As nuclear power operates at high temperature and high pressure, corrosion is considered as one of the issues that threaten the safe operation, though corrosion rarely occurs. To fully understand the electrochemical behavior of nuclear key materials and manage the corrosion degradation of these materials in a proactive manner, a great deal of work have been undertaken in lab. Some sulfur-related specie can cause corrosion degradation of metal materials. Steam generator (SG) is one of the most important components in nuclear power plant, and alloys 800 and 690 are the most frequently used as SG tubing alloys. Sulfur-induced corrosion of SG alloys in high temperature and high pressure water is one of the most complicated processes. In this paper, the research progress regarding to sulfur-induced corrosion of alloys 690 and 800 was reviewed from the aspects of thermodynamic calculations and experimental. Thermodynamic calculations are mainly presented by E-pH diagrams, volt equivalent diagrams and species distribution curves. It is concluded that the valences of sulfur and their interactions with metal is mainly affected by temperature, solution pH and electrode potential. Experimental data indicate that sulfur induced corrosion is determined by temperature, solution pH, sulfur species, and other impurities like chloride ions, grain orientation, alloy compositions and stress etc. These factors can interact in a very complicated way. Generally, increasing temperature and decreasing solution pH would increase the corrosion degree of SG tubing alloys. Sulfur at the reduced or intermediate oxidation level are more detrimental than complete oxidation level, to the passivity of SG tubing alloys. Chloride ions have a combined effect with thiosulfate on passive film degradation in the case that chloride's adsorption is dominant; this combined effect is not remarkable if the chloride's adsorption is not dominant. Elements like Cr, Mo and Cu in alloys would weaken sulfur adsorption to some extent and therefore inhibit sulfur-induced corrosion, but increasing Ni content would enhance sulfur-induced corrosion. Both compressive and tensile stress would increase the reactivity of a passive surface of SG tubing. Sulfur would more easily adsorb on the metal surface where it has more defects, resulting in an increased dissolution rate. The crystal orientation can enhance the corrosion rate in the order of (111)<(100)<(110).
Porous Ti not only inherits the physical and chemical properties of titanium alloy, such as higher special stiffness, special strength, excellent corrosion resistance and biocompatibility, but also its unique pore structure gives it the characteristic of ultra-low density and large surface area. It is an alternative material for human body with structural and functional integration. It has been widely used in the field of clinical medicien in recent years. Many research and applications show that the properties and functions of porous Ti strongly depend on the pore structure of porous Ti prepared by different methods. Surface activation technology can significantly improve the surface activity of porous Ti and shorten the healing period after implantation. In this paper the common preparation methods of porous Ti were introduced based on the structure and properties of porous Ti. The surface modification, biological activity, osteoinductive properties of porous Ti and their domestic research status were summrized. The development of biomedical porous Ti and titanium alloys was prospected.
Corrosion of buried pipeline in iron-rich clay mineral, such as the red soil, is a great issue for safety and economy concern in various industrial applications, e.g. oil/gas, water, sewerage disposal systems, which may partly attribute to the active Fe oxides constituents residing in the clay. Although various parameters on metallic corrosion in red soil have been widely studied, some soil properties affecting corrosion are still not fully understood, such as synergistic action of sulfate reducing bacteria (SRB) and Fe oxides in iron-rich clay. Anaerobic SRB, which reduce sulfate to sulfide, have long been associated with corrosion of steel and have been the focus of most research on biocorrosion. Recently, there have been numerous studies showing that SRB can reduce oxidized metals, such as Fe(III), Mn(IV), and some SRB are capable of coupling metal reduction to growth, so Fe(III) reduction in clay minerals by SRB will have great impacts on corrosion processes. Most of previous studies focused on the single parameter, such as microbial activities, Fe oxides, but neglected their synergistic action. In this work, to further mechanistic understanding the synergistic action between SRB and Fe oxides, the indoor immersed experiment was desinged. Open circuit potential (EOCP), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and polarization potential scanning were used to monitor the corrosion electrochemical process of the X80 pipeline steel electrode. Microscopic surface observation was studied by SEM. The results showed that, SRB had no significant effect on the electrochemical process during the environmental adaptation period (the initial 7 d). The decrease of EOCP and electrochemical impedance (|Z|) of the X80 steel was resulted by the SRB iron respiration activity in the growing period, which significantly promoted the corrosion process of the steel. The SRB acts as an electron transport medium to participate in the electron transfer between Fe and iron oxide, which may lead to the electrochemical reduction of the iron oxides in the surface of red soil particles by the action of extracellular iron respiration, and it's the main reason to promote the local corrosion electrochemical process. The relationship between the corrosion of the material in the Fe-rich red soil and the microbial extracellular iron respiration was proposed.
