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CN 21-1139/TG
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The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M2C eu. . .
 Acta Metall, 2014 Vol. 50 (7): 769-776    DOI: 10.3724/SP.J.1037.2013.00621
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 Solidification of Undercooled Co75B25 Alloy PDF (1204KB)
 2018-03-16
 RECENT PROGRESSES IN COMPETITIVE GRAIN GROWTH DURING DIRECTIONAL SOLIDIFICATION PDF (1117KB)
 2018-03-14
 Current Research and Future Prospect On Microstructures Controlling of High Performance Magnesium Alloys During Solidification PDF (1162KB)
 2018-03-12
 Review of controlled solidification and its applications PDF (2163KB)
 2018-03-12
 The Effect of Superheated Temperature And Cooling Rate on The Solidification of Undercooled Metallic Melt Ti PDF (3901KB)
 2018-03-06
Current Issue More>>
 20 February 2018, Volume 54 Issue 2 Previous Issue
Orginal Article
 Select Integrated Computational Materials Science: New Paradigm for Metal Manufacturing Dianzhong LI Acta Metall. 2018, 54 (2): 129-130.   DOI: 10.11900/0412.1961.2018.00038 Abstract   HTML   PDF (1545KB)
 Select Research Progress of Numerical Simulation in Steelmaking and Continuous Casting Processes Miaoyong ZHU, Wentao LOU, Weiling WANG Acta Metall. 2018, 54 (2): 131-150.   DOI: 10.11900/0412.1961.2017.00430 Abstract   HTML   PDF (4741KB) Because of the complexity of steelmaking and continuous casting processes and their limitation condition for direct measuring and testing, numerical simulation has become an indispensable means to analyze the phenomena and mechanisms occurring in the processes, and since the 1980s, it has made a rapid development. For the converter smelting, some new oxygen lances were designed by using the simulation study of the characteristics of the oxygen lance supersonic jet. Some mathematical models have been established to describe the slag-metal-gas multiphase flow behavior in steelmaking converter, and the flow field, mixing efficiency, metal droplet splashing, lining scouring and other physical phenomena. For the ladle refining, the Euler-Euler model gradually replaces the quasi-unidirectional and Euler-Lagrangian models, and successfully describes the phenomena of bubble turbulent dispersion caused by liquid turbulent fluctuation, and bubble-induced turbulence occurring during bubble floating process. So, some new and important inclusion transport mechanisms and phenomena have been presented. The CFD-PBM model was used to predict successfully the inclusion transport, collision growth and removal behavior in the molten steel, which enriches the inclusions removal theory of ladle refining. The CFD-SRM coupled model was used to accurately describe the slag-metal reaction and desulfurization behavior in a gas-stirred ladle, and the effect of the different content of compositions in synthetic slag and liquid steel, arrangement of bottom blowing tuyeres on the slag-metal reactions and desulfurization efficiency were discussed and clarified. For steel continuous casting, as the heat flow model from the solidified shell to the copper plate of mold was coupled with the thermo/mechanical model of the solidified shell, distributions of mold flux and air gap both along circumference and height directions of the mold were successfully predicted, while founded theoretical backgrounds for designing new mold with inner convex surface and controlling the surface corner crack of micro-alloyed steel. The coupled simulation between flow and electromagnetic fields in mold revealed the flow behavior of molten steel with electromagnetic stirring or braking, the fluctuation characteristic of the slag-steel interface and the distribution characteristic of inclusions in the strand. Based on the volume averaged method, multi-field and multi-phase solidification model successfully clarified the formation mechanism of macro-segregation in continuously cast strand and quantitatively predicted central/centerline segregation indexes in the strand under different casting conditions. In addition, the numerical simulation of the evolution of solidification structure of the continuously cast strand mainly focused on the as-cast grain, and its extension to the dendrite structure needed further more endeavors. Generally speaking, the numerical simulation in steelmaking-continuous casting process is moving towards coupling multi-physical/chemistry phenomena and multi-fields and gradually transits to the microscopic scale.
