SiC fibers can be used to reinforce a range of titanium base materials including alloys of the (α+β) type, metastable β type and near α type, as well intermetallic based γ-TiAl and orthorhombic Ti2AlNb. Along the fiber directions the obtained composites possess exceptional strength and stiffness, creating a large room and great flexibility for the design of higher performance components to be used in both aero engine and aircraft. The composites can be used by itself such as in sheet and bar form, or as a reinforcing module embedded in titanium alloy components, e.g., as a ring at the rim of a compressor disk. In this paper, recent progress in the development and application of SiC fiber reinforced titanium matrix composites was reviewed, emphasizing the work conducted at the Institute of Metal Research, Chinese Academy of Sciences. Five aspects of research were covered, the first is fiber manufacture and batch production, in which the influence of the chemical vapor deposition parameters on the quality of the W-core SiC fiber was discussed, and the relationship between the tungsten-SiC interface reaction and the high temperature stability of the fiber was described. The second part covers the composite interface, in which a detailed discussion was given to both the chemical and physical compatibility, followed by the design of different reaction layers between the SiC fiber and different titanium based matrixs. The mechanical property section presents tensile data of a range of composites developed in the authors' group and compares to literature reports where available, together with a comprehensive discussion of failure due to fatigue and creep. The fourth part deals with nondestructive testing, presenting new results of inspection on real size composite components using a combination of several techniques including X-ray, industrial CT and ultrasonic scanning. The limitations of each method were shown and the technical challenges were identified. The last part describes the development of structural parts and their verification testing. Titanium matrix composite sheets with [0/90] laminate prepared by both the foil-fiber-foil and matrix coated fiber methods were highlighted, followed by a description of the development of full size bladed ring and excess revolution testing. Future directions of research on SiC fiber reinforced titanium matrix composites were also discussed.
Owing to the unique amorphous structure, metallic glasses (MGs) exhibit quite distinctive deformation and fracture behaviors from the conventional crystalline materials. The high strength, brittleness and macroscopic homogenous and isotropic structural features make MGs ideal model materials for the investigations of the strength theory of high-strength materials. Hence the fracture behavior and strength theory of MGs have attracted very extensive interests of researchers from the fields of materials, mechanics and physics. This paper is based on the research works of the authors on the fracture and strength of MGs in the past decade, and concentrates on discussing the current knowledge and recent advances on the fracture behavior and strength theory of ductile and brittle MGs. Firstly, the fracture behaviors of ductile and brittle MGs including tension-compression strength asymmetry, fracture mechanism and ductile-to-brittle transition will be briefly elaborated. Then the strength theories of MGs will be discussed, with our emphasis on the foundation, validation, further development and application of the ellipse criterion. At last, some unsolved issues associated with the fracture and strength of MGs are proposed.
Second phase strengthening is a conventional, yet effective hardening methods for metallic materials. However, the resultant improvements on strength always associated by dramatic decreases in toughness. In this paper, the recent research work on applications of such mechanism into several typical advanced metallic materials including high-performance steels, high-entropy alloys and bulk metallic glasses were summarized. It was found that the characteristics of precipitates, i.e., sizes, volume fractions, morphology, etc., could be manipulated by controlling the interface features and mechanical mismatch of the precipitates and matrix, which eventually give rise to much enhanced mechanical performance.
Large forgings are the fundamental parts of many kinds of key equipment, and large ingots are the basis of large forgings. There are severe metallurgical defects in large ingots, such as porosities, shrinkage cavities and gas cavities. The continuity of material is damaged by the defects, which must be eliminated during forging process. Using FEM simulation, it is found that void shape is the most important parameter affecting void closing during hot forging. Height-diameter ratio of the void is defined to describe the effect of void shape. The simulation results show that the larger height-diameter ratio of the void, the harder it is for the void to close. Based on these results, wide anvil radial forging (WRF) method is proposed. WRF method can concentrate the strain on the center of the ingot; make the height-diameter ratio of the voids smallest and heal shrinkage cavities effectively. Another one direction heavy forging method is proposed to be used on smaller forging machines. Using this method, the billet is forged along the same direction for two passes. This method can heal defects effectively with small pressure. Based on interface healing rules, temperature dwelling forging method for forging tube plates are proposed. A tube plate with defects is repaired using this method. These forging methods have been used on industrial experiments, and have been proved to be able to heal the defects in the billets and increase qualified rate of the forgings.