Pipeline steels for sour oil and gas containing H2S generally suffer from either hydrogen-induced cracking (HIC) or sulfide stress corrosion cracking (SSC). Oil and gas containing high concentration H2S are noxious to pipeline steels because of the hydrogen-induced corrosion. In this study, HIC susceptibility of welded MS X70 pipeline steels was evaluated in NACE “A” solution at room temperature. Meanwhile, microstructure and regions near a HIC crack in the MS X70 base steel and its welded joint were analyzed through OM, SEM and EBSD. The hydrogen trapping efficiency was also investigated by measuring the permeability (J∞) and the effective hydrogen diffusivity (Deff). The results showed that both base metal and welded joint were highly susceptible to HIC and the later steel sample was more vulnerable than the former. This higher susceptibility could be primarily attributed to the following effects: the higher hydrogen trapping efficiency of bainitic lath microstructure in the welded joint; the more low angle grain boundary in the welded joint also made it easier to crack by improving the hydrogen trapping efficiency of high angle grain boundary; the less amount of coincidence site lattice grain boundary and Σ13b、Σ29b lead to higher HIC susceptibility by decreasing the resistance to crack of high angle grain boundary.
The radiation-induced segregation (RIS) and microstructure evolution such as dislocation loops and cavities are major microstructural causes for the irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steel (SS) core components. While a couple of studies have been reported on the irradiation induced damage in nuclear grade (NG) austenitic SS, the evolution of dislocation loop density and size and its correlation with the mechanical properties have still remained incompletely understood. In addition, the correlation between the segregation at the grain boundary and that at the dislocation loop has received limited attentions. In particular, there is still a lack of a systematic study of the irradiation damage in domestically fabricated NG austenitic SS. In this work, the proton-irradiation induced microstructural damage in domestically fabricated 304NG SS was characterized, in an effort to correlate the RIS and the dislocation loop density and size with the irradiation dose, as well as the dislocation loop density and size with the radiation-induced hardening. The results revealed that the radiation-induced microstructure damage was mainly dislocation loops with a few micro-voids. The loop density was in the order of 1022 m-3 with an average size of <10 nm. The square root of the product of loop density and size (Nd)0.5, scaled linearly with the square root of irradiation dose with a factor of 6.8×103 dpa-0.5mm-1. The loops were believed to be mainly responsible for the hardening in 304NG SS, which also scaled linearly with (Nd)0.5 with a factor of 1.16×10-2 HV0.025mm. A comparative analysis about the segregation at the grain boundary and at the dislocation loop was conducted. While the depletion of Cr and enrichment of Ni at the dislocation loop and grain boundary showed no difference, the enrichment of Si at the dislocation loop could be of about 6 times of that at the grain boundary. In addition, the loop density and loop size, as well as RIS and radiation-induced hardening were all increased by a higher dose and tended to saturate by a dose of 3.0~5.0 dpa.