 Select Numerical Simulation of Macrosegregation inSteel Ingot Casting Houfa SHEN, Kangxin CHEN, Baicheng LIU Acta Metall. 2018, 54 (2): 151-160.   DOI: 10.11900/0412.1961.2017.00431 Abstract   HTML   PDF (2995KB) Many key forging components of heavy equipment are manufactured by large steel ingots. Macrosegregation in steel ingots is a key defect formed during the solidification process. Over the past few decades, numerical modeling has played a more and more important role in the study of macrosegregation. Various models have been developed and applied to different ingot casting processes. This paper focused on the application of macrosegregation models to the steel ingot. Firstly, the formation mechanism and influencing factors of macrosegregation were introduced. Then, the existing macrosegregation models and their recent development were summarized. Macrosegregation models accounting for such mechanisms as solidification shrinkage-induced flow and mushy zone deformation were analyzed, respectfully. To model macrosegregation due to solidification shrinkage, the key was to solve the free surface. A simple derivation showed that the multi-phase (including gas phase) models were equivalent to the VOF-based segregation models in dealing with the shrinkage-induced flow. Finally, our recent research work on numerical modeling of macrosegregation in steel ingots was illustrated, including application of the developed multi-component and multi-phase macrosegregation model to a 36 t steel ingot, and numerical simulation of multiple pouring process. The carbon and sulphur concentrations at about 1800 sampling points, covering the full section of a 36 t ingot, were measured. By detailed temperature recording, accurate heat transfer conditions between the ingot and mould were obtained. Typical macrosegregation patterns, including the bottom-located negative segregation and the pushpin-like positive segregation zone in the top riser, have been reproduced both in the measurements and the predictions. The carbon and sulphur concentrations predicted by the three dimensional multi-component and multi-phase macrosegregation models agreed well with the measurements, thus proving that the model can well predict the macrosegregation formation in steel ingots. As for the multi-pouring process simulation, the results show a high concentration of carbon at the bottom and a low concentration of carbon at the top of the mould after the multi-pouring process with carbon content high in the first ladle and low in the last ladle. Therefore, the multiple pouring process could get the initial solute distribution with the opposite form of segregation. Such carbon concentration distribution would reduce the negative segregation at the bottom and the positive segregation at the top of the solidified ingot, thus proving the ability of the multiple pouring process for the control of macrosegregation.
 Select Research Status and Prospect on Numerical Simulation Technology of Casting Macroscopic Process Dunming LIAO, Liu CAO, Fei SUN, Tao CHEN Acta Metall. 2018, 54 (2): 161-173.   DOI: 10.11900/0412.1961.2017.00317 Abstract   HTML   PDF (4773KB) In recent years, with increasingly maturing of computer simulation technology, numerical simulation methods are playing an increasing significant role in casting macroscopic process, and the research status on numerical simulation technologies in casting macroscopic processes is summarized in this paper. The differences in casting filling process discribed using different flow models are compared, and it is found that the two-phase flow model can be used to accurately handle the effect of gas phase on filling process. The applicabilities of different stress models to the evolution process of casting stress are also analyzed. The accessing and correcting method of physical property parameters, which is fit for simulation of casting macroscopic process, is explained. And the method is that the alloy composition and solidus/liquidus temperature are measured by experimental means, then physical property parameters are calculated by relevant softwares and adjusted accordingly, at last, the parameters are corrected according to temperature experiment. The boundary conditions of different casting techniques are listed, and, in addition, the boundary conditions of high pressure die casting (velocity inlet) and directional solidi fication (radiation heat transfer boundary) are explained specially. The differences of different mesh types are compared, in combination with which the differences of different numerical solution methods are analyzed. The suitable meshes would be adaptive hexahedral mesh and hybrid mesh, because they fit more for finite volume method (calculation for filling process) and finite element method (calculation for solidification and stress evolution processes). Prediction models and analysis methods of different casting defects are illustrated. In this paper, various methods used in simulation of casting process are introduced, and their application development trends are also predicted. We hope to offer a reliable reference for numerical simulation methods of casting macroscopic process.
