Effect of Gravity on Directionally Solidified Structure of SuperalloysEI北大核心CSCD

分别利用常规下抽拉法与新型上提拉法进行不同方向的高温合金定向凝固实验,对比研究重力对单晶铸件凝固组织的影响。结果表明,在常规下抽拉法实验的向上凝固过程中,容易出现雀斑、γ/γ’共晶上聚和籽晶回熔紊乱等问题。原因是糊状区内液体由于元素偏析引起密度减小,在重力作用下形成了上重下轻的失稳状态并引起对流。而通过新型上提拉法实现的顺重力凝固过程中,密度减小的液体处于糊状区上端,形成上轻下重的稳定状态,使重力的作用由失稳因素转化为维持稳定的因素,抑制了液体对流的产生与发展。采用新型上提拉法制备的单晶铸件中彻底消除了雀斑缺陷,抑制了γ/γ’共晶组织的向上聚集,也保证了低密度籽晶稳定的回熔和外延生长。顺重力定向凝固技术从根本上消除了重力对高温合金定向凝固的不良影响,有希望发展成为新一代的先进单晶叶片成型技术。     

Influence of Ultrasonic Rolling on Corrosion Resistance of 7075 Aluminum AlloyEI北大核心CSCD

表面纳米化处理是一种有效改善耐腐蚀性能的手段,但受表面粗糙度和残余应力等因素的影响,其相关机制并不清晰。 运用透射电镜(TEM)和扫描电镜(SEM)研究经超声表面滚压工艺(USRP)处理后 7075 铝合金的组织和性能。结果表明： 经 1 道次和 15 道次 USRP 处理后,7075 铝合金表面粗糙度减小并且引入了残余压应力。滚压 15 个道次的试样表面能获得平均晶粒尺寸为 52 nm 的纳米晶。相较于未处理试样,经 1 道次和 15 道次 USRP 处理后试样的耐腐蚀性能均显著提高。其中, 滚压 15 个道次试样的耐腐蚀性能提升更为显著。这主要是因为纳米晶可以使材料表面形成更加致密的钝化膜,导致其耐腐蚀性能显著提高,而表面粗糙度降低和引入残余压应力是提升耐腐蚀性能的次要因素。对比分析残余应力、表面粗糙度和表面纳米晶对 7075 铝合金耐腐蚀性能的影响,揭示了 7075 铝合金经表面纳米化处理后耐腐蚀性能提升的机制。     

Preparation and Bioactivity of Micro-Nano Structure on Ti6Al4V SurfaceEI北大核心CSCD

Titanium (Ti) and its alloys have been widely used in the medical field for dental and orthopedic surgeries owing to their excellent mechanical and biological properties. However, much effort has been devoted to the surface modification on Ti-based implants for better biological response in medical applications. Bioactive layers with micro- and nano-scale structures and morphologies can increase the specific surface area of the implants and facilitate rapid osseointegration, which has shown good biological behaviors both in the laboratory and clinical setting. Sandblasting and acid-etching (SLA) technology has become one of the most commonly used surface modification processes for currently marketed dental implants, since it can be easily operated and is efficient. However, studies on etching behavior are still limited. In this study, concentrated hydrochloric acid ((36%-38%)HCl, mass fraction) and mixed diluted acid (20%HCl : 30%H2SO4 = 1 : 1, volume fraction) were used to etch Ti6Al4V, and an ultrasonic field was applied to the acid etching treatment. The influence of different etching parameters on the surface structure and morphology of Ti6Al4V was discussed, including the acid etching reagent, acid etching time, and ultrasonic field. Moreover, through the combination of SLA and induction heating treatment (IHT) oxidation, the micro- and nano-scale hierarchical structure was prepared on the surface of Ti6Al4V. The evolution of surface topography, chemistry, roughness, wettability, and bioactivity of the hierarchical structure was discussed. The micro-scale composite pores combing dozens of micron pores and several micron pores were obtained by SLA. Within a certain etching time range, with the prolonging of the etching time, the step structure on the inner wall of the micro-pores becomes more obvious, and ultrasound can accelerate the acid etching. After the IHT at 800 degrees C, the micro- and nano-scale hierarchical surface with micro-scale composite pores and nanoscale oxide was obtained. Compared with the SLA surface, there was a decrease in surface roughness and an increase in wettability. Furthermore, after soaking in simulated body fluid (SBF) for 14 d, a homogeneous hydroxyapatite (HA) layer was formed on the micro- and nano-scale structured Ti6Al4V surface, suggesting high biological activity of the fabricated structure.     

