Columnar grains show their special characteristics of morphological and crystallographic anisotropies, and thus markedly influence the microstructure and texture evolution during rolling and annealing process in electrical steel. The rolling and annealing microstructure and texture of three columnar grained samples with the long axes arranged along different directions were investigated by means of XRD and EBSD techniques, and the effects of columnar grain boundaries were analyzed from the view point of geometry-induced interaction and orientation-induced interaction. The results indicated that, prominent microstructure and texture gradients caused by the surface shearing during hot rolling inherited to subsequent cold rolling and annealing in columnar grained samples. The difference in morphological anisotropy of initial columnar grain boundaries in the three samples were eliminated after hot rolling, while a same type structure of anisotropic grain boundaries was formed. However, the crystallographic anisotropies of the samples were changed before cold rolling, and it caused that the evolution of the microstructure and texture during subsequent cold rolling and annealing was different with directly cold rolling process in previous work. This feature produced a graded microstructure and texture relationship between hot rolled samples and finally annealed samples. In this work, it mainly focused on the {100}-oriented regions at grain boundaries, because the {100} texture was most beneficial to the magnetic property of electrical steel.
Nano-crystalline (<100 nm) and ultrafine grained (100~500 nm) materials have high strength and toughness, but its work hardening ability and uniform elongation decreased relative to the coarse grained material. Through the deformation, phase transformation and recrystallisation combination mode of development of bimodal grain size distribution of ferrite, bainite steel, the elongation rate is greatly improved. These studies are generally in order to improve the mechanical properties of material through change microstructure, but lack of study for the bimodal grain size distribution formation mechanism. This research work by cold rolling with annealing at 820~870 ℃, in 316L austenitic stainless steel to achieve micro (3~5 μm) and sub-micro (300~500 nm) bimodal grain size distribution. In the austenite deformation process, deformation twinning and strain induced martensite transformation occurred in large deformation stage. Accordingly inferred austenite deformation twinning is the micro mechanism of strain induced martensite. Annealing at 820~870 ℃, the hardness of the samples and the grain size distribution remains nearly constant. Through the comparative analysis of induced martensite austenite evolution driving force and strain deformation during annealing, determined the source of bimodal grain size distribution. The micro scale grains came from the recrystallization of deformed austenite in the cold deformation does not change, and sub-micron grain size is mainly composed of strain induced martensite reverse transformation.
As a kind of clean, efficient and relatively safe energy, nuclear energy has been widely used around the world. The high-level radioactive waste (HLRW) generated in the nuclear has also become a major risk, so the disposal safety of HLRW will be especially important. The planned concept of China's HLRW disposal program is a shaft-tunnel model located in saturated zones in granite. The metal container for sealing the HLRW is the key because its interaction with the ground water will lead to the leak of the HLRW during the long repository time. Beishan is a selected repository area and the ground water contains a bicarbonate (H C O 3 - ) buffer solution. Therefore, as a candidate material of the container, the active/passive state of low carbon steel in the ground water with is of significance, which determines the container's service life. The active state will ensure that the container achieves the designed life under general corrosion, and moreover the passive state will degrade the container's life under stress corrosion cracking (SCC) caused by pitting corrosion. In this work, the effect of H C O 3 - on the corrosion behavior of low carbon steel was examined in deaerated bicarbonate solutions (pH 8.3) over 50 d. The presence of H C O 3 - enhanced both the anodic Fe dissolution and cathodic hydrogen evolution reaction. The situ-measurement of corrosion potential revealed that the increased concentration of H C O 3 - led to the high corrosion potential. When the concentration of H C O 3 - was 0.01 mol/L, the corrosion potential was in the active region. When the concentration of H C O 3 - was higher than 0.02 mol/L, the corrosion potential was in the passive region. EIS results showed that the charge transfer resistance, film resistance and the diffusion impedance increased with the increasing H C O 3 - concentration. Results of XRD analysis illustrated that the key corrosion products were mainly composed of Fe3O4 and α-FeOOH.
