K4169 nickel-based superalloy has been widely used to fabricate high-strength components in aircraft engine. When in service, especially affected by vibration and start-stop process, this alloy is inevitably affected by the external cyclic stress. Therefore, it is of great significance for researchers to understand the microstructure evolution in K4169 while cyclic loading. In the present study, the microstructure evolution of K4169 during cyclic loading has been examined and discussed in detail by using investment casting, cyclic loading and microstructure characterization methods. The cyclic loading test with stress amplitude of 380 MPa was carried out on a pull-push type fatigue machine at room temperature. The dependence of cycle times or fatigue life of specimens with different casting conditions on microporosity content has been discussed. Special emphases have been put on investigating the deformation and fracture characteristics of Laves and δ-Ni3Nb phases under the influence of microporosity. The results show that the cyclic life was mainly dominated by the content of microporosity. The crack initiation occurred mainly near the microporosity of the specimen surface. The specimen with high microporosity content exhibits the characteristic of complete brittle fracture, while the specimen with low microporosity content exhibits obvious transgranular fracture characteristics. In addition, the fracture of Laves phase was not apparently affected by cycle number. At the beginning of cyclic loading, the long-striped Laves phase near the microporosity was easy to crack, which became the sensitive area of crack growth, and extending in the manner of parallel secondary cracks. The δ-Ni3Nb plates near microporosity exhibited two obvious cyclic deformation and fracture characteristics depending on their arrangement (or growth orientation) relative to external loading axis: cracking along length direction (or denoted as branch cracking); and exhibiting slip lines and cracks on the surface of δ-Ni3Nb plates. At the initial stage of cyclic loading, δ-Ni3Nb plates were prone to crack along the length direction, while the surfaces of the δ-Ni3Nb plates far from microporosity appear the characteristics of slipping, bending and fracture in turn with the decrease of microporosity content or increase of cyclic cycles. Edge dislocations have been found within δ-Ni3Nb plates, indicating the transition from screw dislocations to edge dislocations under cyclic loading. Additionally, the twinning deformation of γ-Ni matrix during cyclic loading has been scrutinized through TEM and TKD analyses. The results have been linked to the evolutions of Laves and δ-Ni3Nb phases, i.e., the evolutions were influenced by the increase of strain localization around Laves and δ-Ni3Nb phases.
Due to the excellent high temperature comprehensive performance and cost effective, the second-generation nickel-based single crystal superalloy has been widely used in the high-pressure turbine blades of advanced aero-engines. Microdefects such as micropores and interdendritic eutectic are seriously harmful to the high temperature mechanical properties of nickel-based single crystal superalloys. Hot isostatic pressure (HIP) technology, which has been widely used in powder and casting superalloys, can effectively reduce the micropores, interdendritic eutectic and other structural defects formed in the turbine blades during manufacturing, and improve the service reliability of turbine blades. However, the effect of HIP process on the high temperature stress rupture life of nickel-based single crystal superalloys is still controversial, especially with regard to the initial microstructure state of the nickel-based single crystal superalloys, i.e. the as-cast microstructure state or the as-solid-solution state. In this work, a kind of second-generation nickel-based single crystal superalloy with as-cast state or as-solid-solution state was selected as the research object. Through two-stage heat/booster type heat treatment process, in combination with microdefects quantitative analysis, quantitative characterization of alloying element segregation and high temperature stress rupture tests at 980 ℃ and 250 MPa, the effects of HIP process on the microdefects and high temperature stress rupture life of the used superalloy with different initial microstructures were studied. The results indicated that the solid-solution treatment can significantly promote the diffusion of alloying elements, such as Re, W, Al, and Ta, reduce the area fraction of interdendritic eutectic, but significantly increase the average area fraction and size of micropores in the used alloy with as-cast state. While, HIP process can effectively reduce the average area fraction and size of microspores in the used alloy with as-cast state or as-solid-solution state, but cannot eliminate the interdendritic eutectic as remarkable as the solid-solution treatment. By HIP process of the used alloy with as-solid-solution state, the area fraction of micropores is reduced to 0.005%, the eutectic structure is basically eliminated, and the dendrite segregation of Re, W, Al, Ta and other elements is significantly alleviated, resulting in the higher stress ruputure life of the used alloy, about 40% over that of the used alloy with the standard heat treatment state. Performing HIP process on nickel-based single crystal superalloy alloy with as-solid-solution state is of benefit to the high temperature stress rupture life due to the reduction of microdefects and the homogenization of alloying elements, in comparison with performing HIP process directly on the alloy with as-cast sate.