Influence of Cl- and polarization potential on crevice corrosion behavior of X70 pipeline steel北大核心CSCD

The crevice corrosion behavior of X70 pipeline steel in NaHCO3 solution with varying Cl- concentration was investigated by potentiostatic polarization method in terms of the initiation and development of crevice corrosion. Results show that inside the crevice the X70 steel could suffer from localized corrosion in NaHCO3 solution by polarization potential-0.4 V. The acidification initiated firstly at the crevice mouth and then extended gradually to the bottom. The hydrogen evolution could be observed with the development of corrosion and acidification. The cathodic reaction changed from the reduction of the dissolved oxygen to the reduction of hydrogen ions. The presence of Cl- did not change the crevice corrosion mechanism. With the increase of Cl- concentration, however, the crevice corrosion rate increased. The corrosion region moved towards the crevice bottom gradually and then pitting corrosion occurred with the increasing polarization potential. The initiation of crevice corrosion was determined by the polarization potential. © 2016, Corrosion Science and Protection Technology. All rights reserved.     

Study,on Influence of Processes on the Dehydrogenation Performance of NaAlH4EI北大核心CSCD

In this paper, the effects of preparation process, ball-milling equipments, storing time and ball-milling time on the dehydrogenation performances were analyzed. All the samples were ball-milled by planetary ball mill except for sample 3 which was ball-milled by high-energy vibration ball mill. The results indicate that the above mentioned influence factors present obvious effect on the dehydrogenation performances of NaAlH4. The dehydrogenation amount of the samples turned up and down during ball milling increases by 50wt%. Compared to the samples prepared by planetary mill, the dehydrogenation amount of samples prepared by the high-energy vibration ball mill increases markedly. The results from studying on storing time and milling time show that the dehydrogenation amounts of the samples milled by planetary mill and laid aside for 24 h get an obvious increase. In addition, the amount of the hydrogen release of the samples milled for different time with planetary mill presents significantly difference. The amount of the hydrogen release of the sample milled for 80 min is higher than those milled for 100, 40 and 60 min. However, compared to other influence factors, the effect of ball-milling time on NaAlH4 is smaller.     

Influence of TiC Addition on Properties and Microstructure of Mo-Ti AlloyEI北大核心

Mo-Ti-TiC alloys were fabricated by powder metallurgy process through adding TiH2 powder and ultrafine TiC powder into Mo metal. The influence of the addition of nano-scale TIC particles on the microstructure and tensile properties of Mo-Ti alloy was studied. The results indicate that the tensile strength of Mo-Ti alloy was effectively increased by TiC particles addition. Mo-Ti with 0.05wt% TiC exhibited the highest tensile strength, which is 31.7% higher than that of Mo-Ti alloy. The addition of TiC protects Ti from oxidation, which is produced by decomposition and dehydrogenization of TiH2 particles. The second phase particles containing Mo, Ti, 0 and C in the alloy were formed with TiC addition. The grain size of the alloy decreased with the increase of the TIC content since the second phase particles can inhibit the grain growth.     

Effect of Fe on Microstructure and Coercivity of SmCo-Based MagnetsEI北大核心CSCD

High-temperature permanent magnets have an important application in the aerospace and other high-tech fields, among which 2:17-type SmCo magnets have become the first choice for high-temperature permanent magnets due to the strong magnetic anisotropy and high Curie temperature. Although there are studies on the effect of Fe on the remanence and coercivity, the role that Fe plays on coercivity mechanism of SmCo magnets is still unclear. In this work, Sm(CobalFexCu0.08-0.10Zr0.03-0.033) z (x= 0.10-0.16, z=6.90 and 7.40) magnets are prepared and the magnetic properties under different temperatures are investigated. The magnets with an intrinsic coercivity of 603.99 kA/m and a maximum energy product of 87.30 kJ/m(3) at 500 degrees C. are obtained. It is revealed that at room temperature the coercivity of the magnets increases with increasing Fe content, however, at 500 degrees C. the coercivity shows an opposite dependency on Fe content. Moreover, the effect of Fe on coercivity is more obvious at low z value. The phase structure and composition analyses were characterized by XRD and TEM. The results show that with the increase of Fe content, the size of the 2: 17R cell phase increases, the volume ratio of cell boundary 1: 5H phase decreases, and furthermore, both Fe content in the 2: 17R phase and Cu content in the 1: 5H phase increase. The variations of Fe and Cu contents in both phases lead to the change of the domain wall energy difference. With the increase of Cu content of 1:5H phase, the domain wall energy of 1: 5H phase (gamma(1:6)) drops faster at room temperature, the coercivity is determined by gamma(2:17)-gamma(1:5), so the coercivity increases with increasing Fe content. While at 500 degrees C, due to gamma(1:6) at its Curie temperature, the coercivity is mainly determined by the domain wall energy of 2: 17R phase (gamma(1:17)), which decreases with increasing Fe content. The increase of Fe content at the low z value results in a smaller growth of cell size, which leads to a more significant change in coercivity.     

