Positron Annihilation Study of High-Temperature Oxidation Behavior of Zr–1Nb Alloy

The oxidation behavior of Zr–1Nb alloys exposed at 873 and 973 K in air was investigated by positron annihilation lifetime spectroscopy together with mass gain, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mass-gain results showed that during the oxidation process, a transition of the oxidation rate occurred. The transition times of the specimens oxidized at 873 and 973 K were 30 and 6 h, respectively. In the pre-transition stage, the mass-gain curves obeyed the subparabolic law (n?=?2.3), while at the post-transition stage, the mass-gain curves obeyed the linear law. The positron lifetime measurements indicated that in pre-transition stage, the formed oxide scale mainly consisted of a compact layer that only contained small-size vacancy defects. The accumulation of these vacancy defects together with the high compressive stress might cause the breakaway of the oxide layer. During the post-transition stage, the thickness of the porous oxide layer with more and larger-size defects such as voids and pores increased rapidly as increasing the oxidation time. These large-size defects, together with the cracks produced during the transition from protective to breakaway-type oxide, increased the oxygen absorption rate and accelerated the diffusion of oxygen. The formation of cracks in the porous layer was confirmed by SEM examinations.     

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.     

Micro-Superplastic Behavior of Copper Oxide in AgCuO CompositesEI北大核心CSCD

In the investigation of AgCuO composites, we have found a type of copper oxide particles with a superplastic deformability behavior, which is similar to that of metals. To find the reason of the deformability of the copper oxide particles. SEM, STEM and TEM were used to analyze their crystal structures in AgCuO composites, The results show that the copper oxide particles with micro-superplasticity in the composites have cubic crystal structure, and their maximum elongation can be up to 300%, The copper oxide particles with no micro-superplasticity in the composites have monoclinic crystal structure. But the micro-superplastic behavior and mechanism of the copper oxides with the cubic crystal structure are not clear for the time being, the further investigation could be needed.     

Investigation of Corrosion Behavior of Welded Joint of X70 Pipeline Steel for Deep SeaEI北大核心CSCD

X70 pipeline steel with thick specifications (40.5 mm) for 3500 m deep sea reached the international advanced level in the wall thickness and service depth. Due to the high heat input during the welding process, the corrosion resistance of inside welding and outside welding would vary depending on the microstructure differences. The corrosion resistance of the welded joints of X70 pipeline for deep sea was studied by the immersion test, the weight loss test, the electrochemical test in this work. The components of the passive film were analyzed by XRD and the microstructure was observed by SEM. The results show that the corrosion resistance of the weld metal is the best. The corrosion resistance of the heat affected zone follows. The corrosion resistance of the base metal is the worst. And for the same area, the corrosion resistance of the inside welding is better than that of the outside welding. The formation of dense Fe3O4 passivation film can effectively slow down the progress of the reaction, and the corrosion products of Fe2O3, FeOOH and Fe(OH)(3) which are loose in the outer layer, have no protective effect on the matrix. The microstructure of the weld metal with the best corrosion resistance is mostly the intragranular nucleation ferrite and martensite-austenite (M-A) constituent is fine and uniform. The microstructure gradient of the heat affected zone is the largest, the M-A constituent is coarse and the corrosion resistance is inferior to the weld metal. The base metal consists of ferrite and bainite, the bainite is island-like distribution and the corrosion resistance is the worst. Microstructure of the inside welding is more refined, owing to the influence of outside welding thermal cycle, and the volume fraction of M-A constituent in inside welding is higher than that of the outside welding, so the corrosion resistance is better than that of the outside welding.     

