There exist poor work hardening capacity under medium or low stress condition in conventional Hadfield steels. This poor work hardening capacity together with their low yield strength result in a serious plastic deformation in initial service. To address these two problems, a mechanism had been put forward to explain the unusual work hardening ability of conventional Hadfield steel under heavy stress or high load impact. The formation of deformation twins and its concomitant serious lattice distortion is responsible for their unusual work hardening ability due to the existence of interstitial C atoms. Based on the fact that the same effect can be produced after the formation of stress–induced " martensitic transformation, a high silicon high manganese steel Fe–17Mn–6Si–0.3C was designed. In this alloy the stress–induced " martensitic transformation easily took place under low stress. The mechanical properties and microstructure evolution of the high silicon high manganese steel and a conventional Hadfield steel were studied by OM, XRD and TEM under both static tension and dynamic impact loads. The results showed that under the tension load the high silicon high manganese steel had higher strain hardening rate than the conventional Hadfield steel. Under dynamic impact load the high silicon high manganese steel had lower impact deformation but higher surface hardness than the conventional Hadfield steel. The preferential occurrence of stress–induced " martensitic transformation accounted for this difference. This result also indirectly confirmed that the formation of deformation twins and its concomitant serious lattice distortion due to the existence of interstitial C atoms led to the unusual work hardening ability of conventional Hadfield steel.
Strain hardening behaviour and mechanism of a cold–rolled dual phase steel DP1180 under quasi–static tensile condition at a strain rate of 0.001 s−1 by electronic universal testing machine, and dynamic tensile condition at strain rates of 500 and 1750 s−1 by split Hopkinson tensile bar (SHTB) apparatus were systematically studied. According to the modified Swift true strain–stress model, the experimental data was regressed by using nonlinear fitting method, and strain hardening exponent in the modified Swift model was calculated by a modified Crussard–Jaoul method. The results revealed that there are two stage strain hardening characteristics of DP1180 steel at the strain rate range of 0.001—1750 s−1, the strain hardening ability of the stage I was enhanced with increase of strain rate, while the strain hardening ability of the stage II was weakened, and the transition strain was decreased. The ferrite near the martensite regions formed cell blocks with dislocation structures, with a size of 90 nm, due to the limit of deformation compatibility, and the existence of geometrically necessary boundary (GNB) made DP1180 steel not instantly damaged under deformation at high strain rates. In addition, the adiabatic temperature rise of ΔT=103 ℃ made martensite easy to have plastic deformation at a strain rate of 1750 s−1.
Improved mechanical properties of ferritic stainless steels (FSSs), such as toughness and high temperature or creep resistance, have been attained through the addition of stabilizing elements such as Nb and/or Ti. Therefore, stabilized ferritic stainless steels are good candidates to replace the conventional Cr–Ni austenitic stainless steels for specific applications to save the higher price of Ni. As compared to austenitic stainless steels, however, ferritic stainless steels possess lower formability which is closely depends on the γ–fiber recrystallization texture. Hence, improvement of formability is desired for further wide applications of FSSs. The stabilizing effects of alloying elements work by consuming not only the interstitial atoms in solid solution but also forming the carbide and nitride precipitates such as TiC, TiN and NbC. The precipitation takes place in steel making processes such as slab reheating, hot rolling and coiling. The parameters involving these processes have their effects on the size, shape and distribution of the precipitates that influence the γ–fiber recrystallization texture. Many papers intended to clarify the effect of precipitates. However, there were differences concerning the effect of precipitates, which may hinder further improvement of formability. In the present paper, precipitate size and dispersion were changed by controlling hot rolling processes and the effect of precipitate size and dispersion on the development of recrystallizaton texture in a 17%Cr ferritic stainless steels was investigated. Mechanical properties were measured by tensile tests. The characteristics of precipitate were observed by transmission electron microscopy, and X–ray diffraction was used to characterize texture evolution processes. The results show that low temperature finish rolling promotes the formation of a large number of fine and dispersed TiC precipitates in the hot band. After rolling and annealing, the state of fine and dispersed precipitation can be inherited in the cold rolled and annealed sheets. Strong γ–fiber recrystallizaton texture is developed in the specimen with sparsely distributed and coarse precipitates. Fine and dispersed precipitates promote the nucleation of randomly oriented grains, strongly suppress the growth of recrystallized grain, and thereby weakening γ–fiber recrystallizaton texture and impairing the formability of the cold rolled and annealed sheets. The precipitates have significant effects on the nucleation of randomly oriented grains and pinning grain boundary mobility during recrystallization annealing after cold rolling, which plays an important roles in controlling the γ–fiber recrystallizaton texture in a ferritic stainless steels.