Influence of Multi-Thermal Cycle and Constraint Condition on Residual Stress in P92 Steel WeldmentEI北大核心CSCD

P92 steel is a typical 9%similar to 12% Cr ferrite heat-resistant steel with good high temperature creep resistance, relatively low linear expansion coefficient and excellent corrosion resistance, so it is one of important structural materials used in supercritical thermal power plants. Fusion welding technology has been widely used to assemble the parts in thermal power plant. When the supercritical unit is in service, its parts are constantly subjected to combination of tensile, bending, twisting and impact loads under high temperature and high pressure, and many problems such as creep, fatigue and brittle fracture often occur. It has been recognized that welding residual stress has a significant impact on creep, fatigue and brittle fracture, so it is necessary to study the residual stress of P92 steel welded joints. The evolution and formation mechanism of welding residual stress in P92 steel joints under multiple thermal cycles were investigated in this work. Based on SYSWELD software, a computational approach considering the couplings among thermal, microstructure and mechanics was developed to simulate welding residual stress in P92 steel joints. Using the developed computational tool, the evolution of residual stress in Satoh test specimens was studied, and welding residual stress distribution in double-pass welded joints was calculated. In the numerical models, the influences of volume change, yield strength variation and plasticity induced by phase transformation on welding residual stress were taken into account in details. Meanwhile, the hole-drilling method and XRD method were employed to measure the residual stress distribution in the double-pass welded joints. The simulated results match the experimental measurements well, and the comparison between measurements and predictions suggests that the computational approach developed by the current study can more accurately predict welding residual stress in multi-pass P92 steel joints. The simulated results show that the longitudinal residual stress distribution around the fusion zone has a clear tension-compression pattern. Compressive longitudinal residual stresses generated in the fusion zone and heat affected-zone (HAZ) in each pass, while tensile stresses produced near the HAZs. In addition, the numerical simulation also suggests that the transverse constraint has a large influence on the transverse residual stress, while it has an insignificant effect on the longitudinal residual stress.     

Influence of Inter-Pass Temperature on Residual Stress in Multi-Layer and Multi-Pass Butt-Welded 9%Cr Heat-Resistant Steel PipesEI北大核心CSCD

9% Cr heat-resistant steels have been abundantly used in boilers of modern thermal plants. The 9% Cr steel components in thermal plant boilers are usually assembled by fusion welding. Many of the degradation mechanisms of welded joints can be aggravated by welding residual stress. Tensile residual stress in particular can exacerbate cold cracking tendency, fatigue crack development and the onset of creep damage in heat-resistant steels. It has been recognized that welding residual stress can be mitigated by low temperature martensitic transformation in 9% Cr heat-resistant steel. Neverthe-less, the stress mitigation effect seems to be confined around the final weld pass in multi-layer and multi-pass 9% Cr steel welded pipes. The purpose of this work is to investigate the method to break through this confine. Influence of martensitic transformation on welding stress evolution in multi-layer and multi-pass butt-welded 9% Cr heat-resistant steel pipes for different inter-pass temperatures (IPT) was investigated through finite element method, and the influential mechanism of IPT on welding residual stress was revealed. The results showed that tensile residual stress in weld metal (WM) and heat affected zone (HAZ), especially the noteworthy tensile stress in WM at pipe central, was effectively mitigated with the increasing of IPT. The reasons lie in two aspects, firstly, there is more residual austenite in the case of higher IPT, as a result, lower tensile stress is accumulated during cooling due to the lower yield strength of austenite; secondly, the higher IPT suppresses the martensitic transformation during cooling of each weld pass, thus the tensile stress mitigation due to martensitic transformation was avoided to be eliminated by welding thermal cycles of subsequent weld passes and reaccumulating tensile residual stress. The influence of IPT on welding residual stress relies on the combined contribution of thermal contraction and martensitic transformation. When the IPT is lower than martensite transformation finishing temperature (M-f), thermal contraction plays the dominant role in the formation of welding residual stress, and tensile stress was formed in the majority of weld zone except the final weld pass. While, compressive stress was formed in almost whole weld zone due to martensitic transformation when the IPT is higher than martensite transformation starting temperature (M-s).     