Effect of Pre-Oxidation Treatment on the Behavior of High Temperature Oxidation in Steam of G115 SteelEI北大核心CSCD

In order to improve the steam oxidation resistance of G115 steel (9Cr3W3CoVNbCuBN) at 650., pre-oxidation treatment was carried out in argon environment with low oxygen partial pressure. The oxidation behaviors of the pre-oxidized and untreated samples were simultaneously investigated by a cyclic oxidation experiment. Weight gains of samples were measured by analytical balance, phases of oxide products were identified by XRD and EDS, morphology and structure of scales were characterized by SEM and EDS. The result showed that pre-oxidation treatment significantly decrease oxidation weight gains in 1800 h. After pre-oxidation treatment, the oxidation kinetics transformed from cubic into linear form, and the scale structures transformed from duplex layers into triple layers. In the scale of preoxidized samples, the outermost layer was enriched in Fe, the middle layer was enriched in Cr, and the innermost layer was transformed from the matrix metal. The middle layer had chromium content as high as 46% (mass fraction) and was considered to be conformed of chromite (FeCr2O4). This layer was the most protective layer due to its highest Cr content, and the diffusion of O and Fe though it was the main controlling process of the whole oxidation. It suggested that the stable structure of the middle layer improved the oxidation resistance of pre-oxidation samples. The thickness of the middle layer nearly kept constant during the whole oxidation process, which was the main reason why the pre-oxidized sample had linear oxidation kinetics. The long term effect of the pre-oxidation treatment was evaluated based on the scale structure and oxidation mechanism.     

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.     

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.     

Corrosion Behavior of GH4169 Alloy in NaCl Solution Spray Environment at 600°CEI北大核心CSCD

The corrosion behavior of engine materials of airplanes working in marine environments is accelerated by the synergistic effects of NaCl particles and water vapor at high temperatures. This work examined the corrosion behavior of GH4169 alloy with a NaCl solution spraying at 600 degrees C using an oxidation kinetics test and micro characterization technology in the aspects of corrosion kinetics, corrosion layer phase composition, and microstructure. The weight gain of the GH4169 alloy corroded in the NaCl solution spraying environment was much lower than that in solid NaCl + wet O-2 after 20 h corrosion at 600 degrees C. The corrosion products of the GH4169 alloy in the NaCl solution spray environment were less complex than those in the solid NaCl + wet O-2 environment, but they were denser. In addition, Cl was concentrated in the inner layer of the corrosion products and accelerated the corrosion of GH4169 alloy via an active oxidation mechanism at the initial stage. When NaCl deposition was increased, the corrosion mechanism of GH4169 alloy changed gradually to Cl-induced active oxidation. The sensitivity of GH4169 alloy in the NaCl solution spray environment at 600 degrees C was analyzed. Overall, the sensitivity of elements in GH4169 alloy to chlorine activated corrosion was Ti > Al > Nb, Cr > Fe > Mo, Ni, whereas the sensitivity of the oxides was TiO2 > MoO2 > Cr2O3(Nb2O5) > Fe2O3 > Al2O3 > NiO.     

Precipitation Behavior of delta Phase of Deformation Induced GH3625 Superalloy Hot-Extruded TubeEI北大核心CSCD

GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the gamma" is a metastable phase, easily transformed into stable delta phase, or d phase directly formed in the gamma matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of d phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 degrees C for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of d phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that d phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of delta phase at the deformation bands increases with the increase of cold deformation. The morphologies of delta phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of delta phase increases which grows according to LSW theory. At 800 degrees C, the relationship between the precipitation content of delta phase and ageing time follows Avrami equation. As cold deformation increases, the content of delta phase increases, the time index n decreases, whereas the d phase precipitation rate increases. Cold deformation promotes the precipitation of delta phase. The solute drags of Nb in soild solution and pinning of delta phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at epsilon = 35% but no obvious change at epsilon >= 50%.     

