Since quenching-partitioning-tempering (Q-P-T) process was proposed in 2007, our research group have realized the enhancement of both strength and ductility of Q-P-T martensitic steels by increasing the carbon from low content to medium content range. The recent work devoted every effort to extending carbon content to high carbon range. Based on failure of our many trials, a design idea of anti-transformation induced plasticity (anti-TRIP) effect was proposed and the composition and process of high carbon low alloying martensitic steel were designed according to the idea of anti-TRIP effect so that the strength and ductility of high carbon Q-P-T martensitic steel are higher than those of medium carbon Q-P-T martensitic steel, which fulfills the desire of investigators for a century. This paper will mainly expound the background of anti-TRIP effect, the design of composition and process of high carbon Q-P-T martensitc steel as well as its microstructure, the mechanism of high strength and ductility for high carbon Q-P-T martensitic steel, and finally analyze the principle that Q-P-T process makes the enhancement of both strength and ductility by increase of the carbon content.
In order to control physicochemical characteristics of inclusions in steel through appropriate heat treatment process, solid-state interface reaction between solid alloy deoxidized by Mn and Si and MnO-SiO2-FeO oxide during heat treatment was studied. Using confocal scanning laser microscope (CSLM) and high temperature induction furnace, the reaction between the Fe-Mn-Si alloy and MnO-SiO2-FeO oxide during heat treatment at 1473 K and its influence on the compositions and phases in the alloy and oxide were investigated by diffusion couple method. A suitable method for pre-melting oxide and producing diffusion couple of Fe-Mn-Si alloy and MnO-SiO2-FeO oxide was proposed to obtain good contact between them. After that, the diffusion couple sample with Ti foil for reducing oxygen partial pressure and bulk alloy containing the same compositions was sealed in a quartz tube for carrying out subsequent heat treatment experiment. In addition, equilibrium compositions and phases of the oxide and alloy during solidification and the solid-state reaction mechanism between them were analyzed and discussed. Quantitative analysis of each element in alloy and oxide was calibrated by standard sample before analysis. Results showed that solid-state interface reaction and element diffusion between the Fe-Mn-Si alloy and MnO-SiO2-FeO oxide were observed which indicated that the alloy and oxide in the diffusion couple was not equilibrated at 1473 K, even though the liquid phases of them were equilibrated at 1873 K. The activity of FeO in MnO-SiO2-FeO oxide decreased with the decrease of temperature and excess oxygen diffused from oxide to alloy. Mn and Si contents in the alloy were consumed by the chemical reaction and some MnO-SiO2 particles in the alloy near the interface generated. As the heat treatment time increased from 10 h to 50 h, the widths of particle precipitation zone (PPZ) and manganese depleted zone (MDZ) increased from 79 and 120 μm to 138 and 120 μm, respectively. During the heat treatment, the width of MDZ was always greater than that of PPZ. Moreover, due to the separation of the FeO, pure Fe particles formed in the oxide. The MnO and FeO contents in the oxide increased and decreased respectively with the increase of the heat treatment time.
At present, the quality of commercial non-oriented electrical steels is improved mainly by optimizing deformation and recrystallization textures, but the most desirable {100} texture for the magnetic properties of sheets is normally no more than 20% in volume fraction. Throughα→γ→α transformation, however, the percentage of {100} texture can be up to 50%, even as high as 80% or more. The characteristics of transformation microstructure in ultra-low carbon non-oriented electrical steel are basically revealed in this work, and the formation mechanisms are analyzed and discussed. The cold-rolled sheets of electrical steels are heated inγ single phase region,α→γ→α transformation occurs in hydrogen and nitrogen atmosphere, respectively. The results indicate that strong {100} texture with monolayer pancake grains is developed in hydrogen, and the size of {100} oriented grains reaches more than 1 mm; whereas near {100} and {110} textured columnar grains are formed at the surface layer of the sheets in nitrogen, and the equal-axed grains with {111} and {114} textures in the center layer are obtained finally. Σ3 grain boundaries generally appear in the transformation microstructure where grain orientations are preferred, and its formation mechanism is closely related to K-S relationship which is followed during variant selection induced by surface-effect. There is an approximate linear orientation gradient in the columnar grains at the surface of the sheet annealed in nitrogen, and this phenomenon should be resulted from the accumulation of transformation strain induced by the suppression of the growth of surface grains withγ→α transformation along the normal direction.