Anisotropy in Microstructures and Mechanical Properties of 2Cr13 Alloy Produced byWire Arc Additive ManufacturingEI北大核心CSCD

2Cr13 martensite stainless steel has been widely used for the manufacturing of surgical tools and turbine blades. Contrary to the conventional fabrication technologies, there are several remarkable advantages in the fabrication of 2Cr13 parts by adopting wire arc additive manufacturing (WAAM) technologies, such as excellent metallurgical bonding, high production efficiency, near-net-shape production, and limited environmental contamination. In this work, the effect of interlayer dwelling temperature (110-550 degrees C) on microstructural and mechanical properties has been revealed, providing a new approach for the active control of the performances of 2Cr13 buildups produced by wire-arc additive manufacturing. The part with a dwelling temperature of 550 degrees C was featured by elongated acicular martensite features, with a slightly enhanced fiber-like texture, along with minor fine irregular-reverse austenite structures, dispersed among martensite gaps. This special martensitic distribution was mainly caused by the grain-broken effect under the intensive thermal shock from liquid melting pool. Consequently, the enhanced tensile strength and microhardness were obtained due to grain refinement, although exhibiting an obvious anisotropy in tensile properties. The parts with dwelling temperatures of 110-180 degrees C were characterized by relatively coarsened martensite laths, with a random texture type, within block-shaped ferrite matrix. The average martensite size was gradually refined due to the increased cooling rate by lowering interlayer temperature. The isotropic mechanical properties of all three parts (110-180 degrees C) were similar because of the similar martensite laths.     

Effects of W Concentration on Creep Microstructure and Property of Novel Co-Based SuperalloysEI北大核心CSCD

以γ’相强化的Co-Al-W高温合金(Co-9Al-xW,x=8、9、10,原子分数,%)为研究对象,耦合CALPHAD和晶体塑性本构关系,建立了高温加载时微观组织演化的三元弹塑性相场模型,考察了W含量对蠕变过程中γ’相演化行为和蠕变性能的影响。结果表明,随W含量增加,γ’相体积分数增加,γ基体塑性变形降低,筏化形成并提前,导致蠕变性能提高。不变矩分析表明,9W和10W合金中筏组织形成是出现稳态蠕变阶段的主要原因。应力/应变分析表明,高W合金γ基体中较大的错配应力减小了塑性变形。     

Effect of High-Energy and Instantaneous Electropulsing Treatment on Microstructure and Properties of 42CrMo SteelEI北大核心CSCD

42CrMo steel was widely used in many industry fields for its excellent hardenability and high temperature strength. Many transmission mechanisms and fasteners, such as roller and heat-resistant gear, are made of this steel. However, the ductility of 42CrMo steel is relatively low after quenching and tempering. During high tempering Mo riched carbides at grain boundary and undecomposable martensite at low tempering are the main reasons for poor ductility of 42CrMo steel. Grain refinement can enhance both strength and ductility significantly, but traditional refinement technology will cause intergranular oxidation so that strengthening effect was weak. Although thermomechanical treatment can achieve dynamic recrystallization, its refinement effect is unstable. Elecropulsing treatment, which makes significant change in microstructure and properties of metals, has been applied in many fields such as, modification of solidified microstructure of liquid metal, healing of fatigue crack, nanocrystallization of amorphous materials and so on. Moreover, this process can produce superior mechanical properties in metals. In order to improve the mechanical properties of 42CrMo steel better, high-energy and instantaneous electropulsing treatment was applied. In this contribution, 42CrMo steel was subjected to traditional and electropulsing treatment individually. It was found that EPQ treatment (480 ms electropulsing treatment, water cooled) results in finer grain, promoting the formation of retained austenite and twin martensite; EPT treatment (180 ms electropulsing treatment, air cooled) can stabilize retained austenite in EPQ specimen and induce multiphase structure. Mechanical properties results indicate that strength-ductility balance of EPQ and EPQ+EPT specimen are 32% and 13.9% higher than that of TQ (traditional quenched) and EPQ+TT (traditional tempered) specimen respectively.     

