Austenite/Ferrite Interface Migration and Alloying Elements Partitioning: An OverviewEI北大核心CSCD

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.     

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.     

Study and Development on Numerical Simulation for Application of Electromagnetic Field Technology in Metallurgical ProcessesEI北大核心CSCD

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 solidified 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.     

Galvanic Series of Metals and Effect of Alloy Compositions on Corrosion Resistance in Sanya SeawaterEI北大核心CSCD

With the development of ocean engineering, various metallic materials have been applied to the marine environment. It is an urgent requirement to study the galvanic series and alloy composition optimization of metallic materials in the tropical marine environment. In this work, open circuit potentials (OCP) and galvanic series of 36 kinds of metallic materials in Sanya seawater were studied. By considering the response of OCP to tidal changes, the anti-corrosion effects of alloying elements were also ana lyzed. The results show that the OCP of metallic materials in Sanya seawater has a large range. The galvanic series order of metallic materials from high to low in Sanya seawater is: nickel alloy, duplex stainless steel, austenitic stainless steel and pure copper, ferritic stainless steel, martensitic stainless steel, copper alloy, low alloy steel, carbon steel, cast iron, aluminum alloy and aluminum anode. Low-carbon high-alloy content carbon steel and high Cr, Ni contents stainless steel have higher OCP. The potential fluctuations of carbon steel with tidal changes involves two phases: (1) under the dynamics control, the OCP of carbon steel is more negative at high tide; (2) under the diffusion control, the OCP is more positive at high tide. The potential fluctuations of metallic materials reflect the effect of the corrosion product film on the change of ionization balance, and metals with less potential fluctuations have better inhibition on ion diffusion. In Sanya seawater, the carbon steel, which has more alloying content and less carbon content, has less potential fluctuations with the tidal changes and has good oxygen diffusion resistance. The potential fluctuations of austenitic stainless steel with tidal changes are less than that of ferritic stainless steel and martensitic stainless steel. After 2700 h immersion, austenitic stainless steel and martensitic stainless steel, which have a higher content of Mo, have more stable OCP. In other words, the corrosion film gets a better corrosion resistance. The OCP of aluminum anode in Sanya seawater environment increases when the oxygen content is brought up. The OCP of Zn-containing or Ga-containing aluminum anode remains relatively stable. Al bronze and T2 copper have less potential fluctuations with tidal changes, and perform good corrosion resistance in Sanya seawater.     

Study on Heat Generation Mechanism and Melting Behavior of Droplet Transition in Resistive Heating MetalWiresEI北大核心CSCD

With the development of space technology, the ability of manufacturing in space is a necessary guarantee for a long-term space mission. To achieve the repair and maintenance of spacecraft structure in space, a metal additive manufacturing method named resistance heating metal wire additive manufacturing process has been proposed in this work. During the experiments, the wire and the base plate are short-circuited, the current output from the programmable power source flows through the wire and the base plate to generate resistance heat, and then the wire begins to melt and transfer to the base plate. A real-time synchronization system has been used to record the current, voltage and image of metal wire synchronously, to study the melting process of metal wire by resistance heating. The direct current and pulse current with different amplitudes which were supplied by programmable power source have been used to study the effect of the current style and value on the melting process and transition behavior of metal wire. The change characteristic of the resistance in the wire and base plate has been analyzed during wire melting, to study the relationship between the current resistance and the wire state. The effect of gravity on the wire melting process has been studied by the wire transfer experiments at different space locations. The results show that when the metal wire was heated by the constant current, the total heat of metal melt could be controlled by controlling the current value, but it was difficult to precisely control the heating speed and the heat input. When using pulse current heating, both the heating speed and the heat input could be precisely controlled by pulse frequency and pick value. In the melt transfer stage, the constant current provides a fixed force on the molten wire, but the pulse current makes the molten wire swing by the intermittent force. The real-time resistance of metal wire during heating could be used to reflect the melting state of wire in both current styles. On the ground environment, the surface tension and electromagnetic contraction force make the melting wire against the gravity and transfer to the base plate, which illustrated the feasibility of using this process in space environment.     

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。     

Effect of Brazing Coating Induction Parameters on Temperature FieldEI北大核心CSCD

重熔处理技术作为提高基体与涂层界面结合强度的方法之一,具有加热速度快、工作环境清洁等优点,但仍存在热影响偏大时造成基体损伤的风险。以感应重熔涂层为研究对象,建立二维有限元传热模型,研究涂层感应重熔过程中的温度场变化规律。以 TC11 钛合金基体表面感应重熔钛基涂层 Ti49Zr49Be 为典型材料,研究发现：集肤深度分别为 4.0 mm、1.5 mm 和 0.6 mm 时,三种集肤深度下涂层熔化界面推移方式均不同,且基体热敏感温度区深度和持续时间随着集肤深度的减小呈现递减趋势;感应重熔功率分别为 35 kW、45 kW 和 55 kW 时,发现界面推移方式相同,均为涂层表面、涂层 / 基体界面处向涂层内部双向推移,且基体热敏感温度区深度和持续时间随加热功率的增加呈现递减的趋势。涂层感应重熔过程传热模型和温度场变化规律研究表明,增大功率、减小集肤深度有助于工程应用中抑制基体热影响。     

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.     