With the increase of the strength of steel plate, the welding performance of the steel decreases sharply and the welding crack susceptibility increases. The properties of welding heat-affected zone of high strength steel lower with increasing the welding heat input. The present work aims to improve the toughness of welding heat-affected zone by rare earth oxide metallurgy technology. The effect of 5×10-6 and 23×10-6 rare earth Ce on the impact toughness, microstructure, austenite grains of heat-affected zone and fracture morphology of welded joint were studied. When the steel contains 5×10-6 rare earth Ce, the inclusions are MgO-Al2O3 spinels surrounded with a small amount of CeAlO3 inclusions. In the case of the steel with 23×10-6Ce, Ce can completely modify MgO-Al2O3 inclusions, resulting in the formation of (CeCa)S+MgO-Al2O3+MnS complex inclusions. The simulation welding of high strength steel was performed. The results show that the Charpy impact energy of heat-affected zone of the steel with 23×10-6Ce is higher at four different heat inputs, in comparison with the steel with 5×10-6Ce. The microstructure analysis shows that the fracture morphology of heat-affected zone of the steel with 23×10-6Ce appears dimples, which is an indication of a higher toughness. With increasing the heat input from 25 kJ/cm to 100 kJ/cm, the average grain size of the original austenite in the heat-affected zones of the steels with 5×10-6Ce and 23×10-6Ce was increased by 75.6% and 52.4%, respectively. It indicates that the growth of the original austenite grain during welding is suppressed with increasing the Ce content in the steel. Comparison of the microstructure shows that rare earth Ce can delay the formation of upper bainite structure in the heat-affected zone. Through the high temperature confocal microscope, it was observed that the rare earth inclusions pinned on the original austenite grain boundary, which can effectively restrain the grain growth during the welding process. It provides an evidence showing the mechanism of improvement in the heat-affected zone in the welding of the high strength steel by rare earth Ce. The present study demonstrates the rare earth oxide metallurgy can improve the weldability of the high strength steel.
Due to the high damping capacity and excellent mechanical properties, Fe-Mn alloy is considered to be a promising high damping alloy, and suitable for constructional and vehicle metal parts application, which can enhance the fatigue property of structures and metal parts, and also improve the working and living environment. It's generally accepted at present that damping capacity of Fe-Mn alloy is influenced by the stacking fault boundaries in γ-austenite and ε-martensite, γ/ε phase boundaries and ε/ε variant boundaries; another view is that boundaries of the above damping sources are made up of partial dislocations, so the damping capacity of Fe-Mn alloy is caused by the motion of partial dislocations, and interpreted by G-L dislocation pinning model and stacking fault probabilities calculation. But there is no distinction between the probabilities of different type stacking faults. Both deformation stacking fault and growth stacking fault can be formed in γ-austenite and ε-martensite, and the change of process parameters has different influence on them, which will lead to different changes of deformation and growth stacking fault probabilities. So it's necessary to analyze whether boundaries of different stacking fault types will have different effects on damping capacity of Fe-Mn alloy. Based on that, a hot-rolled Fe-19Mn alloy is prepared and then solution treated between 950~1100 ℃. Damping capacity is measured by dynamic mechanical analyzer (DMA). The microstructure evolution is observed by OM and TEM, and XRD is used to analyze phase constitution and to measure stacking fault probabilities. The results reveal that Fe-19Mn alloy shows amplitude-dependent damping capacity which almost linearly increases with amplitude, and frequency-independent damping capacity. From G-L plot, the variation of damping capacity below the critical amplitude A' (A'≈30 μm) is interpreted by G-L model, while it's associated with micro-plastic deformation when above A'. As the increase of solution treatment temperature, the damping capacity of Fe-19Mn decreases, and possesses the best performance at 950 ℃; furthermore, it shows different characteristics in different amplitude ranges: when the amplitude is lower than 170 μm, damping capacity decreases in exponential form, which changes similarily with deformation stacking fault probability in ε-martensite, so it can be considered the boundaries of deformation stacking fault as the main damping source; when the amplitude is higher than 170 μm, damping capacity decreases linearly, which changes similarily with the relative length of γ/ε phase boundary, so it can be considered γ/ε phase boundary as the main damping source. Based on TEM observation of stacking faults in γ-austenite, it can be inferred that stacking fault boundaries in γ-austenite have no obvious contribution to the change of damping capacity of Fe-19Mn with amplitude.