Mandrel is an important tool for thermal deformation of the seamless steel tube rolling unit. It requires high heat resistance and toughness due to its application in the harsh environment. H13 steel is commonly used as mandrel materials with excellent comprehensive performance. It is reported that addition of carbide-forming elements, such as Nb, Ti, or Zr, especially the Nb element, can break the dendritic microstructure and refine the cast structure of H13 steel. In addition, Nb can act as a strong carbide-forming element to favor the formation of MC carbide. This stable carbide has low solubility and does not dissolve in austenite even at high temperature, and hence fines austenite grain by pinning effect of carbide on grain boundary. As the stable NbC has stronger ability to improve the fatigue resistance and abrasion resistance than Mo6C and VC, the mandrel steel can be produced by the method of Nb addition. It has been reported that the addition of Nb in H13 can successfully increase heat resistance. Nb element dissolves into the matrix after quenching and tempering, and precipitates in the form of NbC after heat preservation for a long time, and eventually improves the resistance of material to temper softening. However, it has not been widely applied in the production because the primary carbides of NbC can seriously deteriorate toughness of steel. The purpose of the work is to analyze the effect of addition of 0.06%Nb (mass fraction) on segregation, primary carbides and toughness of large size H13 mandrel steel. The different segregation, primary carbides, structure between large size H13 and H13-Nb mandrel were investigated by employing methods of OM, SEM, EDS and EBSD, and the mechanical properties including the hardness and impact toughness were measured at room temperature. The results show that addition of 0.06%Nb aggravates segregation compared with H13. Nb increases the precipitation temperature of MC-primary carbides, and changes the type of MC-primary carbides from mainly VC to mainly (Nb, V)C which easily induces gravitational segregation of H13-Nb. The severe segregation leads to unfavorable structure of the large and nonhomogeneous effective grain size (EGS) of annealed H13-Nb, and the primary carbides do not decrease or change significantly after quenching and tempering. In the impact test, the zone of the chain-shaped carbides gathering is prone to cracking and generates horizontal stripes, resulting in low toughness.
In the continuous pursuit of miniaturization, multifunction and high-reliability of electronic products and devices, the packing density has been increasing and the dimension of solder joints has been scaling down. In electronic packaging, during the soldering process being employed to Sn-based solders, an intermetallic compound (IMC) layer is formed between molten solder and pad (or under bump metallization, UBM), whose morphology and thickness as well as growth kinetics play an important role in controlling the service performance of the solder joints, in particular for solder interconnects with the decreasing size where the interfacial IMC layer takes up a high volume fraction in the solder joint. Thus, characterizing the morphology change and growth kinetics of interfacial IMC layer is very important to optimize the soldering process and evaluate the reliability of solder interconnects. In this study, a multi-phase-field model is applied to intensively account for the effect of grain boundary diffusion coefficient ( D G B ) and IMC/liquid interfacial energy σ ηL on the morphology evolution and and growth kinetics of IMC. The simulation results show that Cu6Sn5 grains grow up and contact with each other exhibiting a scallop-like morphology which can be influenced by both the grain boundary diffusion coefficient and IMC/liquid interfacial energy. The IMC growth process exhibits three stages, including the initial stage associated with Cu6Sn5 grain broadening followed by the transition stage characterized by scallop shape formation and the last normal growth stage dominated by IMC layer thickening and concurrent Cu6Sn5 grain coarsening. It is also found that the IMC layer thickness increases with grain boundary diffusion coefficient but decreases with IMC/liquid interfacial energy, while the scallop average width decreases with grain boundary diffusion coefficient and increases with IMC/liquid interfacial energy. The relationships between IMC layer thickness/width and reaction time can be well fitted by an exponential growth law, in which the large grain boundary diffusion coefficient combined with σ G B = 2 σ ηL (where σ GB is the grain boundary energy) can produce precise growth exponent closing to that in the ideal solid-liquid interface reaction.