 Select Characterization and Modeling Study on Interfacial Heat Transfer Behavior and Solidified Microstructure of Die Cast Magnesium Alloys Shoumei XIONG, Jinglian DU, Zhipeng GUO, Manhong YANG, Mengwu WU, Cheng BI, Yongyou CAO Acta Metall. 2018, 54 (2): 174-192.   DOI: 10.11900/0412.1961.2017.00418 Abstract   HTML   PDF (16999KB) Magnesium alloys are widely used in various fields because of their outstanding properties. High-pressure die casting (HPDC) is one of the primary manufacturing methods of magnesium alloys. During the HPDC process, the solidification manner of casting is highly dependent on the heat transfer behavior at metal-die interface, which directly affects the solidified microstructure evolution, defect distribution and mechanical properties of the cast products. As common solidified microstructures of die cast magnesium alloys, the externally solidified crystals (ESCs), divorced eutectics and primary dendrites have important influences on the final performance of castings. Therefore, investigations on the interfacial heat transfer behavior and the solidified microstructures of magnesium alloys have considerable significance on the optimization of die-casting process and the prediction of casting quality. In this paper, recent research progress on theoretical simulation and experimental characterization of the heat transfer behaviors and the solidified microstructures of die cast magnesium alloys was systematically presented. The contents include：(1) A boundary-condition model developed based on the interfacial heat transfer coefficients (IHTCs), which could precisely simulate the boundary condition at the metal-die interface during solidification process. Accordingly, the IHTCs can be divided into four stages, namely the initial increasing stage, the high value maintaining stage, the fast decreasing stage and the low value maintaining stage. (2) A numerical model developed to simulate and predict the flow patterns of the externally solidified crystals (ESCs) in the shot sleeve during mold filling process, together with discussion on the influence of the ESCs distribution on the defect bands of die cast magnesium alloys. (3) Nucleation and growth models of the primary α-Mg phases developed by considering the ESCs in the shot sleeve. (4) Nucleation and growth models of the divorced eutectic phase, which can be used to simulate the microstructure evolution of die cast magnesium alloys. (5) The 3D morphology and orientation selection of magnesium alloy dendrite. It was found that magnesium alloy dendrite exhibits an eighteen-primary branch pattern in 3D, with six growing along $<112?0>$ in the basal plane and the other twelve along $<112?3>$ in non-basal planes. Accordingly, an anisotropy growth function was developed and coupled into the phase field model to achieve the 3D simulation of magnesium alloy dendrite.
 Select Progress and Application of Microstructure Simulation of Alloy Solidification Tongmin WANG, Jingjing WEI, Xudong WANG, Man YAO Acta Metall. 2018, 54 (2): 193-203.   DOI: 10.11900/0412.1961.2017.00428 Abstract   HTML   PDF (3126KB) Solidification structures are the interaction links between the alloy components and their mechanical properties. Scientifically comprehending about the formation mechanisms, dominant factors and control methods in alloy solidification has a significant effect on the structure control and optimization. Dendritic structure is the most frequently observed solidification microstructure of alloys and controlled by heat, solute, melt flow, capillary and many other factors. Modelling and simulating can accurately quantify various phenomena and evolution rules in the process of solidification, thus play an increasingly important role in the design, preparation, processing and performance optimization of alloy materials. Over the past two decades, remarkable progress has been made and various models have been proposed in microstructure simulation during alloy solidification process, such as deterministic method, phase field (PF), Monte Carlo (MC) and cellular automaton (CA). With the advantages of clear physical meaning, easily programming and high calculation efficiency, CA method has been widely applied in the study of solidification structure simulation and exhibits great advantages. Considering the current development level of computer hardware, numerical model and calculation method, microstructure simulation of large components mainly adopts macro-microscopic coupling calculation method, such as CA-FD/FE model. The heat transfer and other multi-physical fields are calculated at the level of coarse mesh, where as nucleation and dendritic growth are simulated at a much finer grid level. This paper reviews the main models and development of CA method used for nucleation simulation. The key aspects in the simulation of dendritic growth including mean solid-interface interface curvature, growth kinetics and the algorithm for eliminating “pseudo anisotropy” are discussed. Based on this, the development and application status of macro-micro coupling model during casting, directional solidification and other manufacturing fields are summarized. Finally, the existing problems and future tendency for simulation of solidification structures are analyzed.
 Select Recent Progresses in Modeling of Nucleation During Solidification on the Atomic Scale Jincheng WANG, Can GUO, Qi ZHANG, Sai TANG, Junjie LI, Zhijun WANG Acta Metall. 2018, 54 (2): 204-216.   DOI: 10.11900/0412.1961.2017.00425 Abstract   HTML   PDF (8443KB) Nucleation, the starting point of first-order discontinuous phase transformations, has long been an important issue in condensed matter physics and materials science. It plays a key role in determining the microstructures and mechanical properties of crystalline materials. As nucleation occurs at the atomic length scale and the diffusional time scale and is a typical stochastic event, investigating such kind of multiple scale issues will be taken up an enormous challenge. Because of the limitations of present experimental methods, it is still very hard to observe the nucleation process in situ. With the development of computational materials science, a deeper understanding of nucleation process has been obtained with the numerical modeling of nucleation process on the atomic scale. In this paper, some recent developments in modeling and simulation of nucleation process during solidification on the atomic scale are reviewed. Firstly, the development of classical nucleation theory and the step nucleation theory are reviewed. Then the developments in modeling of nucleation process by using the phase field method, Monte-Carlo method, Molecular dynamics method and the phase field crystal model are discussed. After that, some recent progresses in modeling of nucleation process during solidification in our research group by using the phase field crystal model are demonstrated. Finally, the outlooks of the future study on the nucleation during solidification are also presented.