In order to optimize the hot working technology of Incoloy 800H, the dynamic recrystallization (DRX) behavior of Incoloy 800H at temperatures ranging from 850 ℃ to 1100 ℃ and strain rates from 0.01 s-1 to 10 s-1 was investigated by single–pass compression tests on MMS–300 thermo–mechanical simulator. The evolutions of microstructure and nucleation mechanisms of DRX were analyzed combined with the technique of EBSD and TEM. The results show that when the deformation temperature is below 950 ℃, the behavior of DRX is obviously restrained by precipitation of Cr23C6 and Ti(C,N). Therefore the hot deformation constitutive equations in two temperature intervals (from 850 ℃ to 950 ℃and from 950 ℃ to 1100 ℃) were established by regression analysis with the deformation activation energy of 465.394 kJ/mol and 427.360 kJ/mol respectively. The inflection points were determined by fitting a third order polynomial to the lnθ–ε curves, which makes the prediction for the ratios of critical stress to peak stress and critical strain to peak strain more accurately. Accordingly, the mathematical models of critical stress and critical strain vs Z parameter were deduced. The DRX nucleation mechanisms of Incoloy 800H during hot deformation mainly include strain induced grain boundary migration, grain fragmentation and subgrain coalescence.
42CrMo heat–resistant steel is a kind of structural steel, which is widely used in structural components such as crane weight–on–wheel, automobile crank shaft, locomotive gear hub and so on, for its good hardening ability, high temperature strength, good creep resistance, and little quenching deformation. However, in industry application, mismatching between the strength and the toughness always occurs for 42CrMo structure components. In order to solve the problem that the strength does not match the toughness in the manufacturing process for the polar crane for the nuclear power station, the effect of tempering temperature on the morphology and distribution of carbides and the impact toughness has been investigated for steel 42CrMo in this study. The experimental results indicated that the microstructure of the quenched steel 42CrMo after 500—650 ℃ tempering was characterized by tempering sorbite. As the tempering temperature increased, the Charpy absorbed energy at –12℃ initially increased and then decreased. The flake carbides after 500 and 530 ℃ tempering are not evenly distributed on the original martensite boundaries, the Charpy absorbed energy are 26 and 44 J, respectively. While the granular carbides are evenly distributed in the microstructure after 600℃ tempering, the Charpy absorbed energy reaches a maximum value of 104 J. When the tempering temperature is higher than 600 ℃, granular carbides coarsened obviously and the Charpy absorbed energy reduced notably. The morphology and distribution of carbides is the key factor that influences the impact toughness of steel 42CrMo. Morphology and structure analysis for the carbide was carried out by TEM together with EDS analysis, the results showed that the carbide after tempering treatment is (Fe, M)3C and Fe, Cr, Mo are the main alloy element in the carbide. When the tempering temperature is in the range of 560—600 ℃, the uniformly–distributed granular carides forms on the matrix and the impact toughness is over 60 J. As the tempering temperature continues increasing, the carbides will coarsen and the impact toughness will decrease. In order to obtain the good strength and toughness matching, for 42CrMo structure, it is recommended that the tempering temperature should be in the range of 550—590 ℃.