With the increasing demand and exploitation depth for offshore oil and gas, offshore platforms are becoming larger and the performance requirements and size for offshore platform of ultra-heavy plates are also increasing. Due to the large plate thickness and the limitation of manufacturing techniques, inhomogeneous microstructures and mechanical properties along thickness direction are great challenges for offshore platform of ultra-heavy plates. In this work, variation of microstructure and its effect on mechanical properties for the 117 mm-thick Ni-Cr-Mo-B industrial ultra-heavy plate were investigated by means of OM, SEM, TEM and EBSD observation, in combination with the tensile and impact toughness test. The results show that yield strength reduces gradually from the surface (798 MPa) to the center (718 MPa) and elongation almost keeps constant around 20.0%~22.0% for the 117 mm-thick plate. It is noted that impact energy at -60 ℃ increases first from 35 J at the surface and reaches its peak 160 J at the depth of 1/8T (T—thickness of plate), and then drops to the minimum about 20 J at the center, which suggests that impact energy curve along the whole section varies sharply and exhibits like letter 'M'. Lath width, boundary carbide size and intragranular carbide size are all gradually increasing from the surface to the center, i.e., from 198.7 nm to 500.6 nm, 130.6 nm to 226.6 nm, 45.8 nm to 106.2 nm, respectively, and there are also some blocky areas at the center, all those indicate that refinement strengthening and precipitation strengthening would decrease, as well as the gradual decrease of yield strength. Also, from the surface to the center, effective grain size (EGS) decreases first and then increases. The surface and the center have larger EGS (2.2 μm and 2.7 μm, respectively), which indicates that they have weaker resistance to cleavage crack and exhibit lower impact energy. However, the 1/8T position has smaller EGS (1.7 μm) while obtains higher impact energy.
TiB2 is a promising strengthening phase in steels applying in lightweight transportation systems due to its high Young's modulus and low density. However, the density difference between TiB2 particles and matrix leads to segregation during solidification. TiB2 particle-reinforced steels were solidified with a vertical linear-type electromagnetic stirring device. The effects of electromagnetic stirring on the morphology and size distribution of TiB2 particles were studied. Vickers hardness, mechanical properties in the tensile test were also discussed. The results show that electromagnetic stirring effectively refined the primary TiB2 particles in the steel, and the average particle size decreased with the increase of exciting current. The particles distributed dispersively and the structure was more homogenous under a higher exciting current. Besides, the defects of crackle around particles were eliminated under high current. Electromagnetic stirring reduced the macrosegregation of TiB2 particle-reinforced steels, which decreased the hardness discrepancy in the ingot at various heights. A higher exciting current attributed to higher average hardness, and the steel reached a hardness of 275 HV under 350 A exciting current. The ultimate tensile strength and the strain at break were both enhanced by electromagnetic stirring, and reached 520.2 MPa and 8.5% respectively under an exciting current of 350 A. The refinement of particles was caused by the smashing process under a strong convection driven by the moving magnetic field, and the effect of electromagnetic force acting on the particles. The influence factors of electromagnetic force were analyzed, which show the force increases with increasing magnetic intensity, decreases with increasing temperature of melt, and increases with increasing particle size.