Accelerator driven subcritical (ADS) system has been recognized to be the most promising technology for safely treating the nuclear wastes by now. In China, ADS system has achieved great progress in both fundamental research and engineering practice. This system is composed of three parts, which are accelerator, spallation target and reactor. The biggest challenge exists in the structural material for the spallation target is to possess not only good heat-resistance and radiation resistance but also a resistance to liquid metal corrosion. A novel martensitic heat-resistant steel, SIMP steel, has been developed against this challenge. By negotiating the effects of the contents of those important elements such as C, Cr and Si in the (9%~12%)Cr martensitic heat-resistant steel on heat resistance, radiation resistance, and liquid metal corrosion resistance, an optimized chemical composition was obtained for SIMP steel and a good balance was reached among these three properties. The test results conducted on 1 t and 5 t grade SIMP steels showed that this novel steel is much potential as a candidate structural material for the spallation target in ADS system.
This paper simply introduced the research progress in friction stir welding (FSW) of dissimilar materials, high melting point materials and Al matrix composites, thermal-field simulation, and friction stir processing (FSP), especially based on research results of the authors. Some hotspots like the key factor of FSW dissimilar materials and bonding mechanism on interface, microstructure evolution during FSW of steel and Ti alloys and tool development, microstructure and properties of FSW Al matrix composite joints and tool wear, heat resource model of thermal-field simulation and effect of FSW parameters on thermal-field, microstructure and properties of nano-composites and ultrafine-grained materials prepared by FSP, were summarized and discussed. At the same time, the further research and development direction in FSW are suggested.
The development of external field assisted milling technologies and their application in materials fabrication have been briefly described. A recent developed milling method named as dielectric barrier discharge plasma assisted ball milling (DBDP-milling) was introduced. A combination of heating effect and high energy electron bombardment effect produced by plasma, as well as the milling mechanical effect was induced simultaneously in the DBDP-milling, which can effectively promote the powder refinement, activation and chemical reaction. On this basis, the DBDP-milling method was applied in the fabrication of cemented carbide, anode materials for lithium ion batteries, hydrogen storage materials, and so on. The studies have indicated that DBDP-milling could improve the efficiency of mill, produce unique structure and thus enhanced properties. In addition, DBDP-milling is also possible to establish a new material production process. Research results have demonstrated that the DBDP-milling method has a great potential in refinement, surface modification, mechanical alloying, composite fabrication and gas-solid reaction of powder materials for different applications.
The nanostructured metallic multilayers (NMMs) are widely used as essential components of high performance microelectronics and interconnect structures. The deformation and damage of NMMs is the essential factor leading to the structural failure of these systems. In this paper, based on these experimental results achieved by the authors, as well as the state-of-the-art and progress at home and abroad in the plastic deformation behavior of micropillars of Cu-based NMMs, the correlation of microstructure-size constraint-mechanical performance in the Cu-based nanolayered micropillars is illustrated. The universality of their deformation modes and internal damage mechanisms are revealed, and the work hardening /softening behaviors of two types of nanolaminates, including crystalline/crystalline and crystalline/amorphous NMMs, are summarized. Finally, a brief prospect on the studies of NMMs in future is suggested.