Numerical Simulation of Liquid-Solid Conversion Affecting Flow Behavior During Casting Filling ProcessEI北大核心CSCD

Misrun and cold shut are common defects in casting productions, which could make surface accuracy of castings poorer, even leading to cracking and casting scraps in them. The formation process of misrun and cold shut is hard to be observed directly only by experiment measures, since casting filling process is in a state of high temperature flow inside mold. The key to predict the defects accurately is the way to handle the effect of liquid-solid conversion on flow behavior. On the basis of existing methods for treating liquid-solid conversion, a calculation model of mushy region flow behavior through measurement of solid-fraction is developed, which can effectively investigate the flow behavior of mushy region in different stages. Generally, the critical solid-fraction method is adopted for mushy region with high solid-fraction, in consideration of that only the speed of high solid-fraction region is supposed to be zero during casting filling process. The variable viscosity method is applied for mushy region with low solid-fraction, due to casting filling process being unlikely to form toothpaste-like flow. However, the porous medium drag-based model is used for mushy region with middle solid-fraction, because only the middle solid-fraction region can be equivalent to porous medium. Combining the above three methods, a flow-field calculation program considering the effect of liquid-solid conversion on flow behavior during casting filling process is developed, in which finite volume method (FVM) is included for discretization equations; the pressure implicit with splitting of operator (PISO) algorithm is added for coupling pressure and velocity; the volume of fluid (VOF) algorithm is also combined for interface tracking. An numerical simulation of water-filled S-shaped channel is performed in the case of taking no account of liquid-solid conversion, and the simulated results coincide better with the experimental results, which certifies for its accuracy as an adopted model. Since the bottom filling casting craft is commonly used in single-shape casting, a comparison between the calculated results obtained using other single models and those using this model at different control parameters, is needed. The better agreement between them indicates that this new model is appropriate for calculating the flow behavior in mushy region.     

Research on Hot Working Behavior of Low-Nickel Duplex Stainless Steel 2101EI北大核心CSCD

The thermal deformation difference of two phases for duplex stainless steel (DSS) makes hot working difficult, 2101 DSS substitute Mn, N for Ni to stabilize austenite phase, which will significantly affect hot deformation behavior. Hot compression tests in the temperature ranging from 1123 to 1423 K and strain rate ranging from 0.001 to 10 s(-1) were carried out on a Geeble-3800 thermal simulator for 2101 DSS. At the same strain rate, the flow curve characteristics of 2101 DSS changed from dynamic re-crystallzation (DRX) to dynamic recovery with increasing deformation temperature. Increasing defomation stain rate from 0.001 s(-1) to 0,01 and 0.1 s(-1) increased DRX temperature range, but higher strain rate of 1 and 10 s(-1) is not beneficial to DRX occurrence. In the deformation temperature region of 1253 similar to 1323 K and low strain rate of 0.01 similar to 0.1 s(-1), the smaller strain value corresponding to the peak stress, the austenite DRX is more likely to occur, which is beneficial to the equiaxed recrystallized grains formation. At low strain rate, the recrystallization grain grows up with the increase of deformation temperature, the worse effect of austenite DRX is related to weakened austenite stabilized ability of Mn substitution for Ni at high Zener-Hollomon parameter values. Based on the thermal deformation equation, the apparent activation energy Q was calculated as 464.49 kJ/mol, which is slightly higher than that of 2205 DSS, and the constitutive equation of the peak flow stress was established. By combining with flow curve and microstructure analysis, the processing map exhibits the optimum processing conditions are in deformation temperature ranging from 1220 to 1350 K and strain rate ranging from 0.001 to 0.1 s(-1) with high power dissipation co-efficient of 0.40 similar to 0.47, under which the austenite DRX obviously occurred.     

Research Progress of Modified Polyaniline in Anticorrosive CoatingsEI北大核心CSCD

聚苯胺因其可逆的氧化还原特性在金属腐蚀与防护领域具有广阔的应用前景,目前有关改性聚苯胺对涂层附着力、阻隔性能以及钝化机理的研究比较零散,缺乏系统总结。通过对单一聚苯胺分散性差、疏水性弱等缺陷的分析,报道近年来改性聚苯胺在防腐涂料领域中的研究思路和研究进展,比较不同条件下改性策略的优劣,归纳聚苯胺结构与涂层耐腐蚀性之间存在的联系。进一步论证柔性、疏水基团取代聚苯胺有利于提升涂层抗渗性,改变掺杂剂以及与纳米氧化物、石墨烯等原位聚合制备复合填料也是提升涂层防腐性能的有效途径。展望该行业未来研究和发展的趋势,可为今后聚苯胺的改性工作提供理论指导。     

Numerical Simulation of Macrosegregation in Steel Ingot CastingEI北大核心CSCD

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.     