Along with the increasing pace of marine resource development and strategic deployment of China, the infrastructure materials and deployed aircraft were facing severe salt fog corrosion during the construction process of the South China Sea. Materials damage in this environment is much more serious than that in other marine atmospheric environment. Owing to its location near the equator and the direct impact of solar radiation, Nansha marine atmosphere is a representative and typical climate with high temperature, high humidity, high salinity and high radiation. However, there has been lack of material corrosion data and relevant fundamental research until now. Carbon steel is usually one of the most widely used infrastructure materials and reference materials, and its corrosion data exposed to Nansha Islands marine atmosphere is much more important. These corrosion data can not only provide important basis for environmental corrosivity category, but also provide reference for indoor accelerated corrosion test. Therefore, in order to obtain useful information on selected construction materials, adopting the appropriate corrosion protection methods, and predicting the life of metallic structures under service, the exposure test was conducted on carbon steel Q235 and weathering steel Q450NQR1 in Nansha Islands for 2 and 5 months. Thickness loss analysis, macroscopic observation, SEM, XRD, optical profiler and tensile tests were conducted to study the initial corrosion behavior on both sides of Q235 and Q450NQR1 in Nansha marine atmosphere. The results showed that the initial corrosion behavior of both steels at this site was more serious than those at most areas, such as Wanning and Xisha Islands, and the corrosion of skyward of both steels was more serious than that of field-ward. The rust layer formed on field-ward was easier to fall off. After exposure for 2 months, the thickness loss of Q235 was the same as that of Q450NQR1, and corrosion products on both sides were mainly composed of γ-FeOOH, α-FeOOH and Fe3O4; while after 5 months' exposure, the thickness loss of Q235 was much larger than that of Q450NQR1, and corrosion products were mainly composed of γ-FeOOH, α-FeOOH, Fe3O4 and β-FeOOH. The relative composition of β-FeOOH and γ-FeOOH was fewer on the field-ward, and the relative composition of Fe3O4 was fewer on the skyward.
Due to the temperature rising or cooling stage in thick plate, non-isothermal ageing has been the research hotspot of heat treatment for Al-Zn-Mg-Cu alloy thick plate. It is possible to replace the isothermal ageing by non-isothermal one because of the high efficiency and practicability. As one of the Al-Zn-Mg-Cu alloy, 7B50 aluminum alloy and its thick plate have supposed to manufacture the wing in "Yun-20" big plane. In this work, hardness test, electrical conductivity test, room temperature tensile test, DSC analysis, exfoliation corrosion test and TEM observation were used to study the influence of non-isothermal ageing on microstructure and corrosion resistance of 7B50 aluminum alloy hot rolling thick plate. Results revealed that, after 480 ℃, 1 h solution and quenched in room temperature water, followed by ageing from room temperature to 215 ℃ at 1 ℃/min heating rate, and furnace cooling to room temperature immediately, the inner precipitates of 7B50 aluminum alloy are fine and dispersed while the ones on grain boundary are coarsened and discontinuous. The tensile strength and exfoliation corrosion grade reached to 605 MPa and EB level, respectively. Comprehensive performance of 7B50 aluminum alloy are excellent overall those of isothermal peak ageing (T6) or isothermal double stages over ageing (T76), but similar to that of retrogression and re-ageing (RRA) treatment. The non-isothermal ageing realized the short process preparation and the measure removing isothermal stage is more suitable for thick plates.
