压水堆控制棒驱动机构CANOPY密封焊缝服役环境与失效

综述了国际上对PWR核电站控制棒驱动机构密封焊缝(CSWs)结构、载荷、应力状态、失效分析的研究。研究认为CSWs的结构设计易造成高溶解氧服役环境,在CSWs死水区空腔存在微量氯离子污染的可能,导致较大应力腐蚀倾向。目前研究确认的失效模式有穿晶应力腐蚀(TGSCC)、沿晶应力腐蚀(IGSCC)、SCC+点蚀。计算出CSWs死水区空腔理论浓度可达230×10~(-6),分析认为溶解氧和低浓度水平的氯污染是可引起上述腐蚀的环境因素;CSWs应力腐蚀裂纹扩展模式与材料敏化、服役溶液环境、初始应变、应力状态等因素相关;点蚀可能成为应力腐蚀的起源也可发展成为独立的破坏形式。     

Formation of a novel nanocrystalline coating on AZ31 magnesium alloy by bias sputtering

A novel protective coating was deposited on AZ31 magnesium alloy by bias sputtering with a Ti/Al composite target in this study. It was determined by the analysis of EDS and XRD that the coating was mainly composed of Ti₃Al phase. The calculation, based on the Scherrer formula, further revealed the formation of nanocrystalline structure in the coating. SEM and AFM observations showed that the coating was compact and no obvious columnar structure occurred. In corrosion tests, it was found that the coating was more cathodic than the AZ31 substrate and had a good corrosion resistance in a 3.5 wt.% NaCl solution. Accordingly, the AZ31 substrate was protected by this obtained coating.     

Stress corrosion cracking susceptibility of ultrafine grained AZ31

In this study, stress corrosion cracking (SCC) behavior of AZ31 magnesium alloy was carried out using slow strain rate testing (SSRT) technique in 3.5 wt% NaCl solution. The influence of microstructural scale on the stress corrosion behavior was investigated in AZ31 alloy with three different mean grain sizes. Single-pass and two-pass friction stir processing (FSP) was employed to obtain fine grain and ultrafine grain microstructures, respectively. For FSP, SSRT specimens were extracted from the processed region. SSRTs were carried out in air and solution at an initial strain rate of 10⁻⁶/s. A significant decrease in the ultimate tensile strength was observed for FSP specimens tested in chloride solution as compared to specimens tested in air. More than 75 % loss in total elongation was observed for the specimens tested in chloride solution as compared to the ones tested in air. In comparison with base material, lower time to failure was observed for processed samples. The higher SCC susceptibility of processed microstructure is attributed to increased hydrogen adsorption and favorable basal texture.     

Fabrication of Al and Al/Ti coatings on magnesium alloy by sputtering

Multi-magnetron sputtering was applied to prepare aluminum coating and aluminum/titanium multilayer coating on AZ31 magnesium alloy. FESEM, AFM and XRD were used to investigate the morphology and phase structure of these obtained coatings. Aluminum coating presented a (111) preferred texture and this texture was strongly strengthened with the Ti(002) plane as template in Al/Ti multilayer coating. The top surface of Al/Ti-coated sample took on a round roof-like morphology compared to the pyramid-like morphology of Al-coated sample. The result of polarization tests showed that both Al coating and Al/Ti multilayer coating could improve the corrosion resistance of AZ31 magnesium alloy in 3.5 wt.% NaCl solution.     

Corrosion behavior of Ti–Al–N/Ti–Al duplex coating on AZ31 magnesium alloy in NaCl aqueous solution

In this study, Ti–Al–N/Ti–Al duplex coating was deposited on AZ31 magnesium alloy by magnetron sputtering with a Ti/Al composite target. Scanning electron microscopy and Auger electron spectroscopy were applied to investigate the morphology and elemental concentration of the obtained coating, respectively. The top layer was Ti–Al–N film with a Ti:Al:O:N ratio of 0.32:0.84:0.08:1, and the bottom layer was Ti–Al film with a Ti:O:Al ratio of 1.94:0.12:1. Each layer of this coating presented a developed columnar structure. The polarization test and immersion test were used to investigate corrosion behavior of the coated sample in 3.5 wt.% NaCl aqueous solution. The results showed that this duplex coating could protect the substrate effectively in NaCl aqueous solution. Nevertheless, several through-thickness micropores in the coating finally induced the failure of the coated AZ31 in the immersion test.     