For the manufacture of complicated metallic structural components in power plants, aerospace and defense industry, Inconel 718 superalloy has been widely employed. High-temperature fatigue resistance and creep rupture strength of Inconel 718 superalloy are susceptible to the microstructure evolution in manufacture processing. Previous research work is generally focused on the parameter optimization of hot working processes (directional solidification, heat treatment, forging and welding). Relationships between the cold deformation, hot working, welding and the high-temperature mechanical performance, are seldom discussed, especially in the light of precipitate control . In this work, various types of secondary phases in Inconel 718 alloy are reviewed, including the primary strengthening phase (γ'' phase), secondary strengthening phase (γ' phase), equilibrium phase of γ'' phase (δ phase), MX-type carbonitride and Laves phase. Precipitation mechanisms of secondary phases in Inconel 718 alloy are also reviewed, as well as the effects of different types of precipitates on high-temperature performance of the Inconel 718 alloy. With respect to the high-energy electron beam welding of Inconel 718 alloys, factors contributing to the cracking in heat affected zone are indicated.
The {101?2} deformation twinning with extremely low activation stress is considered to be one of main reasons for the low strength of magnesium and its alloys at room temperature. In addition, it was found that those generally adopted age-strengthening methods are less effective for magnesium alloys in which postmortem investigation found that {101?2} deformation twinning is still profuse. The formation and propagation mechanism of {101?2} deformation twinning, which are of great importance for designing high strength magnesium alloy, remains elusive or under fervent debate. This paper reviewed the classical definition of deformation twinning, the existing twinning mechanisms, and the recent achievements through in-situ TEM studies on {101?2} deformation twinning. It was found that the {101?2} deformation twinning observed in magnesium are distinct from the classical definition on twinning. It is indeed a brand new room temperature deformation mechanism that can be carried out through unit-cell-reconstruction, without involving twinning dislocations. In addition, the boundaries generated through unit-cell-reconstruction are composed of {0002}/{101?0} interfaces (BP interfaces) and exhibit a terrace-like morphology in 3D space. The unit-cell-reconstruction is essentially different from the traditional dislocation-based twinning mechanism. As a consequence, to develop an effective strengthening strategy based on the nature of this new deformation mechanism would be the key for designing high strength magnesium alloy.
As a typical hexagonal close-packed structure metal, the dendritic morphology and preferential orientation of Mg would be influenced by many factors. Current investigations still fall short on the thorough description of the diversity and complexity of dendrites growth patterns and their origination, therefore, this paper re viewed recent research progress of this group on 3D characterization of microstructure in solidified magnesium alloys. Using synchrotron X-ray tomography and phase-field modeling, the formation mechanism of the diverse α-Mg (X) dendrites and the affections of alloying element (such as Al, Ca, Zn, and Sn), solute concentration on the growth selection and evolution of α-Mg dendrites during solidification were studied. The results indicate that the alloying elements and solute concentration would impose a significant influence on the morphology and orientation selection of the primary α-Mg dendrites. In Mg-Ca and Mg-Al (hcp-fcc) alloys, dendrites tend to grow with preferred orientation of <112?0> or <224?5> which is in good agreement with the traditional expected direction. The equiaxed growth dendrites in Mg-Sn (hcp-bct) alloys evolve as a structure with 18 branches, six of which grow on the basal plane along <112?0> and the remaining 12 along <112?X> (X≈2) off the basal plane. For the case in Mg-Zn alloys, an orientation transition from <112?0> on the basal plane to <112?1> off the basal plane are observed with the increasing addition of Zn alloying element, a hyperbranched seaweed structure is also revealed with an interim composition. A probable explanation is that the addition of high anisotropy Zn would slightly alter the anisotropy of interfacial free energy in front of the growth interface which results in a dendrite orientation transition (DOT). These findings partially reveal the underlying formation mechanism and origination of the diversity dendritic morphologies and branching structures of α-Mg dendrites in Mg alloys. Furthermore, with the fast X-ray imaging facility, in situ observations of the 3D microstructure evolution in Mg alloys during solidification are also carried out and the evolution of α-Mg dendrites are obtained for further analysis.