The severe wear and uneven wear will happen on the surface of ductile cast iron such as the traction wheel of elevator, in the long-term working under the serious wear and impact conditions. Laser cladding can be applied to reinforce its surface, which can improve the properties and life of the cladded components, save materials and manufacturing cost and raise the economic efficiency, but as to the surface of different materials, especially the ductile cast iron, the alloy powder and process parameters for laser cladding need to be chosen carefully by the experimental studies because of the rapid melting and solidification, the difference of thermo-physical properties between the laser cladding layer and the matrix, the laser absorption of the cladding layer and matrix and so on. In this work, laser cladding is employed to fabricate Co-based composite coatings reinforced by TiC particles by a 6 kW CO2 laser. The effects of the technical parameters of laser on the composition, phase and microhardness of the laser cladding layer are investigated by OM, SEM, EDS, XRD and microhardness tester, with the emphases of analyzing the changes of the distribution, morphology and size of TiC in the laser cladding layer. It is shown by the results that the cladding layer is mainly composed of γ-Co, TiC/(Ti, W)C1-x, Cr-Ni-Fe-C and a small amount of Cr7C3 phase, and its microstructure changed from the fine dendrite crystal near the cast iron matrix to the equiaxed dendrite in the middle then to the fine dendrite crystal near the surface of laser cladding layer with the dispersed distribution of TiC at the root or tip of secondary dendrite arm, even at the branch of primary dendrite arm. The number of dendrite and the dendrite arm spacing both increase in the microstructure of the laser cladding layer and the morphology of TiC is transformed from the smooth circular shape to the irregular polygon shape, and its content obviously increases with the particle size of TiC decreasing and its distribution more uniform, when the laser power is decreased from 3.6 kW to 3.2 kW or the scanning rate increased from 350 mm/min to 410 mm/min. The growth of the primary dendrite or secondary dendrite can be inhibited by the precipitation of TiC after its dissolution in the melt pool of laser cladding. In this experiment, the hardness at the surface of laser cladding layer gradually increases with the decrease of laser power or the increase of scanning rate, in which the maximal microhardness is 1246.6 HV0.2, up to increasing by 5 times of the matrix.
Continuous silicon carbide (SiC) ?ber-reinforced titanium metal-matrix composites (TMCs) are potential candidates for high temperature application in jet engines because of their high specific strength and stiffness. However, severe interfacial reactions caused by high temperature manufacture and service have a detrimental effect on the mechanical properties of composites. Furthermore, the phase transition occurred in matrix at elevated temperature is unfavorable to the properties. In this work, the interfacial reaction, matrix phase transformation and thermal stability of SiCf /Ti65 composites were investigated. The composites were prepared by the combination of magnetron sputtering and hot isostatic pressing (HIP) method. Matrix-coated precursor wires prepared by sputtering were aligned, degased and encapsulated, then consolidated by HIP. And the densified composites were subjected to long-term thermal exposure at 650, 750, 800 and 900 ℃, respectively. Reaction products and element diffusion of SiCf /Ti65 composites in different conditions were studied. The results show that the elements diffuse and participate in both interfacial reaction and matrix phase transition during HIP and thermal exposure process. In the as-processed SiCf /Ti65 composites, TiC is the main product of interfacial reaction layer, and (Zr, Nb)5Si4 is the product of matrix phase transition. With the continuous consumption of C-coating layer in the process of thermal exposure, Ti5Si3 and (Zr, Nb)5Si4 form in the interfacial reaction layer, while Ti3(Al, Sn)C and TiC precipitate in the matrix. The results of thermal stability study indicate a parabolic correlation between interfacial reaction layer thickness and exposure time, and the activation energy of reaction layer growth estimated by Arrhenius equation is 93 kJ/mol. The interface of SiCf /Ti65 composites is stable below 650 ℃.