Cavitation behavior of active thermal-sprayed coatings

As a first stage in the development of active coatings to improve both localized corrosion resistance and cavitation resistance in sea water, we studied flame-sprayed coatings of aluminum, zinc and Zn-15 wt.% Al applied to steel substrates. Cavitation was carried out in distilled water and in 3.5 wt.% NaCl solution, and was characterized by weight loss and by scanning electron microscopy. Open-circuit potentials in 3.5 wt.% NaCl solution were measured in an attempt to monitor internal fracture, which would allow seepage of solution through the coating to the more noble steel substrate.Cavitation damage rates were found to be significantly higher in the saline solution for the alloy and the aluminum coatings, whereas little sustained change in damage rate was found for the zinc coatings. The data indicate that the strength of the particle matrix essentially governs the overall strenght of the coating. However, in saline solution the effects of alloy strengthening are slightly reduced by the tendency of localized corrosion attack to initiate coating flaws and eventual failure.     

Improvement on the Corrosion Resistance of AZ91D Magnesium Alloy by Aluminum Diffusion Coating

By combination of magnetron sputtering deposition and vacuum annealing, an aluminum diffusion coating was prepared on the substrate of AZ91D alloy to improve its corrosion resistance. The microstructure and composition of the diffusion coating was investigated by scanning electron microscopy and X-ray diffraction. The diffusion coating was mainly comprised of β phase-Al12Mg17. The continuous immersion test in 3.5 wt pct neutral NaCl solution indicated that the specimen with diffusion coating had better corrosion resistance compared with the bare AZ91D alloy specimen. The potentiodynamic polarization measurement indicated that the diffusion coating could function as an effectively protective layer to reduce the corrosion rate of AZ91D alloy when exposed to 3.5 wt pct NaCl solution.     

Effect of extrusion ratio on mechanical and corrosion properties of AZ31B alloys prepared by a solid recycling process

Some AZ31B magnesium alloy bars were prepared by a solid recycling process with different extrusion ratios. A reference specimen was processed by extruding an as-received AZ31 ingot. The microstructures, mechanical and corrosion properties of AZ31B magnesium recycled specimens were investigated. With increasing extrusion ratio, the yield strength, tensile strength and yield ratio increases. The reliability of the recycled alloy is poorer than the reference specimen. The corrosion rates of recycled AZ31B magnesium specimens increase immersed in both alkaline and neutral 4% NaCl solution with a decrease extrusion ratio. The corrosion resistance of recycled AZ31B magnesium specimens is improved with increasing pH of immersed solution. The recycled specimens show superior corrosion resistance than reference specimen.     

Microstructure and texture studies on twin-roll cast AZ31 (Mg-3 wt.%Al-1 wt.%Zn) alloy and the effect of thermomechanical processing

The microstructure and texture of the twin-roll cast (TRC) AZ31 (Mg-3 wt.%Al-1 wt.%Zn) sheet, with a thickness of 6 mm, have been investigated. The TRC AZ31 exhibits a dendritic microstructure with columnar and equiaxed grains. These contain Al-Mn and Mg-Al-Zn second-phase particles that are approximately 1 μm in size. This heterogeneous structure is attributed to the effect of the cooling rate, which varies from 325 °C/s on the surface to ∼150 °C/s in the mid-thickness of the sheet. No surface segregation, but a certain degree of macrosegregation is observed in the mid-thickness which persists after annealing and rolling. Recrystallization at 420 °C leads to a bimodal grain-size distribution, while a fine-grain structure is obtained after rolling and annealing. The TRC AZ31 sheet exhibits basal textures in the (i) as-received, (ii) rolled and (iii) rolled-annealed conditions. However, post-annealing of the TRC AZ31 at 420 °C produces a relatively random texture that has not been previously observed in the conventional AZ31 sheet. The texture randomization is attributed to the particle-stimulated nucleation of new grains in the TRC structure. The preliminary evaluation of mechanical properties indicates that such annealing treatment slightly increases the ultimate tensile strength (UTS), but significantly improves elongation.     

