature during service. Generally, B is present in two different forms. Besides the solute state in matrix, B tends to react with transition elements at high temperatures, giving rise to various borides including M2B, M3B2 and M5B3 phases. An accurate knowledge of the microstructural characterizations of these borides is of great importance for a better understanding of the structure-property relationship and designing materials with improved properties. By means of various advanced techniques based on the aberration-corrected transmission electron microscopy (TEM), microstructural features of above borides have been systematically investigated. Various defect features which were controversial in the past have been clarified. In this paper, after a brief review on the studies of borides, the atomic-scale information on the microstructural features has been presented. Finally, some prospects for future studies have been proposed.
The concurrence of solid-state phase transition and grain growth is ubiquitous in thermal processing of metallic materials; understanding the concurrence is significant for manipulation of microstructure and design of structured materials with high strength and good ductility. This work will briefly review the recent progresses on the concurrence in nanocrystalline metallic materials, with particular attention to the physical origin, typical examples, underlying mechanisms, as well as microstructures, of the concurrence. On this basis, perspectives on scientific understanding of the occurrence in nanocrystalline materials are addressed.
The research status and application of powder metallurgy (PM) titanium alloys in connection with near net shape forming technology using hot isostatic pressing (HIP) are reviewed in this paper. A brief summary of historic developments with production of clean prealloyed powder and the use of computer simulation techniques in powder densification as important milestones is presented first. The bulk of the paper is concerned with progress made in the last 15 years, especially in the last decade, citing examples from the authors' group. Four types of alloys are covered: a cryogenic titanium alloy, Ti-5Al-2.5Sn with extra-low interstitial (ELI), which is used to make impeller for hydrogen pump of rocket engine, a high temperature titanium alloy Ti55, which is intended for long term service at 550 ℃ in engine applications, and two Ti-Al based intermetallic compounds including both γ-TiAl and an orthorhombic alloy based on Ti2AlNb. Comparisons in mechanical property were made between the PM alloys and their wrought and cast versions wherever possible. Key issues influencing densification, such as powder size segregation and gas pores in large powders, variation in powder surface oxygen content with powder store time, oxygen layer on γ-TiAl powder surface due to abnormally high fraction of the α2-Ti3Al phase as a result of rapid solidification of the powder, were discussed. The final section is dedicated to finite element modelling of powder densification, taking into account such factors as tooling design and stress shielding effect during HIPing. Future research directions are suggested in the summary section.
A new alloy design concept, high-entropy alloys (HEAs), has attracted increasing attentions and becomes a new research highlight recently. Different from traditional alloy design strategy which usually blends with one or two elements as the principal constituent and other minor elements for the further optimization of properties, HEAs are multicomponent alloys containing several principle elements (usually ≥5) in equiatomic or near equiatomic ratio. Due to their unique atomic structure, HEAs possess a lot of distinguished properties. Since the discovery of HEAs, a variety of HEA systems have been developed and shown unique physical, chemical and thermodynamic properties, especially the promising mechanical properties such as high strength and hardness, abrasion resistance, corrosion resistance and softening resistance. Here in this short review manuscript, starting from the research challenges for understanding the deformation mechanism of HEAs, this work briefly summarized the mechanical properties and deformation behavior of HEAs, reviewed the proposed strengthening-toughening strategies and their corresponding deformation mechanism in HEAs. A brief perspective on the research directions of mechanical behavior of HEAs was also proposed.
Properties of landing gear are closely related to the service safety of aircraft. Thus, it is essential to improve the comprehensive properties of the material used for landing gear. This article briefly introduces the application status and existing problems of currently used landing gear materials, and then proposes future developing directions of landing gear materials. Finally, a new maraging stainless steel with high strength, high toughness and good corrosion resistance, which can be a promising steel for the new generation landing gear material, is introduced.
Due to the superiority in high thermal conductivity, low thermal expansion and good resistance from heat and corrosion, diamond/Cu composites show great prospect in thermal management applications. However, the thermal properties of diamond/Cu composites are impeded by their interface incompatibility. Interface modification is an effective method to enhance interfacial bonding and reduce interfacial thermal resistance. Based on the principles and factors related with interface design, this paper briefly reviewed some hot topics in diamond/Cu composites, including the main research progress, issues remained to be solved and nanoscale interface design with layer thickness lower than 200 nm, and its prospect of the future development.