In fission or fusion nuclear reactors, the interaction between the high energy particles (e.g. neutrons) and atoms would result in the radiation damages in materials, affecting the performance of materials and lifetime of reactors. The radiation hardening, embrittlement, swelling, creep, fatigue and so on are all related with the radiation damages in materials. Therefore, it is necessary to understand the underlying physics for developing the radiation resistant materials in future. Until now, the displacement cascade process has been studied for decades through computer simulation method. The other properties listed above have also been focused by different groups. However, in addition to these properties, it is also important to understand the degradation of mechanical properties induced by the supersaturated point defects and their clusters, especially the contribution from the interaction between dislocations and radiation defects. Under irradiation, in addition to the self-interstitial dislocation loops formed by the clustering of the self-interstitial atoms, the micro-crack can also be formed in bulk and near the surface of the materials, related with the gas bubble, grain boundary and blister, which would seriously influence the application of materials used in nuclear power plants. Under irradiation, in addition to the normal crack expansion under the effect of stress, these cracks would be key factors to irradiation assistant stress corrosion crack (IASCC) phenomena, which have been studied for decades. In previous studies, although the properties of self-interstitial dislocation loops and micro-crack have been studied independently, the interaction between them under the condition of irradiation has not been fully studied. In this work, the detailed interactions between self-interstitial dislocation loop and micro-crack in bcc iron have been studied at atomic scale with molecular dynamics (MD) simulation method. The results indicate that the relative position between them, the slope of micro-crack, the size of loop and existence of free surface, would all affect the interaction between loop and micro-crack and the related finally micro-structure after the interaction. Under different conditions, the interactions either induce the formation of micro-scale complex radiation damage structure, or the absorption of dislocation by micro-crack, resulting in the rugged crack tips, which would affect the growth and expansion of micro-crack, the degradation of mechanical properties of materials under irradiation. Furthermore, the results can also compare with the formation of dislocation and dislocation loop free zone near the crack tip observed experimentally, providing new understanding to these phenomena. Therefore, all these results provide new possible understandings of radiation damages.
UO2 is widely used as fuel in various nuclear reactors, and the sintering of UO2 ceramic powder under high temperature is one of the most important processes during the preparation of UO2 fuel. However, sintering is a very complicated process which is controlled by many simultaneous mechanisms. The phase field method was used to simulate the sintering process of UO2 ceramic powder in the present work. In the modified phase field model, the influence of three anisotropic diffusion mechanisms, including surface diffusion, grain boundary diffusion and lattice diffusion, on the microstructure evolution during sintering was considered, and the effect of the interface energy between different ceramic particles on the sintering morphology was also considered. Based on the experimental conditions and thermodynamic parameters, the sintering process of UO2 ceramic powder at 2000 K was simulated. The simulation results showed that the initial morphology of the ceramic powder affects the sintering kinetics; large grains grow more easily, and small grains disappear at the last stage of sintering; the GB diffusion mechanism is the dominant mechanism during the sintering; the equilibrium dihedral angle between GB and phase boundaries can be strongly affected by the GB energy. In addition, the sintering process of the polycrystalline UO2 ceramic powder was also simulated, and the simulation results were in good agreement with the experimental results.
Elastic modulus is one of the key properties for the application of biomedical β titanium alloy as human bone replacement because the elastic modulus of the alloy has to match that of the bone so as to avoid the stress shielding effect. Alloying of Nb is commonly used in biomedical β titanium alloys. In the present work, the lattice parameter, free energy and elastic modulus of β-Ti1-xNbx alloy were investigated by using first-principles method based on density functional theory. The random distribution of Nb atoms in the alloy were described by using both special quasirandom structure (SQS) and the coherent potential approximation (CPA) techniques, in combination with first principles plane-wave pseudopotential (VASP) and exact muffin-tin orbital (EMTO) methods, respectively. The results showed that the lattice constants from both VASP-SQS and EMTO-CPA calculations increase linearly with Nb content x, while the influence of the local lattice distortion is negligible. The calculations of the free energies demonstrated that EMTO-CPA predicts reasonably the phase decomposition of β-Ti1-xNbx at relatively low temperature whereas VASP-SQS does not, which might be ascribed to the fact that the free energy depends strongly on the detailed SQS structures. The elastic constants C11 and C12 calculated by using EMTO-CPA and VASP-SQS without atomic relaxation increase with Nb content whereas C44 decreases. EMTO-CPA overestimates the elastic stability of β-Ti1-xNbx. At low Nb content, the local lattice distortion is abnormally large due to the lattice instability of the β-Ti1-xNbx, making the free energy and elastic constant against x from VASP-SQS calculations with atomic relaxation deviate significantly from the general trend.