Development of Composition and Heat Treatment Process of 2000 MPa Grade Spring Steels Assisted by Machine LearningEI北大核心CSCD

The rapid development of rail transit has led to the proposition of higher requirements for the mechanical properties of springs and spring steels. Thus, bogies have been identified as the key components for trains to achieve high speed since they are connected with train bodies and wheel sets through springs. Alternatively, since the properties of spring steel materials have an important effect on the safety and comfort of high-speed trains, the development of spring steels with ultra-high strength and good plasticity has attracted the attention of researchers and industrial circles. However, simultaneously improving strength and plasticity has remained an important challenge for the research and development of high-end steels. Notwithstanding, machine learning has recently made substantial progress in designing and predicting various materials, and is expected to become a powerful tool for clarifying the relationship between the composition, process, and properties of complex alloys like steels. Based on the above background, this study reports the realization of rapid chemical composition and heat treatment process-design parameters for new spring steels, using a performance-oriented machine learning design system with high strength and good plasticity (tensile strength (2050 +/- 50) MPa, elongation 10.5% +/- 1.5%) after collecting literature data on spring steels and other typical quenched + tempered steels. Experimental studies were also carried out to obtain a further optimized heat treatment process (heating at 950 degrees C for 30 min and oil quenching + tempering at 380 degrees C for 90 min and water cooling). Investigations revealed that the tensile strengths of the two new spring steel materials developed were 2183.5 and 2193.0 MPa, their yield strengths were 1923.0 and 2024.5 MPa, their elongations after fracture were 10.5% and 9.7%, and the area reductions were 42.4% and 41.5%, respectively, with grain boundary strengthening and dislocation strengthening being the main strengthening mechanisms of the new spring steels. It was also observed that the fine grain size and appropriate amounts of austenite made the spring steels maintain good plasticity and have ultra-high strength. Moreover, compared with the existing ultra-high strength steels at the same strength grade, the new spring steels had significant technological and cost advantages. Hence, based on the above research, a new method and theory are provided to design chemical composition and heat treatment processes for quenched and tempered steels.     

Study on the Galvanic Current of Corrosion Behavior for AH32 Long-Scale Specimen in Simulated Tidal ZoneEI北大核心CSCD

The environment of the tidal zone is very complex. The interactions of dry-wet alternation and sea erosion lead to serious corrosion of steel structures, which makes it difficult to adopt protective methods. Therefore, it is of great significance to study the corrosion and protection methods of steel in tidal zone. For long-scale steel through the tidal zone and immersion zone, there is a big difference in corrosion behavior with complete immersion condition, the potential of the steel surface changes due to the influence of oxygen concentration difference and tidal fluctuations or other factors. In this study, the galvanic current and open circuit potential of the long-scale AH32 steel were monitored in simulated tidal zone. The results shows that the potential at different tide levels and different immersion depths for a long-scale AH32 specimen is not unified, with the macro cell was formed by the difference of oxygen supply, which caused internal galvanic current. The essence of the galvanic current is the net current that was generated by the sum of anode and cathode current. Galvanic current at different positions on the long-scale AH32 specimen varies with the tidal movement periodically in tidal zone. When tide is at the highest level, the galvanic current of all parts accesses a maximum value, and among these maximum values, the largest one is at the middle part of specimen, which causes the biggest anodic dissolution current density. According to the variation of the galvanic current, the time distributions of the drying, wetting and immersion states were calculated, and the results showed that the corrosion scale of the long-scale AH32 specimen at different positions depends on the time all location of wetting and immersion in tidal zone. The macro cell caused the galvanic current when all parts of the specimen were immersed. At wetting state, the solution resistance of the thin liquid film is very large, which leads to the change of the driving potential of the macro cell into the potential drop. Thus, macro cell is ineffective in the wetting state and cannot produce the galvanic current. According to the relation between wetting time and quantity of electricity at wetting state, the maximum wetting time of the long-scale AH32 specimen is shown above average mean tide level in tidal zone, which indicates that the corrosion loss of this part is maximum due to wetting state. In addition to weight loss measurements, maximum of it for long-scale AH32 specimen was obtained at the average mean tide level caused by immersion state. It can be indicated the maximum weight loss of the long-scale AH32 specimen should appear upper the average mean tide level part in tidal zone. These results were consistent with measurements of corrosion rates.     