The nanocrystalline Ni thin films with high density nano-scale growth twins were synthesized by direct electrodeposition technology. The five-fold twinning structure in electrodeposited nano-twin Ni was systematically investigated by TEM. The remarkable diffraction pattern and HRTEM images obtained from the cross-section observation demonstrate directly that the electrodeposited nano-twin Ni has five-fold twinning structure with five {111} twinned subcrystals and systematically analyzed the 7.35° intrinsic structural gap. In this work, the 7.35°gap was at least inset in two twin boundaries of the five-fold twin, the twin boundaries which share the 7.35° gap always broaden and was decomposed into other twins, so that, the grain present a irregular shape. cross-sectional TEM micrograph revealed that the electrodeposited nano-twin Ni had columnar grain structure with a strong {110} texture. By means of comprehensive structure characterization, a new space structural model of the five-fold twin was proposed.
Increasing the steam temperature and pressure of boilers in super-ultracritical power plant is an important approach to increase the plant efficiency. The steam temperature of the most efficient coal power plant is now around 620 ℃, representing an increase of about 80 ℃ in the past 40 years, which owes to the high temperature properties improvement of boiler components, such as the superheater and the reheater. Nickel base superalloy, for example Inconel 740 and Inconel 617, is being developed by some countries for the material requirement of 700 ℃ super-ultracritical power plants. Meanwhile, weldability investigation is necessary for the developing materials since welding plays a key role on the construction of coal power plant boilers. In this work, the weldability of a kind of Ni-Fe base superalloy, one of the candidate materials for the high temperature components of 700 ℃ ultra-supercritical coal plant is studied. By welding thermal simulator (Gleeble 1500) experiments, the variation and evolution of mechanical properties and microstructure were analyzed for this Ni-Fe base superalloy, under welding thermal cycle treatment condition and aging treatment condition after welding thermal cycle. After the welding thermal cycle with a peak temperature of 1249 ℃, both the yield strength and tensile strength for solutioned Ni-Fe base superalloy at 25 and 700 ℃ were decreased, along with the increasing of ductility. After aging treatment to the Ni-Fe base superalloy experienced a welding thermal cycle, the yield strength and tensile strength at 25 ℃ were similar with those of the aged base metal. At 700 ℃, the strength of the heat affected zone (HAZ) after aging treatment is higher than that of the aged Ni-Fe base superalloy. Microstructure analysis showed that the γ' phase and MC carbide in Ni-Fe base superalloy dissolved during the high temperature welding thermal simulation experimental process. The solution of carbides in the grain boundaries caused a loss of a pinning effect on the migration of grain boundary and a decreasing of the strength. After the aging treatment to the Ni-Fe base superalloy experienced a high temperature welding thermal cycle, γ' and M23C6 carbide were precipitated. The precipitation of M23C6 at the grain boundaries during aging treatment was mainly due to the supply of the carbon from the MC which had been dissolved partially during former welding thermal cycle.
Using Inconel 625 wire to weld high yield strength steels or stainless steels that commonly used in nuclear power plant components and gas turbines can significantly improve high temperature mechanical properties and corrosion resistance of weld structure. However, toughness, fatigue strength and creep rupture strength of weld would decline obviously because of the precipitation of δ phase during service at elevated temperatures for a long time. This work aims to investigate nucleation mechanism of δ phase in Inconel 625 deposited metal by means of SEM and TEM. Meanwhile, coarsening inherent law of δ phase during post-weld heat treatment (PWHT) at 850 ℃ for 2, 4 and 8 h respectively was revealed. The results indicate that a large number of needle-like δ phase precipitates in Inconel 625 deposited metal after PWHT at 850 ℃. These δ phases appear a grid-like distribution in γ-matrix, and there are some poor γ" phase regions appearing near δ phase. Formation process of δ phase is a solid phase transformation process which is like bainite transformation in steels. Crystal nucleus of δ phase form in the close-packed plane of γ" phase by shear mode, and coarsening behavior of δ phase is a diffusion-controlled growth process. When PWHT holding time is shorter, actual average size of δ phase is in line with LSW theory. With PWHT holding time extending, its actual average size deviates from the predicted value of classical LSW theory, because of the high-density and non-directional precipitation characteristics of δ phase.