Cu has been adopted to replace Al for conduction lines and contact structures in very large-scale integrated circuits due to its low resistivity. However, Cu could rapidly react with the SiO2-based dielectric under 300 ℃ and form deep level impurities which are strong sink for carriers, leading to the dielectric degradation of the devices. Therefore, it is important to insert a stable barrier between the Cu wiring and SiO2-based dielectric for suppressing Cu diffusion and improving the adhesive strength. The prediction of international technology roadmap for semiconductors that the thickness of diffusion barrier would be further reduced to 3 nm for 22 nm technology node indicates the widely being used Ta/TaN barrier would be incompetent in the future, since Ta/TaN barrier at the limited thickness exhibits a high resistivity and a columnar grain structure which provides lots of vertical grain boundaries for Cu diffusion. Therefore a directly platable amorphous single barrier with low resistivity is highly desired. In this work, MoC are chosen as impurity to expect for amorphous Ru-based films. The RuMoC films with different components were deposited by RF magnetron co-sputtering with different deposition power ratios of MoC versus Ru targets. The sheet resistances, microstructures and components of the RuMoC films in RuMoC/Si and Cu/RuMoC/p-SiOC∶H/Si structures were studied. The sheet resistances, residual oxygen contents and microstructures of the RuMoC films have close correlation with the doping contents of Mo and C elements which can be easily controlled by tuning the deposition power on MoC target. When the deposition power ratio of MoC versus Ru targets was 0.5, amorphous RuMoC II film with low sheet resistance and residual oxygen content was obtained. After annealing at 500 ℃ the Mo-C and Ru-C bonds were well-preserved and co-suppressed the recrystallization of the film and the increasing of the oxygen content, contributing to excellent thermal stability and electrical properties of Cu/RuMoC II/p-SiOC∶H/Si film.
Titanium and its alloys have been widely used in the biomedical field, and have a great potential in making orthopedic implants due to their high specific strength, low elastic modulus, excellent biocompatibility and corrosion resistance in the human body environment. However, important titanium alloys currently used including extra low interstitial (ELI) Ti-6Al-4V (hereafter all in mass fraction, %), Ti-5Al-2.5Fe and Ti-6Al-7Nb are all at risk of releasing toxic Al and V ions in vivo. In addition, the elastic modulus (about 110 GPa) of these alloys are still much higher than those of cortical bones (about 20 GPa), which may bring severe ‘stress shielding’ for implantation failures. In order to solve these problems, much effort has been made to develop Al- and V-free lower-modulus β-Ti alloys. Considering that Fe is one of most effective and low-cost β-phase stabilizing element in titanium, binary Ti-Fe alloys have been selected and an assessment of the potential for biomedical applications has been conducted from the perspectives of their manufacturability, mechanical properties and biocorrosion performance. In this study, Ti-xFe (2%≤x≤20%) alloys were fabricated by powder metallurgy, and their microstructure and compression properties were characterized. In particular, the corrosion properties in four different simulated physiological electrolytes at (37±0.5) ℃ were investigated according to ASTM 59-97, compared with the performances of two commonly used titanium-based materials Ti-6Al-4V and commercially pure (CP) titanium. The results show that the content of β phase gradually increases with Fe content increasing. When Fe content goes up to 20%, the alloy samples are only composed of single β-phase grains. The PM-fabricated Ti-(2~20)Fe alloy is provided with a superior combination of mechanical properties, with the compressive strength range of 2096.2~2702.3 MPa, the compression ratio of 20.6%~33.2% and the elasticity modulus of 62.7~85.5 GPa. Higher Fe content tends to lead to the higher strength and ductility, but lower elastic modulus. In comparison, Ti-15Fe sintered at 1150 ℃ exhibits the superior mechanical properties, including the elastic modulus of 64.6 GPa, the compressive strength of 2702 MPa, and the compression rate of 32.7%. With the rise of Fe content in 2%~15%, the corrosion potential of alloys moves to a positive position, and the corrosion current density decreases, corresponding to the increase in the polarization resistance, which suggests the improvement of their corrosion properties. The binary alloy with 20%Fe possesses the similar corrosion performance to that of Ti-15Fe. The corrosion rates of Ti-15Fe alloy in simulated oral solution (FAS), phosphate buffer solution (PBS), simulated body fluid solution (SBF) and 0.9%NaCl solution (SS) are 1.7×10-3, 7.1×10-4, 1.2×10-3 and 3.5×10-4 mm/y, respectively. Compared with CP Ti and Ti-6Al-4V, Ti-15Fe alloy exhibits a more positive corrosion potential, smaller corrosion current density and higher polarization resistance, indicating a superior corrosion resistance.