All the parameters that may affect void closure behavior are judged using finite element method (FEM). The simulation results show that temperature, strain rate, friction coefficient, sample size and void size do not affect the void closure behavior. Height–diameter ratio of the sample and void position will affect the strain around the void and the void will be easier to close when the strain around it are higher. Of all the parameters, void shape is the most important one. Height–diameter ratio of the void is defined to describe the effect of void shape. The simulation results show that the larger height–diameter ratio of the void, the harder it is for the void to close. Based on these results and the sectioning results of a 100 t ingot, a new forging method, wide–anvil radial forging (WRF) is proposed. WRF method can concentrate the strain on the center of the ingot; meet the optimum height–diameter ratio condition of the void closure and heal shrinkage cavities effectively. Experiments on continuous casting billets prove the effectiveness and applicability of this method
The present work investigates composition characteristics of martensitic precipitation hardening stainless steels using a cluster–plus–glue–atom model. In this kind of steels based on the basic ternary Fe–Ni–Cr, the lowest solubility limit of high–temperature austenite corresponds to the cluster formula [NiFe12]Cr3, where NiFe12 is a cuboctahedron centered by Ni and surrounded by 12 Fe atoms in fcc structure and Cr serves as glue atoms. New multi–component alloys were designed by adding C, Mo, Nb and Cu into the basic [NiFe12]Cr3 with self–magnification of cluster formula and similar element substitution. These alloys were prepared by copper mould suction casting method, then solid–solution treated at 1323 K for 2 h followed by water–quenching, and finally aged 753 K for 4 h. The experimental results show that the microstructures and properties of the serial solid–solution treated and aged alloys vary with alloying elements and their contents. Among them, the {[(Ni13Cu3)Fe192](Cr45Mo2.5Nb0.5)}C1 alloy has higher microhardness and tensile strengths, the hardness is 397 HV, yield strength is 971 MPa and ultra strength is 1093 MPa after aging treatment. {[(Ni13Cu3)Fe192](Cr45Mo2.5Nb0.5)}C1 exhibits good corrosion–resistance in 3.5%NaCl solution.
Sintering is a process of bonding between solid particles which typically occurs under high temperature. Currently, simulation of sintering process is mainly concentrated on the single–phase polycrystalline materials. As there are a lot of materials which are biphasic porous system, it is of practical significance to simulate the microstructural evolution of biphasic porous system during sintering process. In this work, a new phase field model is established to simulate sintering process in biphasic porous system. The evolution of the component is governed by Cahn–Hilliard equation, while the orientation field by the time–dependent Allen–Cahn equation. A function is established to describe the relationship between atomic diffusion coefficient and grain boundary diffusion, surface diffusion and volume diffusion. A group of phenomenological coefficients are obtained by analyzing the characteristic of the phase–field model. The simulation results show that the new phase–field model can effectively simulate the sintering process in biphasic porous system. The formation and growth of sintering neck, the seal spheroidization and disappearance of pores as well as the mergence and growth of grains are observed during simulation. The sintering necks between the parent phase and the second phase grow very fast at the early stage of simulation, while at the late stage, because of the pinning effect, the growth rate of the sintering neck slows down obviously, pores become isolated by the grains, and its shape change from concave to convex, the relative small pores are eliminated, which leads to densification. As the sintering proceeds, the grain size of the second phase gradually decreases and the parent–phase grains are wrapped by the second–phase grains. Because of the pinning effect of the second phase, the migration rate of the grain boundary of the parent phase is restrained. The evolution course of pores depends largely on the interaction between the second phase and the pores. The evolution rate of pores is quantitatively compared between the biphasic porous system and the single–phase system. In the case of biphasic porous system, the evolution rate of pores is slower than that in single–phase system. The simulating growth exponents of the parent phase are calculated with different volume fractions of the second phase. As the volume fractions of the second phase increase from 15% to 25%, the grain growth exponent changes from 2.9 to 3.4.
It is well know that static recrystallization (SRX), which occurs during post–deformation annealing, is greatly affected by the deformation formed during cold working. Therefore, to investigate and predict the SRX microstructure and SRX texture numerically with high accuracy, it is necessary to simulate the SRX process taking the deformation microstructure into consideration. A model that couples the crystal plasticity finite element method and microstructure evolution model is believed to be the most promising approach for SRX microstructure design. In this paper, the subgrain structure evolution is firstly studied by using the multi–state phase field (MSPF) model coupling with the lattice deformation model including the stored energy distribution of deformed alloy. The initial subgrain growth through the mechanism of mergence and swallow during recrystallization process are simulated by MSPF. The different amount of deformation effecting on subgrain distribution and subgrain growth rate are studied systematically. The calculated results show that in the region with higher stored energy, for example, around grain boundaries, there are very dense finer subgrains which recrystallize earliestly in the higher stored energy region during recrystallization process, and grow up by mergencing and swallowing, while the distribution of subgrains inside the deformation grain is relative uniform and with relative large subgrains which grow up slowly. The distribution of grains obtained by the weighted frequency shows that the grain distribution changes from small to large grain is fast for the larger deformed alloy, while the change is slow for the less deformed alloy. All the results are agreement with experimental ones.