In recent years, the increasing application demand for Mg alloys in automobile, rail transport, aviation and aerospace industries brings about the growing prominence of seeking reliable techniques to join Mg alloys. As a solid state welding method, friction stir welding (FSW) exhibits unique advantages in joining Mg alloys, and thus arouses widespread research interest. This paper emphatically reviewed the research status of conventional friction stir butt-welding of Mg alloys, and highlighted the welding process, microstructure evolution, texture characteristics, mechanical behavior and their interaction mechanisms. It was indicated that the texture plays a vital role in FSW joint performance of wrought Mg alloys, which is quite different from that in the FSW Al alloy joints. The specific strong texture formed in the weld is the main factor that gives rise to the impediment to achieving equal-strength joints to base materials. At the same time, some focuses like the weldability and the factors that influence joint performance in other types of FSW like lap welding, spot welding and double-sided welding; the weldability, interface bonding mechanism, joint performance and its affecting factors and optimization methods in dissimilar FSW between Mg alloys and other materials like Mg alloys of other grades, Al alloys and steels, were summarized and discussed. Finally, the future research and development directions in FSW of Mg alloys were prospected.
There have been numerous attempts to achieve superplasticity in light alloy materials for improving the formability and manufacture efficiency of them. However, the superplasticity of light alloy is difficult to realize for the uniform fine equiaxed grains, which are generally required by superplasticity, tend to rapidly grow during high temperature deformation. That means the superplasticity of light alloys not only requires an equiaxed fine-grained structure, but also needs to ensure the high-temperature structural stability. Thus, adding a second phase or alloying elements become one of the current research hotspots on superplasticity of light alloy materials. Currently, the main strategies for improving stability of the fine-grained structure of superplastic light alloy materials can be summarized as: introduction of second-phase particles pinning grain boundaries, phase structure of dual-phase alloys to inhibit growth between each other and reinforcement of composite materials inhibiting grain growth as well as utilizing solute segregation of single-phase alloys. This paper summarizes the research status of superplastic microstructure stability of light alloys including second-phase-containing alloys, duplex alloys, metal matrix composites and single-phase alloys. Finally, the paper proposes the development trend of superplastic light alloy materials from the perspective of industrial applications and cost-reduction requirements. Increasing the variety of alloying element, decreasing the content of alloying element, simplifying the process of manufacture and achieving low temperature superplasticity and high strain-rate superplasticity will be the development trend of superplastic light alloy materials.
This paper reviews the current progresses on the fabrication of TiAl-based composites produced by reaction annealing of elemental Ti and Al matrix composite foils. This technique includes deformation and reaction annealing of the multilayer Ti/Al metal matrix composite (MMC) sheet, which prevents traditionally direct deformation of brittle TiAl intermetallic, and TiAl-based composites sheets with good strength-ductility synergy have been produced. The research on microstructure evolution and forming mechanism of the TiAl-based composites sheet during reaction annealing has been summarized, with the focus on the reaction mechanism between Al-MMCs and Ti during reaction annealing, and the method to eliminate Kirkendall voids is proposed. A feasible proposal is provided to fabricate large scale TiAl-based composite sheets.
Cast superalloy is widely used in aerospace and energy industry. The research and development of these alloys is correlated with a large variety of materials and disciplines. The technology readiness level (TRL) of advanced cast superalloys is generally a mirror of the industry base of a country. China has made great progress in the field under the strong pull of demand in recent years. However, many issues are emerging in the industrial applications, reflecting a low TRL of advanced materials and a large gap between China and the developed countries. We present (1) development of directionally solidified and single crystal superalloys, (2) processing techniques of complex castings and (3) service behavior of blades as examples in this paper to explain the important role of basic research in research and development of cast superalloys.
Here some critical issues existed during forging process of Inconel 718 disks involving recrystallization mechanisms, grain growth, δ-phase morphology control and residual stress are explained. Based on the potential application prospect of selective laser melting in additive manufacture of aerocraft engine components, the specialized anisotropic microstructure and mechanical performance resulted from the rapid solidification process in selective laser melting are analyzed. Furthermore, the importance and difficulty of heat treatment in eliminating Laves-phase as well as tailoring substructure and related mechanical behavior are also discussed. The deformation mechanisms of Inconel 718 alloy at high temperature are illustrated in detail, comprising of dislocation planar slip, twinning and dislocation-shearing γ″ precipitates in complex modes. At last, a newly developed wrought nickel superalloy (Allvac 718Plus, with a increase in service temperature of 55 ℃ as compared to that of Inconel 718) is introduced, and some recent progresses aimed at modifying chemical compositions and phase compositions to improve service temperature on the basis of Inconel 718 alloy are also reviewed. The results indicate that the more stable γ″-γ' composite structure is important for the further design of next-generation wrought nickel superalloys.
Single-wall carbon nanotubes (SWCNTs) with a unique tubular structure have exhibited excellent electrical, thermal and mechanical properties. However, their attractive applications in microelectronic devices and sensors are still pending due to the lack of high-quality, structure-defined SWCNTs. It is still a great challenge to grow pure and ordered SWCNTs with designated structures and properties. The key of breakthrough is to understand the fundamental nucleation and growth mechanism of SWCNTs under reaction conditions. In this article, we analyze the influences of physical and chemical properties, such as electronic structure, melting point, carbon solubility, and diffusivity, of catalyst nanoparticles on the productivity, purity, and fine structures of grown SWCNTs. The progress, current situation, and challenges on the controlled growth of SWCNTs are summarized. Finally, perspectives on future directions are presented and a strategy of structure-controlled production of SWCNTs is proposed.