 Select Austenite/Ferrite Interface Migration and Alloying Elements Partitioning: An Overview Hao CHEN, Congyu ZHANG, Jianing ZHU, Zenan YANG, Ran DING, Chi ZHANG, Zhigang YANG Acta Metall. 2018, 54 (2): 217-227.   DOI: 10.11900/0412.1961.2017.00465 Abstract   HTML   PDF (2925KB) Phase transformation is one of the most effective methods to tailor microstructure of steels. In order to develop high performance steels, microstructure has to be precisely tuned, which requires a deep understanding of phase transformation. The austenite to ferrite transformation in steels has been of great interest for several decades due to its considerable importance in the processing of modern high performance steels, and it has been investigated from various aspects. Mechanism of interface migration and alloying elements partitioning during the austenite to ferrite transformation was regarded as one of the most significant and challenging topics in the field. This paper briefly summarized the recent progress in the understanding of this topic from both theoretical and experimental perspectives, and would also provide discussions and outlook of the unresolved issues.
 Select Study and Development on Numerical Simulation for Application of Electromagnetic Field Technologyin Metallurgical Processes Qiang WANG, Ming HE, Xiaowei ZHU, Xianliang LI, Chunlei WU, Shulin DONG, Tie LIU Acta Metall. 2018, 54 (2): 228-246.   DOI: 10.11900/0412.1961.2017.00360 Abstract   HTML   PDF (9779KB) The application of electromagnetic fields is an important way to control the physical and chemical changes of heat transfer, mass transfer, fluid flow and solidification in metallurgical and material preparation processes. It is of great significance to improve the production efficiency and product quality. In this paper, the authors summarize the research contents and progress of numerical simulation on several typical applications of electromagnetic technology in metallurgical fields in recent years, including the electromagnetic steel-teeming technology using induction heating and induction heating technology of a tundish, the applications of electromagnetic force such as the electromagnetic swirling technology in submerged entry nozzle, the soft-contact mold electromagnetic continuous casting technology and the electromagnetic metallurgical technology for tundish, the influence and control of electromagnetic force on so lidified structure evolution, and also the electromagnetic cold crucible technology with comprehensive utilization of induction heat and electromagnetic force. Numerical simulation, as an important research method, is a very important tool in finding out the mechanism and rules of electromagnetic fields during metallurgical and material preparation processes to predict, analyze, and optimize metallurgical processes.
 Select Progress on Numerical Simulation of Vibration in the Metal Solidification Shiping WU, Rujia WANG, Wei CHEN, Guixin DAI Acta Metall. 2018, 54 (2): 247-264.   DOI: 10.11900/0412.1961.2017.00424 Abstract   HTML   PDF (5912KB) The application of vibration technology to the metal solidification process can not only effectively improve the solidified structure and the performance of castings, but also have the advantages of low cost, energy saving and environmental protection. Therefore, the application of vibration technology in metal solidification has been extensively studied in experiments. However, due to the high temperature and opacity of the metal melt, hindering its measurement and observation, the mechanism how the vibration affects the solidification is not fully understood. Numerical simulation can provide the variation law of various parameters such as flow field, temperature field and stress field under vibration condition, which helps us understand the mechanism of vibration more thoroughly. Meanwhile, the numerical simulation of the influence of vibration on the solidification of metal melt has been much less systematically studied. This paper introduces the research progress of numerical simulation of vibration applied in metal solidification. The main vibration modes include ultrasonic vibration, mechanical vibration and pulsed electromagnetic vibration. The application mainly includes melt processing, filling, solidification, purification and ageing process of numerical simulation. The current research status of numerical simulation theory and technology of vibration applied in all aspects of casting was summarized systematically. Furthermore, the future research directions of numerical simulation of vibration in metal solidification process were prospected.