The structure formation of superalloys is very complicated because of their multicomponent composition and multiphase transition processing. Duo to the limitation of some pre-conditions, the structure formation can not be accurately determined by thermodynamic calculation method. Knowledge about the structure is critical for the design of the following heat treatment process. In this work, a single crystal (SC) sample of superalloy CM247LC was directional solidified in a labor Bridgman furnace with a pulling rate of 0.2 mm/min and then water quenched, to investigate the solidification sequence including MC carbide and γ/γ′-eutectic. It was observed that the γ-phase is firstly formed in the form of dendrites; it is then followed by the homogeneously precipitation of MC carbides from the liquid behind dendrite tips. Near the end of solidification the interdendritic residual liquid transits into γ/γ′-eutectics. It is interesting to found that the γ/γ′ eutectics do not nucleate on the existing γ -phase, but preferably on the MC carbides which have completely different chemical composition and crystal structure. The result of EBSD examination indicates that the γ/γ′ eutectics formed on the MC substrates have random crystal orientations compared to the SC γ -matrix, exhibiting the misoriented multi-crystal microstructure in the so called "single crystal" superalloy casting.
Nickel-based alloy is a good choice for materials used in ultra-supercritical power plant, which is subjected to high temperature about 700 ℃ and high pressure in service. In order to meet the requirements above, a nickel-based alloy was designed and the tubes were successfully manufactured. Based on the hot/cold working processes of tube components of the nickel-based alloy, microstructural evolution, especially the recrystallization mechanisms during hot working processes of the alloy were systematically investigated by using a series of hot compression tests, solution annealing tests, OM and TEM analyses. The results showed that dynamic recrystallization was dominated by discontinuous dynamic recrystallization mechanism involving grain boundary bulging, whereas the nucleation mechanism with strain inducing grain boundary migration was the driving force of static recrystallization. In addition, the essence of different forms of step grain boundary producing during dynamic recrystallization and static recrystallization was to make the surface deviate from low index surface, which could ensure more interface to be low energy close-packed surface, so that the energy of grain boundary interface would be reduced. The morphology of step grain boundary depended on the crystallographic relationship of crystal interface and the Burgers vector of grain boundary dislocation. Moreover, the presence of step grain boundary could also promote grain boundary migration and accelerate the recrystallization process. When the recrystallization process was completed, step grain boundaries still remained partially to minimize interfacial energy and continue to promote subsequent grain growth processes.
Recently, a new Co-Al-W-based alloy with ordered L12 structure has been attracted much attention of researchers, these alloys have higher melting point than Ni-base superalloys with morphologically identical microstructure, but grain defect formation caused by thermosolutal convection has become an important problem for its application. Magnetic field is always applied to damp the convection which reduces the formation of defects. However, there are hitherto few papers to investigate the effect of magnetic field on grain defects during Co-Al-W-based alloy directional solidification. In this work, The effect of high magnetic field on the solidification structure and macrosegregation in directionally solidified Co-Al-W-based alloy was investigated. The results showed that the application of longitudinal magnetic field can induce convection and cause deformation of the solid-liquid interface shape, forming the macrosegregation and the stray grains in the mushy zone at the pulling rate of 5 μm/s. With the increase of pulling rate, the macrosegregation and the stray grains disappeared gradually at 2 T magnetic field. While the transverse magnetic field was applied, the macrosegregation became serious and the number of the stray grains increased. The macrosegregation further became more serious and the columnar-to-equiaxed transition was induced after adding the Ta element. The main reason of undercooling nucleation and columnar-to-equiaxed transition (CET) was the microsegregation induced by thermoelectric magnetic convention.