Microstructure evolution during solidification is a complex process controlled by the interplay of heat, solute, capillary, thermodynamics and kinetics. Computational modeling can provide detailed information about the interactions between transport phenomena and phase transformation. Thus, it has emerged as an important and indispensable tool in studying the underlying physics of microstructural formation in solidification. During the last two decades, extensive efforts have been dedicated to explore the numerical models based on the methods of phase field (PF), cellular automaton (CA), front tracking (FT), and level set (LS), for the simulation of solidification microstructures. The CA approach can reproduce various realistic microstructure features with an acceptable computational efficiency, indicating the considerable potential for practical applications. It has, therefore, drawn great interest in academia and achieved remarkable advances in the simulation of microstructures. This paper gives an overview of CA based models, spanning from the meso-scale to the micro-scale, for the prediction of microstruc ture evolution during alloy solidification. The governing equations and numerical algorithms of CA based models and derived coupling models are summarized, including the calculations of nucleation, growth kinetics, interface curvature, surface tension anisotropy and crystallographic orientation, thermal and solutal transport, melt convection utilizing the lattice Boltzmann method (LBM), the coupling of CA with control volume (CV) method, the coupling of CA with CALPHAD approach for multi-component alloy systems, as well as the approaches for eliminating the artificial anisotropy caused by the CA square cells. The main achievements in this field are addressed by presenting examples encompassing a wide variety of problems involving dendritic growth in pure diffusion and with melt convection, eutectic solidification, microstructure formation in multi-component alloys, dendritic growth with gas pore formation, and multi-scale simulation. Finally, the future prospects and challenges for the CA modeling of solidification microstructures are discussed.
In this article, the history of study on Ising model was first reviewed briefly, including a brief introduction of Ising model, the advances in the study of two-dimensional (2D) and three-dimensional (3D) Ising models, with a special interest in the exact solution of the 2D Ising model. Then two conjectures and putative exact solution of the 3D Ising model were introduced, and the mathematical structure of the 3D Ising model was investigated from the aspects of topology, algebra and geometry. The transfer matrices of the 3D Ising model, the knot theory in the topology, the relations between the Yang-Baxter equations and the tetrahedron equations were analysized. The non-local effect in the 3D Ising model, the relation between quantum field theory and gauge theory, the physical significance of weight factors, the singularity and the topological phase transition at/near infinite temperature in the 3D Ising model were also discussed. Finally, it was pointed out that some approximation techniques (for examples, low-temperature expansions, high-temperature expansions, renormalization group and Monte Carlo simulations) have disadvantages for studying the 3D Ising model.
Thermo-mechanical control process (TMCP) plays a key role in the manufacturing of hot-rolled low-alloy steels, as well as the optimization of microstructures and properties. However, the various phase transformations involved in the TMCP of steels and its impact on the microstructures/properties are still not fully understood. In the present work, on the basis of classical theories of phase transformations and previous experimental results, the key parameters controlling the phase transformation processes are analyzed, from which the correlation between thermodynamics and kinetics of the phase transformations are proposed; then, this correlation in the phase transformations of low-alloy steels and its effect on the competing mechanisms of transformations are analyzed; based on well-established theories (i.e. the first-principles calculations and the double well potential in phase field methods), the energetics of the Bain path of Na and the fcc/bcc transformation of Fe are calculated to demonstrate the correlation between thermodynamics and kinetics. Eventually, the current work is summarized and the potential applications of the correlation between thermodynamics and kinetics of phase transformations are proposed.
Inconel 690TT and Incoloy 800MA have been widely used as steam generator heat transfer tubes in nuclear power plants (NPPs). The corrosion behaviors of these two alloys in high temperature high pressure water have to be fully addressed. This work systematically studied the microstructures of the as-received Inconel 690TT and Incoloy 800MA steam generator tubes (SGTs) and compared the oxide films formed on the tubing materials in high temperature water using several analytical methods including SEM, EBSD, GIXRD, SAED and STEM. The results show that from outer surface to inner surface of Inconel 690TT SGTs, the deviation degrees from the ideal Σ3 misorientation and the average value of Kernel average misorientation (KAM) gradually increase. The outer surface of Inconel 690TT SGTs are weakest. For Incoloy 800MA SGTs, the deviation degrees from the ideal Σ3 misorientation are within 0~1°, and the change of KAM average value is small. Exposed to 325 ℃ pure water containing 0.75×10-6 O2 for 720 h, oxide films of both Inconel 690TT SGTs and Incoloy 800MA SGTs have duplex structure. On Inconel 690TT SGTs, the outer layer is Fe-rich spinel and small NiO particles; the inner layer mainly is NiO, porous and less protective with the thickness of 716 nm. On Incoloy 800MA SGTs, the outer layer is big polyhedral spinel; the inner layer is small polyhedral spinel and protective with the average thickness of 150 nm; Cr is enriched at the interface between inner oxide layer and matrix. In high temperature water with dissolved oxygen, due to the preferential dissolution of Cr, Incoloy 800MA is more corrosion resistant than Inconel 690TT.