 Select Numerical Simulation of Heat Generation, Heat Transfer and Material Flow in Friction Stir Welding Chuansong WU, Hao SU, Lei SHI Acta Metall. 2018, 54 (2): 265-277.   DOI: 10.11900/0412.1961.2017.00294 Abstract   HTML   PDF (8067KB) The heat generation, heat transfer and plasticized material flow in friction stir welding determine directly the microstructure evolution and mechanical properties of weld joints. Numerical simulation of these thermo-physical phenomena is of great significance for getting a deep insight into the underlying mechanisms and optimizing the process parameters of friction stir welding. This article reviews the progress status in numerical simulation of heat generation, heat transfer and plasticized material flow behaviors in friction stir welding, and outlines the unsolved problems. The research work targeting these issues, which has been conducted by the authors' group, is introduced. According to the stress characteristics at the tool-workpiece interface, the expressions of sticking rate and friction coefficient are developed, and this measurement-calculation method lays foundation for improving the accuracy of numerical analysis. Through synthetically considering the characteristics of complex-shaped tools, a three dimensional model of friction stir welding process is established. Three types of tools are taken into consideration, i.e., normal CT (conical-pin tool), ST (conical-pin with 4 flats tool) and TT (conical-pin with 3 flats tool). For the cases in application of these tools, the heat generation, temperature profile, and material flow velocity are analyzed quantitatively. A mathematical model for the whole friction stir welding process including plunge stage, dwell stage, welding stage, and cooling stage is established for numerical analysis of transient development in heat generation rate, temperature and material flow fields in each stages. Based on the status review, the trend in numerical simulation of frictions stir welding is outlooked, and the research focus for next step is proposed.
 Select Phase Field Crystal Model and Its Application for Microstructure Evolution of Materials Yingjun GAO, Yujiang LU, Lingyi KONG, Qianqian DENG, Lilin HUANG, Zhirong LUO Acta Metall. 2018, 54 (2): 278-292.   DOI: 10.11900/0412.1961.2017.00336 Abstract   HTML   PDF (1593KB) With the rapid development of computer technology, the roles of computer numerical simulation technology in materials are more and more prominent. Computer numerical simulation technology, real experimental observation and theoretical model analysis are the same important and are known as three great scientific research methods since the 20th century. In this paper, several important computational numerical simulation methods are briefly compared, firstly, in the spatial characteristic resolution scale and the characteristic time scale, for example, for molecular dynamics (MD), traditional phase field (TPF), and phase field crystal (PFC) method. For simulation of microstructure evolution in nano-scale, the PFC method is of the advantage on the characteristic time scale. Then, the PFC model, and its physical and mathematical basises for establishment, as well as the special feature of the method, are introduced. Next, the development of the PFC models are presented, including the PFC model of binary and multi-element alloys, of gas-liquid-solid three systems, of two-mode and multimode systems, as well as the key technology and the main procedure of the numerical calculation of the dynamic equation solution. After that, combining with the research works of the authors' group in the microstructure evolution of materials, several examples of important aspects of application of the PFC model are presented, including the nanostructure of defects of materials, dendritic growth and heterogenous epitxial growth, premelting under deformation at high temperature and dynamic recovery, extension and bifurcation of cracks on nanoscale, matalllic glass transition, defect structures of graphene, voids formation of electromigration in metal interconnects, microstructure in multiferroic composite matrials, and the formation of the structure of the metal foams. Finally, a summary is given and the development direction and future emphasis application and new fields of the PFC model are pointed out.
 Select Chemical Units in Solid Solutions andAlloy Composition Design Chuang DONG, Dandan DONG, Qing WANG Acta Metall. 2018, 54 (2): 293-300.   DOI: 10.11900/0412.1961.2017.00462 Abstract   HTML   PDF (1254KB) Industrial alloys all have specific chemical compositions as standardized in specifications. Understanding the structural origin of special compositions for these solid-solution alloys is significant to shortening the development of new industrial alloys. It is well accepted that all alloys are based on solid solutions characterized by chemical short-range ordering. Previously it was only possible to describe the deviation of solute distribution from average mode in a statistical manner. The lack of an accurate structural tool to address the characteristic short-range-order structures constitutes the major obstacle in establishing an effective structural model that allows precise composition design for alloys. Since alloys with good comprehensive performance do have specific chemical compositions, their compositions should correspond to molecule-like specific structural units. After a long effort of more than a decade, we have developed a new structural tool, so-called the cluster-plus-glue-atom model, to address any short-range-ordered structures. In particular, solid solutions can be understood as being constructed from the packing of special chemical units covering only the nearest-neighbor cluster and a few glue atoms located at the next outer shell, expressed in molecule-like cluster formula [cluster] (glue atoms). Such units represent the smallest particles that are representative of the whole structures, just like molecules do for chemical substances. After introducing Friedel oscillation, the cluster-plus-glue-atom model is turned into the cluster-resonance model that provides also the inter-cluster packing modes. Ideal atomic density is hence obtained which is only proportional to the number of atoms in the unit and the cube of the cluster radius. The calculation of chemical unit is then possible and is conducted in typical binary Cu-based industrial alloys. The calculated formulas give chemical composition that highly agree with the most popular alloy specifications. The work demonstrates its high potential for developing chemically complex alloys.