Anisotropy and Deformation Mechanisms of As-Extruded Mg-3Zn-1Y Magnesium Alloy Under High Strain RatesEI北大核心CSCD

As a very important design principle, the dynamic properties of materials attracted extensive attention in resent years and a bunch of works have been done concerning with the materials deformation behaviors under high strain rates. However, the dynamic behaviors of magnesium alloys are not through understood, especially the rare earth based magnesium alloys. In order to investigate the dynamic and anisotropic behavior under high strain rates deformation of as-extruded Mg-3Zn-1Y magnesium alloy, the split Hopkinson pressure bar (SHPB) apparatus was used to testing the true stress-true strain curves under the high strain rates of 1000, 1500 and 2200 s(-1) of as-extruded Mg-3Zn-1Y magnesium alloy. The OM and SEM were used to analysis the micorstructure evolution and fracture surface morphology of the alloy. The true reason behind the anisotropic phenomenon was revealed based on the deformation mechanism of highly basal-textured magnesium alloy. The results demonstrate that the as-extruded Mg-3Zn-1Y magnesium alloy exhibits pronounced anisotropy during compression according to the loading direction. The anisotropy of the as-extruded Mg-3Zn-1Y magnesium alloy are arised from the variety of the deformation mechanisms. When the loading direction is along extrusion direction, the predominant deformation mode changes from extension twinning at a lower strain to prismatic slip at a higher strain. While compressed along extrusion radial direction (ERD), the predominant deformation mode changes from contraction twinning to a coordination of basal and second order pyramidal slip with the increasing of strain.     

Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based SuperalloysEI北大核心CSCD

为满足不断攀升的两机涡轮动力系统的快速发展,表面冲击强化技术在涡轮转子用高温合金表面强化的应用及相应机制的研究受到了广泛关注。然而,高温合金表面硬化层在高温服役环境下的回复、再结晶行为难以避免,由此引起的表面强韧化、抗疲劳效果的退化,成为制约表面冲击强化技术在先进高温合金关键部件深入应用的瓶颈。本文总结了近年来镍基高温合金表面冲击强化机制及应用研究进展,分析了表面冲击强化对镍基高温合金表面强韧性及抗疲劳的作用规律,探究了高温合金表面冲击硬化层在高温及长期时效过程中的显微组织、微结构演化及其对高温稳定性的作用机理。以期为发展镍基高温合金表面冲击强化、提高两机涡轮转子疲劳抗力提供基础。     

Effect of Superheated Temperature and Cooling Rate on the Solidification of Undercooled Ti MeltEI北大核心CSCD

Undercooling is an important parameter to characterize the process of solidification and the physical properties of the melt. However, the traditional experimental conditions do not provide mature technical conditions and experimental platforms for the study of this subject. Molecular dynamics simulation method can not only study the experimental process and the organization structure, but also break through the limited conditions of the laboratory, and provide advanced prediction for scientific research. In order to study the influences of superheated temperature and cooling rate on the undercooling of the homogeneous nucleation and the solidified structure, the solidification of undercooled Ti melt was studied by molecular dynamics simulation in this work; and the solidified structure was then analyzed by the radial analysis, the H-A key type analysis and the largest groups of cluster analysis. The results show that, the nucleation undercooling of Ti melt increases with the rise of superheated temperature. In the undercooling vs temperature curve there are two inflection points at 2100 K (T1) and 2490 K (T2), which correspond to the breaking-start temperature and breaking-end temperature for bond pair of nucleation cluster. In this temperature range, the number of nucleation clusters decreases with rise of temperature. When the superheated temperature is higher than T2, the nucleation undercooling approaches a constant. On the other hand, the nucleation undercooling of Ti melt increases with the accelerate of cooling rate until an anomalous structure is formed, and in the numbers of the bonds of the structure vs different cooling rate curves, the number of 1541, 1551 and 1431 bond types gradually adds with cooling rate go-ing up. In addition, when the cooling rate is less than 1.0x10(11) K/s, the hcp and bcc inlaid crystalline structures are obtained after the solidification of Ti melt. When the cooling rate is greater than or equal to 1.0x10(13) K/s, two kinds of crystalline structure are reduced, and the microstructures are mainly amorphous. When the cooling rate ranges between 1.0x10(11) K/s and 1.0x10(13) K/s, its structure is a mixture of crystalline and amorphous. From the results of radial distribution, H-A bond type and atomic cluster analysis, it was found that the critical cooling rate for amorphous structure is determined as 1.0x10(13) K/s.     