As a result of increasing energy demands and accelerated environmental problems, there is an urgent need to improve the thermal efficiency of ultra supercritical (USC) power plants. To achieve this goal, advanced ultra-supercritical (A-USC) technologies with the main steam temperature of 700~750 ℃ and pressure of 35 MPa have been developed quickly in recent years. One of the most promising candidate Ni-based superalloys for the main steam pipe of 700 ℃ ultra-supercritical coal-fired power plants is Inconel 740H, which is a modified version of Inconel 740 developed by Special Metals Corp (SMC). Compared with IN740, the Ti/Al ratio in IN740H is lowered in order to stabilise the microstructure at long ageing times. In addition, the Nb content is lowered to improve the weldability. In this work, the microstructure evolutions, the nucleation and propagation mechanisms of microcracks in the nickel base superalloy Inconel 740H at 750 ℃ high temperature were studied by the self-developed in situ high temperature tensile stage inside a SEM. The results showed that under the uniaxial tensile stress at 22 ℃ room temperature and 750 ℃ high temperature conditions, the grain boundaries of Inconel 740H alloy are always the most primary crack sources. The strength of grain boundaries is higher than that of grains under the room temperature, and the microcracks will be nucleated at the grains as well, but the relative strength of grain boundaries will be weaken under the high temperature, which makes the microcracks tend to nucleate at grain boundaries. The experimental results also showed that the influence of high temperature on the mechanical properties is very significant, the high temperature reducing the activate energy of slip and weakening the strength of the grain boundaries, so that more slip systems activated and the grain boundaries occurring bending and sliding deformation, so further enhance the ability of plastic deformation of alloy. However, the reduction of relative strength of alloy grain boundaries leads to microcracks nucleation and propagation more easily from grain boundaries and lower the yield strength and tensile strength of alloy.
From the view of material point, high-temperature protective coatings are divided into the following two categories: ceramic coating and metallic coating. Metallic coating possesses higher toughness and bond strength to the alloy substrate than ceramic coating does. Its protectiveness relies on the formation of a slow-growing and adherent oxide scale at high temperatures. However, with increasing the oxidation time, the oxide scale will experience cracking and spalling as it has grown to the critical thickness. Ceramic coating due to its chemical inertness has been used in many corrosive environments for protection. But the weak interfacial bond and big mismatch of coefficient of thermal expansion with the alloy substrate limit its application in thermal shock environments. Since glass-ceramics combine the generally superior properties of crystallite ceramics with the easy processing of glasses, it is expected that glass-ceramic coating should show a higher spallation resistance than ceramic one under thermal shock. Cast K444 superalloy is widely used in advanced aircraft engine and gas turbine. Its protection from high-temperature oxidation under thermal shock becomes a critic issue. In this work, NiCrAlY and enamel based composite coatings on the K444 superalloy substrate by arc ion plating and spray-firing methods were prepared, respectively. Thermal shock behavior from 900 ℃ to room temperature of these two coatings was studied comparatively. One cycle of thermal shock contained the holding of samples at 900 ℃ for 1.5 h and the following cooling down in air or water. Results indicated that thermal shock resistance of the NiCrAlY coating was low. As the NiCrAlY coating was thermal shocked by water, its oxide scale cracked severely after 30 cyc, and certain crack had already transported the scale and penetrated into the interior of the underlying metallic coating; for the enamel based composite coating, however, its thermal shock resistance was high. No cracks were detected at the coating surface or interior after thermal shock test. Besides, the enamel coating still adhered well with the alloy substrate. The high resistance to thermal shock of the enamel based composite coating originated from: (1) the coefficient of thermal expansion of the enamel based composite coating matched well with that of the alloy substrate; (2) the addition of nano-sized Ni and NiCrAlY metallic particles improved the toughness of the enamel coating, in addition to enhancing its coefficient of thermal expansion.
In the present technology, the brazing of Si3N4 needs a reactive transition layers to resolve the non-wetting problem of usual metal fillers. Aluminum could wet Si3N4 without reaction but the brazing is very difficult due to wetting temperature above 1000 ℃. In this work, the wetting effect of sputtered Al films on Si3N4 and its physics essence were revealed. Based on this, the brazing of Si3N4 ceramic with Al or Al-Ni film fillers was realized near their melting temperature. The results showed that the seams of brazing joints with direct sputtered Al on Si3N4 film were well-stacked and less defects, and well metallurgically bonded to ceramic without reactive transition layers. The shear strength of pure Al/Si3N4 joint reached 106 MPa. The strength increased to 148 MPa with adding 1.0%Ni into film filler due hypoeutectic structure in the seam. With further increasing Ni content to 3.0%, the eutectic structure of the seam slightly decreased the strength of joint to 132 MPa. These joints above all fractured in joint seams. Moreover, the Al-1.0%Ni film filler first sputtered Ni layer was compared. Its brazing joint fractured at the interface between seam and ceramic and the shear strength decreased to only 81 MPa. This comparsion revealed the "wetting" effect of the bombardment of energetic sputtered Al particles. This effect still existed after filler melting and the direct brazing of Si3N4 ceramic without reactive transition layers was realized.