 Select Modeling and Simulation of Hydrogen Behavior in Tungsten Hongbo ZHOU, Yuhao LI, Guanghong LU Acta Metall. 2018, 54 (2): 301-313.   DOI: 10.11900/0412.1961.2017.00414 Abstract   HTML   PDF (4254KB) Based on national strategic needs for fusion energy, our group have investigated the behavior of H isotopes including dissolution, diffusion, accumulation and bubble formation in W using a first-principles method in combination with molecular dynamic method. It is found that the dissolution and nucleation of H in defects follow an "optimal charge density" rule, and a vacancy trapping mechanism for H bubble formation in W has been revealed. An anisotropic strain enhanced effect of H solubility due to H accumulation in W has been found, and a cascading effect of H bubble growth has been proposed. Noble gases/alloying elements doping in W has been proposed to suppress H bubble formation, because these dopants can change the distribution of charge density in defects and block the formation and nucleation of H2 molecule. These works are reviewed in this paper. Our calculations will provide a good reference for the design, preparation and application of W-PFM under a fusion environment.
 Select Numerical Simulation of Temperature Field and Thermal Stress in ZTAp/HCCI Composites DuringSolidification Process Xiaoyu CHONG, Guangchi WANG, Jun DU, Yehua JIANG, Jing FENG Acta Metall. 2018, 54 (2): 314-324.   DOI: 10.11900/0412.1961.2017.00351 Abstract   HTML   PDF (6993KB) As advanced wear-resistant materials, it is important to promote the process and application of high chromium cast iron (HCCI) matrix composite reinforced by zirconia toughened alumina ceramic particles (ZTAp/HCCI composite). For the purpose of wider applications of this kind of composite, it is urgent to optimize the process parameters of casting process for it. Based on the finite element software the temperature field and thermal stress in ZTAp/HCCI composite during casting process were simulated. The temperature fields of castings are investigated using the uniform initial temperature and the non-uniform initial temperature at the beginning of solidification. It is more appropriate to the actual situation at the end of mold filling process when the initial temperature of solidification is considered as an unstable temperature field. The influence from performs with different honeycomb shapes is considered in the calculations of temperature fields of castings. In this work, the thermo-elastic plastic model was used to accurately describe the thermal stress in the castings with different honeycomb shapes of preforms, and the results indicate that the thermal stress in them decreases with the increase of edge number of holes in preforms. Finally, the hot crack in castings is predicted and the shakeout process is optimized. It is concluded that the simulated results are in good agreement with the experimental results.