Zr-Sn-Nb-Fe alloy is one of the high performance zirconium alloys used as the fuel cladding materials for high burnup fuel elements. The corrosion behavior of zirconium alloys were affected by the alloying element, the microstructure and fabricating process. To better understand the effect of Nb on the corrosion behavior of Zr-Sn-Nb-Fe alloy, Zr-xNb-0.4Sn-0.3Fe (x=0~1, mass fraction, %) sheets were prepared by thermo-mechanical processing and tested in static autoclave in 360 ℃, 18.6 MPa pure water, 360 ℃, 18.6 MPa, 0.01 mol/L LiOH aqueous solution, and 400 ℃, 10.3 MPa superheat steam. The characteristics of the microstructure were analyzed by TEM and SEM. It was shown that the corrosion weight gain of specimens was increased when x increaseed from 0 to 1 in pure water and steam. However, it was found that the corrosion weight gain reduced in LiOH aqueous solution as Nb content was increased. The microstructural characteristic indicated the addition of Nb has the effect of refining recrystallization grain of Zr-xNb-0.4Sn-0.3Fe alloy. The mean size of the precipitates in alloy were almost the same even though the Nb was considerably changed, but the area fraction of precipitates and mass ratio of Nb/Fe in precipitates of alloy were increased with the Nb content increasing when all the samples heat-treated in the same condition. The ZrFe or ZrNbFe precipitate of including small amounts of Nb was mainly formed when x was 0.2 or less, and the ZrNbFe precipitate was mainly found when the content of Nb was higher. With the increasing of corrosion rate, there are more cracks in the fracture surface of the oxide films and the size of “Cauliflower-like” structure grows bigger. It was concluded that the contents of Nb in ZrNbFe precipitates will be responsible for the difference of corrosion resistance for Zr-xNb-0.4Sn-0.3Fe alloy.
Dissimilar metal weld joints (DMWJ) widely exist in the nuclear power plants to join the different parts which are made of different structural materials. Among these DMWJs, safe-end DMWJ has attracted much attention of researchers and operating enterprises, as premature failures, mainly stress corrosion cracking failures, have occurred in these kinds of joints. However, DMWJ with 52M as filler metal in the nuclear power plants has no in-service experience. To ensure the structural integrity of the weld joint and the safe operation of the future plants, the microstructure and local properties of a domestic safe-end DMWJ by using hot-wire gas tungsten arc welding (GTAW) technology was studied in detail by OM, SEM, micro-hardness testing, local mechanical tensile testing and slow strain rate tests. The tensile tests were performed at room temperature with the tensile speed of 5 μm/s while the slow strain rate tests were conducted in simulated primary water containing 1500 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH with 2 mg/L dissolved oxygen at 325 ℃. A large amount of type I boundaries and type II boundaries which are susceptible to stress corrosion cracking (SCC) exist in 52Mb near the SA508/52Mb interface and result in the highest SCC susceptibility of this interface. Microstructure transition was found in the SA508 heat affected zone (HAZ). In 316LN HAZ, increasing the distance from the fusion boundary, the number fraction of CSL boundaries increase while the residual strain decreases, resulting in the second-highest SCC susceptibility of 316LN HAZ. In 52M, residual strain distributes randomly but not uniformly, the residual strain is prone to accumulate at the grain boundaries. Dramatic changes of mechanical properties are observed across the joint, especially at the SA508/52M interface. The differences of the local microstructure and chemical composition lead to the differences of the local properties of the weld joint.