Effects of Tool Rotation Rates on Superplastic Deformation Behavior of Friction Stir Processed Mg-Zn-Y-Zr AlloyEI北大核心CSCD

Compared to conventional Mg-Al and Mg-Zn system magnesium alloys, the Mg-Zn-Y-Zr heat-resistant alloy exhibits high thermal stability due to the addition of Y earth element, which is an ideal candidate for producing high strain rate superplasticity (HSRS, strain rate >= 1 x 10(-2) s(-1)). Recently, the HSRS of Mg-Zn-Y-Zr alloy was achieved by friction stir processing (FSP), because the FSP resulted in the generation of fine and equiaxed recrystallized grains and fine and homogeneous second phase particles. However, the study on superplastic deformation mechanism of FSP Mg-Zn-Y-Zr alloy at various parameters is limited relatively. Therefore, at the present work, six millimeters thick as-extruded Mg-Zn-Y-Zr plates were subjected to FSP at relatively wide heat input range of rotation rates of 800 r/min to 1600 r/min with a constant traverse speed of 100 mm/min, obtaining FSP samples consisting of homogeneous, fine and equiaxed dynamically recrystallized grains and fine and uniform Mg-Zn-Y ternary phase (W-phase) particles. With increasing rotation rate, within the FSP samples the W-phase particles were broken up and dispersed significantly and the recrystallized grains were refined slightly, while the fraction ratio of the high angle grain boundaries (grain boundaries misorientation angle >= 15 degrees) was increased obviously. Increasing rotation rate resulted in an increase in both optimum strain rate and superplastic elongation. For the FSP sample obtained at 1600 r/min, a maximum elongation of 1200% was achieved at a high-strain rate of 1x10(-2) s(-1) and 450 degrees C. Grain boundary sliding was identified to be the primary deformation mechanism in the FSP samples at various rotation rates by superplastic data analyses and surfacial morphology observations. Furthermore, the increase in rotation rate accelerated superplastic deformation kinetics remarkably. For the FSP sample at 1600 r/min, superplastic deformation kinetics is in good agreement with the prediction by the superplastic constitutive equation for fine-grained magnesium alloys governed by grain boundary sliding mechanism.     

Phase-Field Modeling of Austenite-to-Ferrite Transformation in Fe-C-Mn Ternary AlloysEI北大核心CSCD

The effect of Mn on the austenite-to-ferrite transformation has been widely studied by both physical models and experiments due to its technological importance for alloy design in steel industries. In recent years, an increasing interest of this issue is moved onto the effect of alloying element on the migrating interface during the austenite-to-ferrite transformation. For ternary Fe-C-Mn alloys, the interfacial condition is more complicated than that of binary Fe-C alloys in view of the large difference in the diffusivity between the interstitial and substitutional alloying elements. Generally speaking, there are two main concepts, i.e. the paraequilibrium model and the local-equilibrium model, which have been proposed to describe the phase transformation kinetics in ternary Fe-C-Mn alloys based on different assumptions about the diffusion of the substitutional elements. And many modeling attempts have been made to study the effect of Mn on the migration kinetics by using these theories. In this work, a multi-phase-field (MPF) model coupling with a Gibbs-energy dissipation model was developed to simulate the isothermal austenite-to-ferrite transformation in ternary Fe-C-Mn alloys. This model has considered the Mn diffusion inside the migrating interface in a physical manner and takes its effect on the transformation kinetics into account. Comparison simulations were made to analyze the difference in the transformation kinetics and ferrite morphologies with and without considering the energy dissipation at the moving interface. It shows that the incomplete transformation phenomenon does occur due to the Mn diffusion inside interface. The modified MPF model was then used to study the effect of Mn contents on the microstructures and kinetics of the phase transformations. It is found that the ferrite growth along the austenite/austenite boundaries is faster than that in the perpendicular direction. This difference is intensified with increasing the Mn concentration, which hence leads to the ferrite morphology changed from elliptical to flat alike. It also produces a slower transformation kinetics and a larger degree of the incomplete transformation when increasing the Mn concentration.     