Effect of N Doping on Microstructure,Mechanical and Tribological Properties of V-Al-C CoatingsEI北大核心CSCD

The crises of resource shortage have prompted ocean exploitation to spring up all over the world. Some crucial frictional components of marine equipment have to be directly faced with the conjoint action of wear and corrosion. Transition metal nitrides or carbides hard coatings have been widely used to improve tribological performance in various applications. However, the poor toughness, wear and corrosion resistance of coatings cannot meet the harsher marine environment, which needs to obtain multi-functional hard coatings providing complex properties. The nanocomposite structure coatings containing nanocrystalline phase embedded in an amorphous matrix allow tailoring their properties to desired value by designing chemical composition and nanostructure. In this work, V-Al-C and V-Al-C-N coatings were deposited on silicon and high speed steel (HSS) substrates by magnetron sputtering. The crystal microstructure, chemical composition, surface morphology, cross-sectional structure, mechanical property and friction behavior of the coatings under different contact conditions (air, distilled water and artificial seawater) were studied by XRD, XPS, SEM, nano-indentation and ball-on-disc tribometer. The results showed that the V-Al-C coating displayed columnar structure with coarse grain. When the nitrogen was incorporated, the coating structure evolved into nanocomposite structure composed of nanocrystallite and amorphous carbon. The hardness increased from (14 +/- 0.48) GPa to (24.5 +/- 0.8) GPa, and the toughness was significantly improved (H/E>0.1). In air condition, the friction coefficient decreased from 0.70 to 0.42, owing to the synergy interaction between V2O5 and amorphous carbon during sliding. The friction coefficients of the both coatings in distilled water and artificial seawater were lower than those in air owing to the boundary lubrication forming lubricative film by absorbed water. The friction coefficient in seawater was lower than those in distilled water, resulting from the formation of Mg(OH)(2) and CaCO3 during sliding. However, the wear rates of the both coatings in artificial seawater were larger than that in distilled water, which demonstrated a synergism between corrosion and wear in artificial water. The V-Al-C coating was all worn out under different contact conditions owing to severe abrasive wear, while the V-Al-C-N coating showed better wear resistance, with a wear rate of 3.0x10(-16) m(3)/(N center dot m) in air and 1.4x10(-15) m(3)/(N center dot m) in artificial water, respectively.     

Progress and Application of Microstructure Simulation of Alloy SolidificationEI北大核心CSCD

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.     

Effect of Transverse Static Magnetic Field on Microstructure of Al-12% Si Alloys Fabricated by Powder-Blow Additive ManufacturingEI北大核心CSCD

Due to the great advantage in manufacturing component with complex structures, additive manufacturing (3D print), essentially the rapid solidification of tiny metallic molten pool (hemisphere like with diameter ranging from dozens of microns to several millimeters) has become an important formation technique. Using powder laser melting, the effect of transverse static magnetic field on the solidified structure of additive manufactured Al-12% Si alloy was studied. The macrostructure was formed by white band (mainly primary alpha-Al phase) and dark grey area (mainly eutectic phase) and no obvious influence was presented with or without static transverse magnetic field of 0.35 T. However, for the microstructure, the primary alpha-Al in dark grey area formed as columnar structure without magnetic field was found to transform to dendritic like with developed dendrite arms when under a static transverse magnetic field. The analysis on thermoelectricity and dimensionless Hartman parameter which used to characterize the restriction of static magnetic field on molten flows show that under a static transverse magnetic field of 0.35 T, the thermoelectric magnetic force can be as high as a magnitude of 10(5) N/m(3), and Hartman values is far more than 10. The results indicate that the Marigoni and thermosolutal convection in laser melting pool was restricted. The transform from columnar to equiaxed dendrite of primary alpha-Al in dark grey area under static magnetic field was attributed to the fragmentation by thermoelectric magnetic force (10(5) N/m(3)) in solid phase. In addition, the formation of high order dendrite arms was supposed to be caused by the restriction of static magnetic field on the melt.     

Modeling and Simulation of Hydrogen Behavior in TungstenEI北大核心CSCD

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 H-2 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.