以Ti-52%Al合金为研究对象, 在Bridgman定向凝固炉中进行不同时间的热稳定处理实验及相关的定向凝固实验, 研究了定向凝固前糊状区的形成及其微观组织演化、界面形态和溶质分布, 并考察了热稳定处理对定向凝固组织的影响. 结果表明, 试样熔化后形成了由α相晶粒、晶粒间富Al液滴和液相通道组成的糊状区, 糊状区前沿为完全液相区. 随着热稳定处理时间的增加, 糊状区内α晶粒逐渐融合且沿温度梯度方向定向排布, 富Al液滴和液相通道逐渐消失, 固/液界面变得光滑平整. 热稳定处理时间足够长时, 糊状区内为单一的α相, 溶质Al的成分在生长方向近似沿着α固相线变化. 根据溶质守恒计算得出完全液相区内溶质Al含量为52.21%, 高于试样原始成分, 与实验测量结果相一致. 进一步研究热稳定处理对后续定向凝固组织的影响, 发现合理的热稳定处理时间(≥30 min)确保了定向凝固启动界面处存在稳定的温度场和浓度场, 柱状晶沿生长方向能够平行并列生长, 试样具有良好的定向效果.
The ability to form external chromia scales on binary Cu-Cr alloys with very small mutual solubility of the two components is strongly increased either by increasing Cr content or by preparing alloys with a very small grain size. The purpose of the present work is to mainly examine the effect of Cr content and especially the influence of the size of the second phase. Equal channel angular pressing (ECAP) was carried out for the grain refinement because it can often provide significant inner deformation and very fine grains. The oxidation behavior of binary Cu-Cr alloys with different nominal Cr contents (Cu-0.5Cr, Cu-7.0Cr and Cu-15.0Cr, atomic fraction, %) was investigated in air at 700 and 800 ℃. At the same time, the oxidation of grain-refined Cu-7.0Cr alloy was compared with the same casting alloy with a normal grain size in order to further reveal the effect of the grain refinement on the oxidation. The oxidation kinetics of all alloys followed the parabolic law. Oxidation of Cu-0.5Cr alloy was basically similar to that of pure Cu and its scales are mainly composed of copper oxides containing a small amount of chromia particles dispersed in the inner layer, even close to the scale/alloy interface. The oxide scales formed on the Cu-7.0Cr and Cu-15.0Cr alloys were complex and were consisted in most cases of the outer layer of CuO and Cu2O plus inner layer of mixed oxides of chromia and double Cu-Cr oxide of Cu2O·Cr2O3, leaving unoxidized Cr particles surrounded by chromia in the scales. Cr depletion was also observed in the alloy. The grain-refined Cu-Cr alloy easily formed more chromia with much lower oxidation rate. The oxidation rate of Cu-Cr alloys decreased considerably with increasing Cr content and reduction in size of β phase is favorable for improvement of anti-oxidation of Cu-Cr alloys. The result indicates that the alloy microstructure affects the oxidation behavior because microcrystalline structures provide numerous diffusion path for reactive Cr component, shorter diffusion distance and rapid dissolution of Cr-riched second phase. All of these favor the formation of the stable chromia. Therefore, it can be deduced that the growth law and microstructure of the oxide scales for the binary alloy are closely related to the reactive component contents, original microstructure, the size and spatial distribution of β phase in Cu-Cr alloys.