Aluminium alloys were widely applied in rail transit, ships and aerospace owing to their unique properties, such as low density, high strength and stiffness, outstanding corrosion resistance and low temperature performance. As a type of structure material, aluminium alloy joining was inevitable. However, these alloys were often considered very difficult to weld using traditional fusion welding technique since the welding seams were often accompanied with metallurgical defects, large deformation and stress. Friction stir welding (FSW), an innovative solid-state welding technology invented at the welding institute (TWI), was seen by designers as an effective joining methods in welding aluminium alloys due to low heat input, small stress-strain and environment friendly. In this work, 0.8 mm thick plate of 6061-T6 aluminium alloy was successfully welded by use of high rotational speed fiction stir welding technology. The microstructure and mechanical property of the butt joints prepared by high rotational speed friction stir welding were analysed in detail. The results show that the well surface topography and excellent bonding interface existed in the nugget zone (NZ) were observed. Both of the microhardness of the weld seam was lower than that of the substrate. The lowest microhardness of the butt joints located between the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Compared with the conventional rotational speed, the number of β-Mg2Si, Al2CuMg and Al8Fe2Si precipitated phases existed in the NZ was more, which made the microhardness in the NZ improved significantly. The rod-shaped precipitates (Mg2Si) have the greatest influence on the microhardness. The excellent mechanical properties were obtained at the rotational speed of 8000 r/min and welding speed of 1500 mm/min. The maximum tensile strength was 301.8 MPa, which was 85.8% of the as-received 6061-T6 (351.7 MPa). And the toughness-brittleness fracture mode appeared.
Ni100-xPx alloys with near-eutectic compositions have a strong glass forming ability (GFA), but the microstructure prototypes and their evolution in various solidification processes are still unclear now. To reveal their unique structures, a series of molecular dynamics simulations for the rapid solidification process of liquid Ni100-xPx (x=19.0, 19.4, 19.6, 19.8, 20.0, 21.0) alloys were performed at a cooling rate of 5×1012 K/s, and their local atomic configurations at 300 K were characterized by Voronoi polyhedron index 〈n3,n4,n5,n6〉and cluster type index (Zni/(ijkl)i...). The results show that the local atomic structures of Ni atoms are mainly Frank-Kasper clusters with high coordination (Z≥12) as well as their distorted configurations. Their chemical short-range orders are mostly NiZ-2P3, and these basic clusters can be further aggregated into medium-range orders (MROs) by intercross-sharing (IS) linkages. The majority of P-centered clusters are bi-capped square Archimedean anti-prism (BSAP) polyhedrons, but lots of Frank-Kasper clusters with higher coordination exist in the amorphous Ni100-xPx alloys. Their typical chemical short-range orders are Ni12P. In these short range orders (SROs) centered by P, all shell atoms are found to be Ni atoms, and no MRO can be detected except for their extended clusters linked by vertex-sharing (VS), edge-sharing (ES) and face-sharing (FS). The BSAP polyhedrons and their correlative structures play a crucial role in the formation of amorphous Ni100-xPx alloy. Their quantity is demonstrated to have a significant impact on the glass transformation of rapidly solidified Ni100-xPx alloys. It is found that the number of BSAP polyhedrons and their deformed structures at eutectic composition point x=19.6 is the largest among Ni100-xPx alloys, and the farther x deviates from the eutectic composition point, the smaller the proportion of BSAP polyhedrons and their structures more related to all P-centered clusters, which are consistent with the variation tendency of GFAs of Ni100-xPx alloys. Maybe, it could be responsible for the existence of the strongest GFAs at the eutectic composition point of Ni100-xPx alloys.