A considerable researches have been conducted to provide rather compelling evidence that the grain size and grain boundary distribution possess much influential effect on mechanical properties and corrosion behaviors in most metals and alloys. However, the effects of grain size and grain boundary distribution on anti-corrosion ability of materials have been independently studied. Some investigations indicate that the occurrence frequency and distribution characteristic of twin-related (especially Σ3n coincidence site lattice (CSL)) grain boundaries play a particularly important role in optimization of grain boundary character distribution. Unfortunately, both of these factors are interactive in annealing processes and there is a need to identify the independent role of the factors in anti-corrosion ability. In this work, a high manganese austenitic twinning-induced plasticity (TWIP) steel was used as experimental material and the anti-corrosion behavior of this steel resulted from both the grain size and grain boundary distribution was studied. The cold-rolled high manganese austenitic TWIP steel sheet was annealed at 700~1000 ℃ for 10~30 min to obtain microstructure with various grain sizes and CSL grain boundaries. The average grain size and grain boundary distribution characteristics for all the annealed steel sheets were obtained by the online analysis of EBSD data with HKL-Channel software. The anodic polarization curves were measured using CorrTest4 electrochemical workstation in 3.5%NaCl solution at 25 ℃ with a scan rate of 0.5 mV/s. The results show that both of the grain size and the occurrence frequency of CSL grain boundary caused by the uniformity of recrystallized microstructure have much effect on the anti-corrosion ability of this high manganese TWIP steel. When the recrystallization process just finished, and grains were inhomogeneous and not start to grow, the average grain size has a great influence on anti-corrosion ability. With increasing the grain size, the anti-corrosion ability of this high manganese TWIP steel was weakened. When the recrystallized grain growth fully takes place, the occurrence frequency of CSL grain boundary has the dominant effect on the anti-corrosion ability. The anti-corrosion ability was optimized with increasing the frequency of CSL grain boundary.
Polishing soft metals using hard abrasives such as diamond, alumina, and silica can easily damage the worn surface by deep scratches and by large material removal due to cutting wear mechanism. An abrasive material with appropriate hardness, hardness/elasticity ratio, and low friction is then highly desirable, which would avoid intense abrasion while at the same time minimize scratching on soft metals. Quasicrystals are characterized by low friction and high hardness/elasticity ratio, making them potentially suitable for use as abrasives for soft metals. It has been pointed out by the authors that AlCuFe quasicrystal abrasive shows a particular smearing dominant wear mechanism and can be used as a special abrasive for flattening soft metals. In this work, the Al62Cu25.5Fe12.5 quasicrystal abrasive was chosen, to compare with conventional hard abrasives such as diamond, alumina and silica, to wear against copper, 2024 aluminum alloy and 304 stainless steel. The surface topography, nano-indentation hardness, smearing coefficient, mass loss and electrochemical impedance were measured and the results indicate that the surface flattening is influenced by the smearing coefficient, a parameter developed to assess the degree of smearing-type wearing. A larger smearing coefficient leads to a more flatten surface at the least expense of mass loss. It is specially noticed that the characteristic smearing mechanism of quasicrystal abrasive produces an obvious surface hardening effect, with the nano-hardness of 304 stainless steel being increased by about 0.3 GPa. The corrosion resistance of the Al alloy is also enhanced due to the formation of a thick and dense passive film.