Modern aero and power industry needs high-performance gas turbine. Directional solidification (DS) columnar grain and single crystal (SX) blade as key parts of gas turbine serve in heavy stress and high temperature conditions. The DS and SX blade are mainly produced by high rapid solidification (HRS) method, and HRS is one of useful DS technology, which has a property that the heat dissipating ways are changing during the process and the temperature gradients will vary correspondingly. The dendrite grain arrays were the substructure of a DS or SX blade. The structure of the dendrite grain arrays influences the mechanical property of the final casting very much, but is seriously affected by the solidification parameters, such as temperature gradient. In this work, the dendrite grain growth of DD6 superalloy was studied based on cellular automaton-finite difference (CA-FD) model concerning the HRS method's macro solidification parameters. Mathematic models for dendrite grain growth controlled by temperature field and solute field were built to describe the competitive growth and morphology evolution of dendrite grains. Then the dendrite calculation model was coupled with the models of DS process calculation, and some HRS solidification parameters were included, such as withdrawal rate, pouring temperature, etc. The coupled models were used to predict the dendrite grain competitive growth of DD6 superalloy during the DS process. The variation of solute distribution and the dynamic adjustment of dendritic spacing during the process could be predicted by simulating calculation. The DS experiment was carried out with a cylinder sample, and dendrite grains' distribution in the transverse and longitude section was observed by OM and SEM. Then the simulated dendritic morphology was compared with that by experiment. The primary and secondary dendritic spacing by experiment and simulation were measured, and the compared results revealed that as the DS process going on the temperature gradient decreased gradually and the primary dendritic spacing was increasing. So simulation results of the DS dendritic competitive growth were validated by the experiment results, and the proposed models could predict the dendrite grain morphology and the adjustments of DS dendritic spacing of DD6 superalloy very well.
Age-hardening effect is considerably strong in magnesium alloys containing Nd, making it possible to develop magnesium alloys with low cost and high strength. Although there have been massive researches about the precipitation product sequence and strengthening models in magnesium, aluminum and other light alloys during their ageing processes, those of NZ30K-Mg alloy, a newly-developed magnesium alloy, has not been carefully investigated. The present work mainly focuses on the model of precipitation kinetics and strengthening of NZ30K-Mg alloy. The precipitation kinetics has been investigated using electrical resistivity testing during continuous heating with different heating rates and formulated based on the isoconversional method. Two related model parameters, modified pre-exponential factor A ' α r and activation energy E α r were respectively determined. The precipitation behavior of NZ30K-Mg alloy during ageing processes can also be intrinsically explained from the variations of E α r and ln A ' α r with the precipitation fraction. This kinetics model with two above-mentioned parameters can accurately describe the precipitation of strengthening phase during different ageing processes. The yield strength of under-aged and peak-aged NZ30K-Mg alloy have been tested and the results show that the testing samples isothermally aged at different temperature from 180 to 250 ℃ have almost the same peak yield strength of about 150 MPa, indicating that the strengthening effect of under-aged and peak-aged NZ30K-Mg alloy is only determined by the precipitation fraction within a certain range of temperatures. The precipitation strengthening model of NZ30K-Mg alloy has been carefully derived, and the parameter C in the model has then been determined by least squares method based on the tested yield strength data. The value of C is about 93 MPa. The prediction of yield strength of under-aged and peak-aged NZ30K-Mg alloy has been performed and fit well with the tested ones, demonstrating the effectiveness of precipitation strengthening model and its engineering application prospects.
In order to alter the overall properties of composites, the reinforcement coatings are commonly implemented to improve wetting behavior and prevent interfacial reaction. Unfortunately, few researches were emphasized on the effects of the sintering temperature of whisker preform (STWP) on the damping behavior of composites, especially whisker with coatings. In the present investigation, Al18B4O33 whisker was coated with Bi(OH)3 by a chemical method. The whisker preform was sintered at the different temperature. The coated whisker-reinforced aluminum matrix composites were fabricated through squeeze casting technique. The damping properties of the coated composites with the different STWP were presented and discussed. The results indicated that the microstructures of coatings on the whisker surfaces and at the interface in the coated composites are strongly dependent on STWP. There are two damping peaks in the coated composites (related to dislocation damping and interface damping), when STWP is 530 and 830 ℃, respectively. Only one interface damping peak occur in the coated composite when STWP is 1000 ℃. When STWP is 830 ℃, the highest damping capacity is obtained in the coated composite, which relate to a special interfacial structure.