Effect of Microstructure Instability on Hot Plasticity During Thermomechanical Processing in PM Nickel-Based SuperalloyEI北大核心CSCD

High alloying Ni-based powder metallurgy (PM) superalloys show excellent fatigue performance and damage tolerance properties, and good creep resistance at 750 degrees C, and are used for advanced gas turbine disks and other hot components. The hot-working window of high alloying PM superalloy is narrow because of its poor workability. The formation of the gamma+gamma' microduplex structure during the thermomechanical processing always results in a decrease in flow stress and a promotion of hot plasticity. However, the stability of the gamma+gamma' microduplex structure has not been evaluated. The high temperature flow behavior of a Ni-based superalloy FGH98 prepared by hot isostatic pressing has been examined by means of uniaxial compression testing isothermally at 1060, 1105, 1138 and 1165 degrees C and at constant true strain rates between 0.01 and 10 s(-1). The microstructural evolution and instabilities during plastic flow have been studied. Under all testing conditions, the as-hipped material exhibits flow hardening, flow softening and steady-state flow sequentially with the true strain increased. The dynamic recrystallization occurs and the gamma+gamma' microduplex structures are generated when steady state flow or highest strains achieved at temperatures below the g' solvus. The formation of the gamma+gamma' microduplex structures results in a remarkable decrease in grain size and a promotion of hot plasticity. The relationships between steady-state grain sizes and steady-state stresses during deformation and the formation mechanism of the gamma+gamma' microduplex structure were analyzed. The possibility of the microstructure controlling during hot working was discussed.     

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.     

Study on Anisotropic Mechanical Properties of Cold Drawn Pearlitic Steel WireEI北大核心CSCD

Cold drawn pearlitic steel wires with ultra-high strength are widely applied in industrial fields such as bridge cables, automobile tire and springs rope. In recent years, the strengthening mechanism and microstructure evolution have been profoundly studied. In order to investigate the influence of microstructure evolution on mechanical properties, the anisotropic mechanical properties of cold drawn pearlitic steel wires were investigated by tensile test, SEM and TEM. Results indicated that the distinctions of tensile strength between three directions (parallel to the tensile axis, inclined to the tensile axis (45 degrees), vertical to the tensile axis) were amplified with increasing strain. The effect of strain strengthening was observed in parallel and inclined directions while the vertical direction remained strength stability in 1320 MPa. The wire rod was isotropic and the fracture mode was transgranular fracture; After cold drawing, the tensile strength reached peaks in parallel direction and valleys in vertical direction. The fracture mechanism of inclined and vertical directions remained transgranular or intergranular fracture while the fracture mechanism of parallel direction was converted into microvoid accumulation fracture. In TEM, the phenomenon was discovered that due to non-homogeneous distribution in vertical direction, dislocations piled up at the boundaries resulting in stress concentration. On the contrary, the dislocations were uniformly distributed which led to homogeneous transformation in parallel direction.     

Characterization and Modeling Study on Interfacial Heat Transfer Behavior and Solidified Microstructure of Die Cast Magnesium AlloysEI北大核心CSCD

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 alpha-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 < 11(2)over bar0 > in the basal plane and the other twelve along < 11(2)over bar3 > 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.     

Numerical Simulation and Experimental Study on Porthole Die Extrusion Process of LZ91 Mg-Li AlloyEI北大核心CSCD

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.     

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.     

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.     

Effect of Laser Oscillation on the Microstructure and Mechanical Properties of Laser Melting Deposition Titanium AlloysEI北大核心CSCD

针对激光熔化沉积冶金组织与缺陷,借鉴激光摆动焊接技术,提出一种激光摆动送粉增材制造TC4钛合金工艺,借助激光原位摆动改变熔池运动轨迹进而影响温度梯度和凝固速率,改善增材制造钛合金的微观组织。利用OM、SEM、EBSD和Vickers硬度计研究了激光摆动送粉增材制造工艺对TC4钛合金微观组织演变及力学性能的影响。结果表明,无摆动激光熔化沉积实验的最佳工艺参数为:激光功率1000 W,扫描速率8 mm/s,送粉速率6.92 g/min;直线型激光摆动的最佳工艺参数为:摆动频率200 Hz,摆动幅度1.5 mm。直线型激光摆动对熔池形貌改善显著,气孔和裂纹等缺陷较少,柱状晶数量和尺寸均有所减小,并且晶粒出现了等轴化的现象。相比无摆动样品,激光摆动后Ti-6Al-4V合金单道区域平均晶粒尺寸从5.20μm减小到4.37μm;硬度从418.00 HV提升到428.75 HV。     

Effect of Manganese Content on Tensile Deformation Behavior of Fe-Mn-C TWIP SteelsEI北大核心CSCD