Tungsten is considered as the most promising candidate for plasma-facing materials in future nuclear fusion reactors. The damage behaviors of tungsten under different He irradiation are one of the main issue of concerns. In this work, the evolution of helium-related defect in polycrystalline tungsten was studied by slow positron beam analysis (SPBA) and SEM as functions of annealing temperature and implantation fluence. The results show that the number of vacancy-type defect induced by the multi-energy He irradiation increases with the increment of irradiation fluence. At the meantime, annealing at the temperature of 220 ℃ induces the recombination of interstitial W atoms with vacancies, thus reduces the number of the vacancy-type defects in the sample. And annealing at 450 and 650 ℃ leads to the formation of He bubbles in the tungsten materials, and the size of He bubbles in tungsten is related to the annealing temperature, and the He bubbles and holes with a diameter of about 600 nm could be observed for the specimen annealing at 650 ℃.
This decade has brought an immense interest in room temperature magnetic refrigeration, because it is considered as a type of potential energy saving material and friendly to environment. MnFe(P, Si) magnetic refrigerants materials shows high-performance and relatively low-cost, which paves the effective way for commercialization of magnetic refrigeration and magnetocaloric power-conversion. The present work is devoted to investigating the effect of Sn addition on microstructure, magnetism and magnetocaloric effect on MnFe(P, Si) alloy. Mn1.3Fe0.7P0.5Si0.5-xSnx (x=0, 0.02, 0.04, atomic fraction) alloys were prepared by mechanical alloying (MA) and solid-state reaction methods. The results show that Sn atoms do not enter into the lattice position of Fe2P. The Sn2(Mn, Fe), (Fe, Mn)3Si, Si-riche (Fe, Mn)2(P, Si) and P-riche (Fe, Mn)2(P, Si) matrix phase are formed in Sn-containing alloys. Two different compositions of (Fe, Mn)2(P, Si) phase result in two ferromagnetic-paramagnetic phase transition and two magnetic entropy change (-ΔSm) peaks at the heating process. This result is in favor of expanding the working temperature and refrigerant capacity (RC) of magnetic refrigeration materials. Mn1.3Fe0.7P0.5Si0.5 alloy shows a maximal magnetic-entropy changes (-ΔSmax) of 12.1 J/(kgK) in a magnetic field change of 0~1.5 T, a maximal adiabatic temperature change (ΔTad) of 2.4 K in a magnetic field change of 0~1.48 T and a thermal hysteresis (ΔThys) of 3 K in vicinity of Curie temperature of 273 K, which can be used as a promising candidate material for room-temperature magnetic refrigeration applications.
Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) have attracted great attention for more than two decades, due to their large magnetic shape memory effect that originates from the rearrangement of martensitic variants under an external magnetic field. Over the past decade, accumulated knowledge on the properties of Ni-Mn-Ga Heusler alloys has allowed people to foresee the possibility of employing these alloys in device applications. However, the low operating temperatures and high brittleness remain the major drawbacks for the industrial application. Consequently, there has been growing interest in the modification of Ni-Mn-Ga alloys by adding a fourth element to increase transformation temperatures and to improve ductility. A recent study shows that the ductility has been effectively improved in Cu-doped Ni-Mn-Ga alloy under the situation of single phase via strengthening grain boundaries. In addition, the crystal structure, martensitic transformation, magnetic properties, high temperature magnetoplasticity and magnetocaloric effect have been reported in Ni-Mn-Ga-Cu alloys. Experimental results have shown that the martensitic transformation temperature (Tm) is drastically increased and the Curie temperature (TC) slightly decreased with Cu addition. As already known, the alloying elements affect both the crystal and electronic structures and hence the stability of austenite and martensite phases. Therefore, knowledge of the effects of Cu addition is of great importance to understand the composition dependence of Tm and TC. First-principles calculation results on Ni8Mn4-xGa4Cux (x=0, 0.5, 1, 1.5 and 2) ferromagnetic shape memory alloys of this research draw following conclusions. The added Cu atom preferentially occupies the Mn site. The formation energy results indicate that ferromagnetic austenite is more stable than the paramagnetic one. The ferromagnetic state becomes instable and paramagnetic state becomes more stable when Mn is gradual substituted by Cu. The evaluated TC decreases with increasing Cu content that is derived from the decrease of total energy difference between the paramagnetic and the ferromagnetic austenite. The experimentally observed decrease of Tm is originated from the decrease of total energy difference between the austenite and the non-modulated martensite. The difference between the up and down DOS is reduced with the increasing Cu content that gives rise to the decrease of the total magnetic moments. The purpose of this work is to explore the influence of Cu addition on crystal structure, Tm, TC and electronic structures of Ni8Mn4-xGa4Cux (x=0, 0.5, 1, 1.5 and 2) alloys by first-principles calculations, aiming at providing theoretical data and directions for developing high performance FSMAs.