Serrated flow in a Ni–Co base superalloy was investigated after subsolvus and supersolvus solution treatments at the temperatures of 400 and 450 ℃ by tensile loading at different strain rates. The results suggested that the sizes of the grain and secondary γ′ phase of the alloy after supersolvus solution were larger than that of the alloy after subsolvus solution. The serrated flow exhibited inverse DSA behavior after two solution treatments, which was caused by the interaction between substitutional solutes and mobile dislocations. The variation of critical strain of serrated flow after two solution treatments may be related to different spacing between secondary γ′ and different densities of mobile dislocations during plastic deformation. The different densities of mobile dislocations and the diffusion controlled by grain boundary were responsible for the variation of stress drop of serrated flow after two solution treatments.
Ni–based single crystal (SC) superalloys are preferential materials for manufacturing blades and vanes of gas turbines, due to their superior mechanical properties resulting from the absence of grain boundaries. However, as the component geometry becomes more complex and the content of refractory elements increases gradually, the forming frequency of stray grains increases significantly leading to the component rejection during directional solidification (DS) of Ni–based SC superalloys. This becomes now one of the major problems encountered during DS and single crystal growth. In the interest of saving the actual manufacturing cost, therefore, the alloys with weak tendency of stray grain formation should be first applied. However, there are still no effective method to quantitatively evaluate the stray grain formation ability of a certain Ni–based SC superalloy. Thus, it is quite necessary to design a new method to do so. In this study, the two model samples with different platform geometries are first designed to investigate quantitatively the tendency of stray grain formation, used to summarize the ability of stray grain formation of different alloys and reveal the mechanism of stray grain formation within platforms. The first model sample with the same platform height but length increasing by degrees along the solidification direction, is used to quantitatively characterize the stray grain formation tendency of different superalloys by using its platform length of stray formation occurring first in time. The second model sample with the same platform length but height decreasing by degrees along the solidification direction, is used to quantitatively characterize this tendency by using the platform height of stray formation occurring first in time. The experimental results show that it is easier for stray grains to nucleate and grow within the platform region with either platform length increasing or platform height reducing. The stray grain formation within outside platform is prior to that within inside platform. This tendencies for the first, second and third generation SC superalloys, however, are different: the first is the weakest, following by the second, and the third is the strongest. Furthermore, the formation of stray grains is dominated by undercooling. The melt undercooling at platform edges is larger than within platform insides and increases gradually with either platform length lengthening or platform height reducing. When the undercooling at platform edges exceeds the critical nucleation undercooling, the stray grain would be able to nucleate and overgrow quickly into the platform insides.
Effects of processing techniques and alloying chemistry on the formation of solidification and homogenization micropores were investigated in this study for two single crystal nickel–based superalloys with different compositions produced by Bridgman (HRS) and LMC processes. The results show that the volume fraction of micropores in as–cast HRS alloy is lower than that in as–cast LMC alloy, and they are mainly influenced by alloy chemistry. After solution heat treatment, external and internal homogenization micropores are observed to form at the Al–depletion zone near the surface region and the internal zone away from the surface, respectively. The number of external homogenization micropores reduces with increasing the distance away from the surface. Responding to high temperature oxidizing environment (in air), the formation of such homogenization micropores is ascribed to fast outward diffusion of Al, which lead to the formation of Kirkendall voids within the Al–depletion zone. Additionally, the formation of internal homogenization micropores at high temperature is due to Kirkendall effect during the diffusion process of alloying elements between dendrite and interdendrite regions. Smaller primary dendrite arm spacing and lower level of solidification segregation in the LMC alloy results in much less internal homogenization micropores in comparison with the HRS alloy.
In the present work, a novel way has been developed to prepare the single–phase nanocrystalline Sm5Co2 alloy. The microstructure and phase constitution were characterized, and it was showed that the nanocrystalline Sm5Co2 alloy has ultra–fine and homogenous nanograin structure with an average grain size of about 10 nm. Moreover, the magnetic and mechanical properties of the Sm5Co2 nanocrystalline bulk were characterized in details, and both properties were enhanced as compared with those of the conventional polycrystalline counterparts.