From the point of view of the application, the service life of component usually relies on the wear resistance of local region, and it is desirable that the local region of component rather than the whole component is reinforced by ceramic particulate to offer high-wear resistance. In this study, the TiC-TiB2 particulates locally reinforced steel matrix composites were fabricated by an SHS-casing route using an Al-Ti-B4C system. The effects of the Al content on the microstructure and wear resistance of the composites were investigated. The results show that the TiC and TiB2 particulates were formed in all the preforms with various Al contents (0~50%, mass fraction) after the high temperature (about 1873 K ) steel melt was poured into the mold and the molten steel, to the different extents, penetrates into the synthesized samples. The Al content in the preforms has a large effect on the constitute of the synthesized products and the quantity, size and distribution of the ceramic phases in both the reinforced region and the transition region. With the increase of the Al content, the quantity and average size of the ceramic particles as well as holes decrease, the type and quantity of the intermetallic compounds in the products increase and the gradient distribution of the ceramic particles in the transition region weakens and finally disappears. The wear resistance of the locally reinforced composites is much superior to that of the unreinforced steel matrix, and the best appears in the sample free of Al composition, and then followed by the samples of 30%Al, 10%Al and 50%Al in turn.
Due to a serious shortage of natural fresh water in many areas all over the world, the seawater desalination has emerged as an effective compensation way to meet the consumption requirements. Due to the good corrosion resistance and low cost, stainless steels have been used extensively to construct the multi effect distillation (MED) plants, especially type 316L stainless steel for the evaporation chambers. However, with the application and development of low temperature MED, there is increasingly need of higher temperature distillation and higher brine concentration in the desalinators to reduce the drainage of hot brine and increase the water production ratio, which may cause more serious corrosion on the stainless steel components in the plants. Pitting corrosion of 316L stainless steel was studied in the concentrated environments of seawater with different temperatures (25, 50, 63, 72, 85 and 95 ℃) and concentration ratios (1, 1.5, 2, 2.5 and 3 times) by using cyclic anodic polarization measurement and SEM surface observation. The results show that both pitting potential and repassivation potential of 316L stainless steel decrease linearly with temperature in the concentration ratio range of 1 to 3 times for seawater, but the change of pitting potential is very slight when the solution temperature is higher than 85 ℃ in the case of concentration ratio larger than 2 times. Both pitting potential and repassivation potential reduce linearly with the logarithm of the concentration ratio of seawater in the range of 25 to 95 ℃. It is apparent that increasing temperature and concentration ratio of seawater will deteriorate the pitting resistance of 316L stainless steel noticeably. The influence of temperature and concentration ratio is analyzed on the basis of the point defect model. Nevertheless, the concentration ratio of seawater has a weaker influence on pitting resistance of 316L stainless steel in comparison with temperature as revealed by the pitting potential changes resulted from the concentration ratio around 1.5 times and solution temperature around 72 ℃. Therefore, compared with temperature, the corrosion resistance of 316L stainless steel for low temperature MED plants may be relatively tolerant of the adjustment or fluctuation of seawater concentration.
The microstructures and mechanical properties of hypoeutectoid U-Nb alloy laser welded joint were investigated by optical microscopy (OM), X-ray diffractometer (XRD), transmission electron microscopy (TEM), split hopkinson pressure bar (SHPB) and other analysis apparatus. The results show that the microstructure of hypoeutectoid U-Nb alloy base metal is α-U+γ-U lamellar pearlite under isothermal heat treatment, while the laser welding seam is composed of α' lath martensite for pre-heated or α' twin martensite for no pre-heated with orthogonal crystal structure. The quasi-static tensile strength of welded joint (about 400 MPa) is much less than base metal and microstructures of weld, for the main reason of incomplete penetration weld and low fracture toughness. Between dynamic impact loading for base and welded joint, the strain rate of welded joint is lower than base metal, and the yield strength of welded joint is higher. Also, the compressive stress-strain curves indicated that the flow stresses for welded joint increased with the increase of strain rate and the obvious effect of strain rate hardening has been observed. At strain rate of 2000 s-1, selected plastic deformation taking place in welded joint is due to the tremendous difference mechanic properties between weld seam and base metal, and the adiabatic shear band(ASB) only appears in the rest of welded joint.