 Select First-Principles Study of Hydrogen Behaviors at Oxide/Ferrite Interface in ODS Steels Yuchao FENG, Weiwei XING, Shoulong WANG, Xingqiu CHEN, Dianzhong LI, Yiyi LI Acta Metall. 2018, 54 (2): 325-338.   DOI: 10.11900/0412.1961.2017.00459 Abstract   HTML   PDF (8124KB) Ferritic oxide dispersion strengthened (ODS) steels, which usually contain a very high density of nano-sized Y-Ti-O particles and oxide precipitates (Y2Ti2O7 or/and Y2TiO5), have been demonstrated to be a leading candidate for promising structural materials in advanced fission and fusion energy applications. By means of first-principles calculations, the defect formation energies and preference sites of hydrogen (H) and helium (He) atoms trapped in Y2Ti2O7, Y2TiO5 and Y2Ti2O7/bcc-Fe interface, were investigated. The calculations uncover that (1) H atoms prefer to occupy the interstitial sites with high pre-exsiting charge densities of Y2Ti2O7 and Y2TiO5, (2) the Y2Ti2O7/bcc-Fe interface trends to attract vacancies in bcc-Fe matrix because of its lower vacancy formation energies, (3) at the Y2Ti2O7/bcc-Fe interface, H at om prefers to occupy the interstitial sites around the bcc-Fe side while He atom prefers to occupy the interstitial sites around Y2Ti2O7 side. All these results demonstrate that both H and He atoms produced by nuclear transmutation reactions would be trapped by oxides precipitates and Y2Ti2O7/bcc-Fe interface in case of the formation of large bubbles. This implies that high density of nanometer-sized oxide precipitates and Y2Ti2O7/bcc-Fe interfaces in ODS steels effectively disperse H atoms and inhibit H clusters in finer size. Besides that, during the growth process of the finer H clusters at interfaces they trap a large number of both H atoms and vacancies, acting as self-healing sites for irradiation damage. These facts potentially corresponds to the excellent capability of ODS steels to resist irradiation damage. Moreover, the calculation results may also interpret the synergistic effect of irradiation damage produced by both H and He to ODS steels.
 Select Numerical Simulation and Experimental Study on Porthole Die Extrusion Process of LZ91 Mg-Li Alloy Liang CHEN, Guoqun ZHAO, Gaojin CHEN, Zhaoqing LIANG, Cunsheng ZHANG Acta Metall. 2018, 54 (2): 339-346.   DOI: 10.11900/0412.1961.2017.00420 Abstract   HTML   PDF (6394KB) Porthole die extrusion is the dominant process to produce hollow profiles due to its high productivity and capacity in producing complex profiles. In this study, the finite element simulation model of porthole die extrusion of LZ91 Mg-Li alloy was established. The effects of extrusion ratio on strain, temperature and flow velocity were studied, and the welding quality was quantitatively evaluated by means of J criterion. The experiments of porthole die extrusion were carried out by varying extrusion ratios. The microstructures of as-cast, homogenized and extruded LZ91 Mg-Li alloy were examined. The results show that the materials near the bridge surface and at the bottom of the bridge have large deformation, while the materials inside the portholes have small deformation. Moreover, with the increase of extrusion ratio, the effective strain of material is increased. Due to the heat generated by plastic deformation and the heat dissipation caused by profile cooling, the temperature of the material on the top of bridge is increased, while that of the material near the die exit becomes lower. The welding quality in the central area of weld seam is lower than that in the edge area of weld seam. With the increase of extrusion ratio, the welding quality is improved. More nucleation is generated in welding zone due to its large strain, resulting in the formation of fine grains. However, the dynamic recrystallization is not complete in the matrix zone, and some coarse grains still remain. Moreover, the material temperature becomes higher with high extrusion ratio, and the phenomenon of grain growth is observed.
 Select Simulation of Gas-Liquid Two-Phase Flow and Mixing Phenomena During RH Refining Process Chang LIU, Shusen LI, Lifeng ZHANG Acta Metall. 2018, 54 (2): 347-356.   DOI: 10.11900/0412.1961.2017.00429 Abstract   HTML   PDF (6076KB) The Ruhrstahl-Heraeus (RH) vacuum system is vitally important in the secondary refining process since it is highly effective on decarburization and degassing, which involves complex multiphase flow and transport phenomena. Many investigations on the flow field in the RH refining process have been reported. However, only several investigations are focused on the bubble expansion in the up-leg snorkel. In this work, the combined mathematical model and physical model were employed to simulate the fluid flow and mixing phenomena in the RH reactor. A water model for a practical 210 t RH reactor was established according to similitude principle, and the flow field on the center section of the physical model was captured by PIV (particle image velocimetry) system. The coupled VOF (volume of fluid) model and DPM (discrete phase model) were used to simulate the multiphase fluid flow in the RH reactor. Both the k-ε model and LES (large eddy simulation) model were performed to describe the turbulent characteristics during the RH refining process. The mathematical model was validated by the water model with the same experimental conditions. It suggests that the calculated results show a good agreement with the measured one. Based on the LES model, the instantaneous velocity distribution and the generate and the dissipate of vortex were computed. Also, the mixing time of different position in the ladle was measured and calculated. The results show that the mixing time near the up-leg snorkel is larger than that near the down-leg snorkel. A model for bubble expansion was developed and used to simulate the bubble behavior in the steel-argon system. The results show that the bubble expansion has a strong impact on the flow field in the RH reactor.