Micromechanism of Cleavage Fracture of Weld MetalsEI北大核心CSCD

Cleavage fracture is the most dangerous form of fracture. Cleavage fracture usually happens well before general yielding at low nominal fracture stress and strain. Cleavage fracture is often spurred by low temperature and determines the toughness in the lower shelf temperature region. This paper describes a new framework for the micromechanism of cleavage fracture of high strength low alloy (HSLA) steel weld metals. Cleavage fracture not only determines the impact toughness in the lower shelf but also plays a decisive role on the impact toughness in the transition temperature region. The toughness is determined by the extending length of a preceding fibrous crack which is terminated by cleavage fracture. Three non-stop successive stages, i.e. crack nucleation, propagation of a second phase particle-sized crack across the particle/grain boundary, propagation of a grain-sized crack across the grain/grain boundary are explained. The "critical event" of cleavage fracture is emphasized which offers the greatest difficulty during crack formation and controls the cleavage process. The critical event indicates the weakest microstructural component and its critical size which specifies the local cleavage fracture stress sigma(f) for cleavage fracture. In toughness-study it is paramount important to reveal the critical events for various test specimens. Three criteria for crack nucleation, for preventing crack nucleus from blunting and for crack propagation are testified. An active region specified by these criteria is suggested where the combined stress and strain are sufficient to trigger the cleavage fracture. It can be used in statistical analyses. A case study, using the new framework of micromechanism for analyzing toughness of 8% Ni steel welding metals is presented to analyze the experimental results.     

Progress on Numerical Simulation of Vibration in the Metal SolidificationEI北大核心CSCD

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.     

Modification Mechanism of Cerium on the Inclusions in Drill SteelEI北大核心CSCD

Fatigue fracture is the main failure forms of drill steel, and the hard oxide with large size is one of the main reasons for the fatigue fracture of drill steel. Therefore, the miniaturization and softening of inclusion can effectively improve the anti-fatigue performance of drill steel and prolong its service life. Rare earth elements have very good affinity with oxygen and sulfur in molten steel, and the hardness of resulting rare earth compounds is very low. In this work, the rare earth element cerium was added into drill steel to investigate the effect of Ce on the MgAl2O4 and sulfides. The composition, morphology, number, and size of inclusions in drill steel were analyzed by using SEM and EDS. The evolution process and modification mechanism of Ce on MgAl2O4 and sulfides were clarified by experimental results and calculated by thermodynamic software. The type of inclusions in drill steel without Ce addition is MgAl2O4 and (Ca, Mn) S. As the Ce content in drill steel reaches to 0.0078% (mass fraction), the type of inclusions changes to Ce-O and Ce-S. In addition, a few complex inclusions, mixture of Ce-O and MgO, were also found. The size of inclusions in drill steel decreases significantly as the oxides and sulfides were modified into Ce-O and Ce-S. The calculated results show that MgAl2O4 and (Ca, Mn) S in drill steel can be effectively modified into Ce-O and Ce-S as the Ce added into molten steel, and the modification sequence of Ce on the MgAl2O4 is as follows: MgAl2O4 -> CeAlO3+ MgO -> Ce2O3+ MgO -> Ce2O3. The content of Ce in drill steel has great influence on the type of inclusions. The modification mechanism of Ce on MgAl2O4 calculated by Factsage 6.3 agrees well with the experimental observations.