Hydrogen embrittlement, the degradation of mechanical behaviors due to the existence of hydrogen, is an industrially and environmentally critical problem in metals and alloys. Yet the fundamental mechanism(s) of embrittlement are still controversial, and the molecular-level damage events shrouded in mystery. In hydrogen embrittlement phenomena, the molecular-level agents of damage are hypothesized to be hydrogen-vacancy complex (Va+nH→VaHn), hereupon called hydrogenated vacancy. Contrary to vacancy, hydrogen-vacancy complex has good thermal stability and low diffusivity. When metals undergo plastic deformation at low homologous temperature in the presence of hydrogen, the mechanically driven out-of-equilibrium dislocation processes produce extremely high concentrations of hydrogen-vacancy complexes. Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. Our work provides the insight on the microscopic mechanism of hydrogen embrittlement. Moreover, this work also helps understanding some unique mechanical behaviors induced by hydrogen.
The fatigue of metallic materials can be divided into high-cycle fatigue (HCF) and low-cycle fatigue (LCF); the damage of these two types of fatigue is commonly evaluated through stress amplitude and strain amplitude of cyclic loading, respectively. The mismatch of the evaluation standards between HCF and LCF leads to difficulties in the design and selection of anti-fatigue materials. Under this condition, systematic researches on fatigue properties and microscopic damage mechanisms of HCF, LCF and extra-low-cycle fatigue (ELCF) for pure Cu and Cu-Al alloys were summarized in this work. On the bases of the experimental results, a three-dimensional fatigue model is proposed, which is simultaneously applicable to both the HCF and LCF properties. The model is built up in a three-dimensional coordinate system of stress amplitude-strain amplitude-fatigue life; it could be associated with the cyclic stress-strain (CSS) curve, S-N curve and E-N curve through the projection method, or be transformed into the Basquin equation, Coffin-Manson equation and hysteretic energy model under specific conditions. In this way, this generally applicable fatigue model helps provide a new viewpoint for the evaluation and optimization of fatigue properties based on the classical fatigue theories.
25Cr-20Ni austenitic heat resistant stainless steels are widely used as structural materials in nuclear industries and power plants for their excellent corrosion resistance and creep properties at elevated temperature. It is generally accepted that the precipitation during creep is a key factor influencing the creep properties. However, the evolution of precipitates is complicated due to the interaction of the alloy elements. To investigate the precipitation behaviors, a modified 25Cr-20Ni austenitic heat resistant stainless steel has been crept at 750 ℃ under different stresses varying from 100 MPa to 180 MPa. The microstructure observation indicates that M23C6 and (Nb, V)(C, N) precipitates are formed during 32.6 h creeping deformation under 180 MPa. M23C6 precipitates are mainly generated at grain boundaries and (Nb, V)(C, N) particles are dispersively distributed in austenitic matrix. The grain boundary M23C6 carbides are significantly coarsened and Ostwald ripening process happens during 98.1 h creeping deformation under the stress of 150 MPa and 353.0 h creeping deformation under stress of 120 MPa, while (Nb, V)(C, N) carbonitrides show high dimensional stability. With the creep rupture time further prolonging to 353.0 h and 752.3 h under the creep stress of 120 and 100 MPa, respectively, σ-phases are generated first at grain boundaries and then at inner grains. Meanwhile, large amounts of σ-phases are formed around (Nb, V)(C, N) particles, indicating the σ-phase precipitation is accelerated by (Nb, V)(C, N) carbonitrides. Composition analysis and thermodynamic calculation are subsequently performed to elucidate the precipitation mechanism of σ-phase. Carbon and nitrogen depleted zone is detected at the interface between (Nb, V)(C, N) precipitates and austenitic matrix. A correlation between σ-phase and C/N contents has been calculated by Thermo-Calc, which shows that the mass fraction of σ-phase increases with the decreasing C/N contents. According to the thermodynamic calculations and experimental results, it is reasonably inferred that the formation of σ-phase is induced by the carbon and nitrogen depletion in austenitic matrix. Additionally, the fracture surfaces of creep specimens show intergranular fracture under all creep stresses. When the creep time is comparatively short, cracks are inclined to propagate along grain boundaries owing to the low cohesion between grain boundary M23C6 precipitates and austenitic matrix, resulting in intergranular creep fracture. With the precipitation of σ-phase at grain boundaries after long time creep, the cracks are primarily generated from σ-phase, further deteriorating the creep elongation.