The preparation of laminar metal matrix composites for immiscible alloy systems was difficult due to the positive formation heat and diffusion difficulty between each other, resulting in no metallurgy combination forming on interfaces. Aiming at this situation, a new irradiation damage alloying method for immiscible and not reacting alloy systems was presented in this work, through irradiation damages were produced by an ion implantation technology in the matrix metal first, and then the metal in the surface layer diffused into the matrix metal through the irradiation damages to form metallurgy combination with the metal. The key of this kind of metallurgy combination is to form amorphous alloy phase in the diffusion layer. By alloying the high–performance Mo/Ag laminar metal matrix composites used in spacecraft were prepared, whose maxium resistance spot welding joint tensile strength reaches 150 MPa, the average 130 MPa. The properties of thees composites exceed the national military standards and the technical requirements specified by astronic users.
The rust/metal structure is one of the multiphase and multiple interface complex systems. The corrosion under rust is the uppermost and longest form of metallic corrosion evolution process. It is difficult to accurately determine the electrochemical parameters because the existence of rust complicates the electrochemical corrosion process. Based on the result of the previous studies of quiescent seawater, the weight–loss method and different electrochemical tests such as polarization curves (PC), electrochemical impedance spectra (EIS) and linear polarization resistance (LPR) were carried out to study the corrosion behavior of A3 carbon steel immersed in flowing seawater for about 280 d. After very short immersing time, there is a thin yellow rust layer on carbon steel, but as time prolonged, the yellow corrosion products are rushed away quickly, and a tense black rust layer cover about the whole electrode. The corrosion rate obtained by weight–loss method show a steady decline and keep stable after about 84 d, but it is higher than that of the static state system data. The cathodic polarization curves show an obvious reduction current peak at about −950 mV, which makes a remarkable overestimating of the cathodic corrosion current. Polarization resistance (Rp), determined by LPR and EIS, increases during the merely short–initial immersion period, then, it decreases gradually with immersion time. This means that the corrosion rate determined by electrochemical tests shows an another pattern compared with the weight–loss result. After a very short immersion time (about 14 d), there is a remarkable deviation between the weight–loss and electrochemical test results, and the longer immersed the greater of this deviation is. So no matter in static state or flowing seawater, electrochemical methods can not get an accurate corrosion rate of carbon steel. And reliable electrochemical measurement and analysis for rusted steel need much more attention.
Stress corrosion cracking (SCC) has become one of the main threats for pipelines safe. The crack initiation is the first step for pipeline SCC. The disbonded coating affects the SCC initiation behavior. In the present paper, the effect of cathodic protection potential on SCC initiation for X70 pipeline steel in the NS4 solution beneath a disbonded coating is studied by a cyclic loading test. The SCC initiation situation was observed at the different distances beneath a disbonded coating area by SEM. The results show that the SCC initiation degree lightens with the increase of distance from the disbonded coating area under the condition of –850 mV cathodic protection potential. When the cathodic protection potential increases to –1000 mV, the SCC initiation degree slightly increases with the increase of distance from the disbonded position. But compared with that under the –850 mV, the crack initiation degree under –1000 mV is lower at the corresponding position. It is shown that the coating disbondment reduces the cathodic protection effect, and in order to obtain the same cathodic protection effect the cathodic protection potential should be appropriately moved in an negative way
Using customized wafer electroplating system, the electrodeposition process of Fe–Ni under bump metallization (UBM) thin film has been developed by modified Watts bath. The major factors which can affect the Fe content in the final UBM films, including the concentration of Fe2+, electrodeposition temperature and current density, were investigated systematically. The growth rate of Fe–Ni film under different electroplating conditions was measured in order to provide a reference for actual production. The microstructure and morphology of obtained Fe–Ni films were characterized by XRD and TEM. Multiple kinds of analytical methods including titration and inductive coupled plasma emission spectrometer (ICP) were used to monitor the content change of bath component under working or storage conditions. Regulations were put forward to maintain the bath daily including the keeping of the main salt content and the inhibition of Fe3+ concentration.