Fe-Ga alloys are attractive magnetostrictive materials due to large magnetostriction along <100> direction and high mechanical strength. However, sharp Goss ({110}<001>) texture and large magnetostriction coefficients were conventionally achieved by secondary recrystallization with centimeter-sized grains under the effects of inhibitor and surface energy, resulting in deteriorated mechanical properties. Texture optimization in relatively fine grained microstructure is an effective way to obtain excellent comprehensive properties. Cold rolling process can determine the difference in number and size of primary recrystallization grains among various texture components, and further influence the texture and grain size evolution during subsequent high temperature annealing. The present work aims to produce strong η texture (<001>//RD, rolling direction) in binary Fe-Ga sheet with relatively fine recrystallization grains by cold rolling parameter modification. Macro- and micro-texture analysis was applied to investigate the effects of primary recrystallization states on texture and grain size evolution during high temperature annealing in binary Fe-Ga sheet. The η grains can gain more numbers and relatively larger sizes in primary recrystallization stage, and preferably grow even abnormally during high temperature annealing. A sharp η texture and large magnetostriction coefficient are successfully developed in primarily and secondarily recrystallized sheets with relatively fine grains. The results provide a prospective route for the efficient recrystallization texture and grain size optimization in binary Fe-Ga and other bcc alloys.
Incoloy800H is a kind of corrosion-resistant Ni-Cr-Fe base alloy with wide application in industrial fields. Vertical continuous casting process was developed to replace conventional mould casting process to increase product rate and decrease energy consumption. However, seriously internal quality issues of the continuously cast Incoloy800H alloy have been revealed. In this work, the square billet of Incoloy800H alloy, whose cross-sectional size were 100 mm×100 mm, were successfully fabricated in vertical continuous casting process with and without electromagnetic field (EMF), and the solidification structure, TiN inclusion distribution and internal crack were investigated. The result showed, without EMF, the Incoloy800H alloy billet had some seriously internal quality issues like coarse column grains, internal cracks and large TiN inclusion. With EMF, The equiaxed grain ratio of Incoloy800H alloy billet increased from 2.45% to 41.45%, the equiaxed grain size decreased from 10.83 mm to 1.28 mm and internal cracks were eliminated. TiN is a kind of detrimental inclusion in Incoloy800H alloy billet, which can act as stress concentration sites to form cracking. Most of TiN inclusions were located at interdendritic area and formed into TiN cluster to block interdendritic feeding channel. The application of EMF reduced the number of TiN inclusion from 3.71×10-4 μm-2 to 1.59×10-4 μm-2 in the center of billet. Further analysis illustrated that the EMF can refine the equiaxed grain size, reduce the degree of element segregation and the number of large TiN inclusion, which can reduce the probability of the crack initiation and inhibit the formation of TiN cluster to enhance the interdendritic feeding, thereby remarkably reduce the internal crack in Incoloy800H alloy billet.
A strongly basal textured AZ31 magnesium alloy sheet with the normal direction (ND) perpendicular to the c-axis has been cryorolled at the liquid-nitrogen temperature to the strain of different amount to analyze the influence of cryogenic rolling temperature. The microstructure and texture of the cryorolled samples have been investigated by using SEM, EBSD and XRD. And the mechanical properties of the cryorolled sheets have also been tested under quasi-static uni-axial tension at the ambient temperature along the rolling direction (RD) and transverse direction (TD) respectively. The microstructural and textual evolutions of the strongly basal textured AZ31 magnesium alloy sheets during cryorolling and the relationship between mechanical properties and the microstructural and textural evolutions of cryorolled samples has also been discussed in this work. The results show that a lot of twins have been observed in cryorolled sheets, and they were found to be {101?2} tension twins. {101?2} tension twins were the dominant twinning type of the AZ31 magnesium alloy sheet during cryogenic rolling. With the increase of cryogenic rolling pass, new texture component with the c-axis paralleled to the normal direction (ND) was strengthened and the breadth of {101?2} tension twins was also increased. Grains were separated by the twin grain boundaries after cryorolling. The mechanical test results show that the strength of the sheets increased while the ductility decreased after cryogenic rolling. The strength of the sheets along RD was higher than that along TD.