Twinning-induced plasticity (TWIP) steels exhibit excellent mechanical properties including high tensile strength and good plasticity owing to their high strain-hardening rate. The high strain-hardening rate results mainly from deformation twinning; in addition, plane slip and dynamic strain ageing also have some contribution to strain-hardening rate. Until now, the influences of some alloy elements such as C, Al and Si on tensile properties of Fe-Mn-C based TWIP steels have received much attention. However, the effect of Mn content on the microstructure and tensile properties of twinning-dominated Fe-Mn-C TWIP steels is still not clear. In this work, the microstructure, tensile properties and strain hardening behavior of two Fe-Mn-C TWIP steels (Fe-13Mn-1.0C and Fe-22Mn-1.0C, mass fraction, %) were studied by using OM, TEM, SEM-EBSD and monotonic tensile tests. The results show that the yield and tensile strengths of the steel decrease while the elongation to fracture increases with the increase of Mn content. At low tensile strains, the increase of Mn content delays the formation of deformation twins. However, at higher strain level, the deformation twinning rate becomes higher and hence more deformation twins are produced in the steel with higher Mn content than that in the steel with lower Mn content. Furthermore, the thickness of deformation twins increases with increasing the Mn content. The twinning and tensile deformation behavior in the two steels are also discussed.     

Effect of Constituent Elements on the Corrosion Resistance of Single-Phase CoCrFeNi High-Entropy Alloys in NaCl SolutionEI北大核心CSCD

High entropy alloys (HEAs) origin from a new alloy design concept with multi-principal elements, which have attracted significant interests in the past decade. The high configurational entropy in HEAs results in simple solid solutions with fcc and bcc structures. Especially, the single solid solution CoCrFeNi alloy exhibits excellent properties in many aspects, such as mechanical properties, thermal stability, radiation resistance and corrosion resistance. The excellent corrosion resistance of CoCrFeNi alloy is ascribed to the single-phase structure and uniform element distribution coupled with much higher Cr content than stainless steel. The single-phase structure and uniform element distribution can prevent the occurrence of localized corrosion, and higher Cr content can protect the alloy surface better with the form of oxidation film. Moreover, the corrosion resistance of CoCrFeNi-based HEAs, such as CoCrFeNiAlx, CoCrFeNiCux, CoCrFeNiTix, have also been extensively investigated. In most CoCrFeNi-based HEAs, the elements of Co, Cr, Fe and Ni are with equal-atomic ratio. However, the equal-atomic ratio is not necessary to obtain satisfactory properties and to ensure the single fcc structure in Co-Cr-Fe-Ni system. Accordingly, it is essential to further consider the effect of alloying elements on the corrosion resistance in Co-Cr-Fe-Ni HEA. In this work, the effect of Co, Fe and Ni elements on the corrosion resistance of single fcc Co-Cr-Fe-Ni system with concentrated constitution but different atomic ratios in 3.5% NaCl solution are investigated by using LSCM and EIS. The potentiodynamic polarization results indicate that the increase of Fe and the decrease of Ni will decrease the passivation current density of the alloys when the Co and Cr contents are equal. With the increase of Co and the decrease of Ni, the alloys show smaller passivation current density and better corrosion resistance when the Fe and Cr contents are equal. With the decrease of Co and the increase of Fe and Ni, the alloys show higher corrosion potential and smaller corrosion tendency when the Cr content is constant. These results will be helpful for the design of corrosion resistant HEAs in NaCl aqueous solution.     

Effect of Zr Addition on the Grain Refinement Mechanism of Mg-Gd-Er AlloysEI北大核心CSCD

In recent years, Zr is widely used as an important additive element in magnesium alloys containing rare earth (RE), to improve the mechanical properties of Mg-RE alloys such as strength, ductility, creep resistance and corrosion resistance property. Heterogeneous nucleation mechanism and peritectic reaction mechanism are recognized as the main grain refining mechanisms. Whereas, during the solidification process, the melt wetting angle and nucleation energy are important factors which influence the nucleation. In this work, the effect of Zr on the solidification microstructure of the Mg-Gd-Er alloy was analyzed by using OM and EBSD; the undercooling of alloy melts was tested by using DSC; and the Mg/Zr interface relationship and interfacial energy were investigated by using HRTEM. Moreover, the effects of Zr on the wetting angle and nucleation activation energy of the Mg-11Gd-2Er and Mg-11Gd-2Er-0.4Zr alloys were investigated; the refinement mechanism of Zr on the alloys was discussed. The results indicates that the addition of Zr element can significantly refine the grain, and the grain size decreased from 1000 mm to 50 mu m. Compared with the Zr-free alloy, the nucleation wetting angle of the present alloy melt decreased from 18.3 degrees to 11.1 degrees, and the activation energy of nucleation decreased by 44.4%. The (1010) plane of Mg was completely coherent with the (1100) plane of Zr, reducing the interfacial energy between the (1010) Mg and the (1100) Zr. The grain refinement of Mg-Gd-Er alloy was ascribed to the decrease of melt wetting angle and the fully coherent interface relationship between Mg and Zr.