Automobile lightweight can effectively save fuel consumption and reduce CO2 emissions. Aluminum and its alloys are desirable for the automotive industry due to their excellent high-strength to weight ratio. However, due to the introduction of the welding seam, it has brought new changes to the forming process, especially to the forming limit. To establish a reasonable forming limit curve (FLC) analysis method of friction stir welding (FSW) aluminum alloy blank, a new theoretical model was proposed based on the new second order function constitutive model. The main idea is using the differences in mechanical property between the welding and heat affected zone substitution for the hypothesis of geometry groove in the classic M-K theoretical model. The new second order function constitutive model was applied to M-K theoretical model. Eventually, a new FLC theoretical model for FSW aluminum alloy blank was established. Such theoretical model also overcomes the low strain hardening exponent of aluminum alloy material, which leads to a poor regression accuracy by power-exponent function model. The forming limit test for FSW aluminum alloy blank was performed, and the real-time strain was measured by three-dimensional digital speckle strain measurement system (XJTUDIC). Finally, the results of experiments and the theoretical analysis are compared. Compared with the traditional power law, the regression result of the new second order function constitutive model on the stress-strain curve no matter in the initial yield stage or in late deformation stage has a good fitting precision. The maximum fitting error of the power law on the stress-strain curve is more than 12%, but the fitting error of the new second order function constitutive model is less than 1%. The theoretical prediction based on the new second order function constitutive model is significantly better than the theoretical predictions based on power law in predicting the forming limit of FSW aluminum alloy blank. The prediction error of the first principal strain based on the new second order function constitutive model is less than 0.01. While the maximum prediction error of the first principal strain based on the power law is 0.14.
Nanometer precipitation is of great importance to the mechanical properties of the low carbon micro-alloyed steel. Precipitation process is controlled by the driving force for precipitation and the diffusion rate of atoms. Under the influence of these two factors, the fastest precipitation temperature for (Mx1Mv2M1-x-v3)(CyN1-y) phase is available, which is also known as nose temperature. The maximum number density of precipitates can be obtained through isothermal treatment at the nose temperature. The most effective tool for getting the value of nose temperature is the precipitation-temperature-time (PTT) curve. Due to that the diffusivity of substitutional atom is several orders of magnitude smaller than that of interstitial atom, the nucleation process and growth process of complex precipitation are controlled by the diffusion of substitutional atoms. So far no model has been established for calculating PTT curve of complex precipitation. All the existing models are established for simple precipitation. In this work, a kinetic model, based on the classical nucleation and growth theories and Johnson-Mehl-Avrami equation, employing Adrian thermodynamic model and L-J model, using average diffusivity to demonstrate the effects of forming elements on precipitation process, has been adapted to describe the precipitation kinetics following high temperature deformation in micro-alloy steels alloying with V, Nb and Ti. Using this model, the PTT curves for the kinetics of second phase were easily obtained. In the experiment, within the temperature range from 660 to 540 ℃, the nose temperature of carbonitride precipitation is equal to or slightly higher than 620 ℃. The value of nose temperature estimated from PTT curve is 628 ℃ which is consistent with the experimental observation. There are enough reasons to believe that the model proposed in this work can estimate accurately the nose temperature information in relatively small experiment case. This model has outstanding advantages in comparison with existing models: the mole fraction of precipitation and the driving force for precipitation per unit volume ?Gv can be calculated directly without calculating the solubility formula of complex carbide in matrix; The proposed model can also be used to calculate the absolute solution temperature and the constituent of initial complex precipitation forming at critical temperature of precipitation, which can be used as the iterative initial values for calculating the equilibrium information between matrix and precipitation at relatively low temperatures.