Influence of texture on corrosion rate of AZ31 Mg alloy in 3.5 wt.% NaCl

In this study, AZ31 Mg alloys with different grain orientation were prepared to investigate the influence of texture on corrosion rate in 3.5 wt.% NaCl. Experimental results showed that texture had significant influence on the corrosion rate of AZ31 in 3.5% NaCl. The corrosion rate of AZ31 dramatically increased with the (0 0 0 1) texture intensity decreased and the (1 0 ?1 0)/(1 1 ?2 0) texture intensity increased. The corrosion rate reached a maximum at about 3 h immersion and subsequently decreased slightly due to the formation of a Mg(OH)₂ corrosion product layer. This study indicates that the corrosion rate of AZ31 Mg alloy in NaCl solution can be modified to some extend by controlling texture.     

Corrosion of Artificial Aged Magnesium Alloy AZ80 in 3.5 wt pct NaCl Solutions

The corrosion morphologies of aged magnesium alloy AZ80 were investigated by immersion corrosion tests, scanning electron microscopy （SEM）, electrochemical measurement. The T5 heat treatment was carried out in a vacuum furnace, holding for 16 h at 177℃, and then cooling in air. The results showed intergranular corrosion （IGC） occurred as an aged AZ80 sample was immersed in 3.5 wt pct NaCI aqueous solution for 1 h and the narrow path attack progressed predominantly along the bulk β phase in the grain boundaries or took place in the eutectic areas. IGC was attributed to the network distribution of β phase along the grain boundaries, the depleted aluminium in the precipitation areas and the breakdown potential.     

Grain refinement of Mg–Al alloys with Al4C3–SiC/Al master alloy

A novel Al₄C₃–SiC/Al master alloy for grain refinement of Mg–Al–Zn alloys has been developed in the present work. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) results show the existence of Al₄C₃ and SiC particles in this master alloy. The master alloy presents good grain refining efficiency on both AZ31 and AZ63 alloys, but little effect on AZ91 alloy. After addition of 0.5 wt.% Al₄C₃–SiC/Al master alloy, the average grain size of AZ31 and AZ63 decreased dramatically from 1300 to 225 μm, and from 300 to 200 μm, respectively. However, no further refinement of grain size was achieved with additional amount of Al₄C₃–SiC/Al master alloy exceeding 0.5 wt.% for both AZ31 and AZ63 alloys in the present investigation. Duplex phase of Al₄C₃ and SiC was found to be located at the grain center of α-Mg and is proposed to be the nucleating agent during solidification of α-Mg.     

温度对PAAM高吸水树脂吸液与保水性能的影响

研究了自制聚丙烯酸-丙烯酰胺(PAAM)高吸水树脂在不同温度下对蒸馏水0、.9%NaCl溶液的吸液性能和泥土与砂土中自来水的保水性能。当被吸收介质为蒸馏水时,吸水倍率随温度升高而下降;而被吸收介质为0.9%NaC1溶液时,吸液倍率随温度升高而上升。吸液初期,溶液温度越高,PAAM树脂在0.9%NaCl溶液中吸液速率越快。PAAM树脂在高温下的恒温保水性能较好,泥土的保水性能优于砂子;加PAAM树脂的泥土或砂子对自来水的保水率较未加PAAM树脂的更好,干燥更慢;泥土和砂子中PAAM树脂的保水率与恒温时间呈线性关系,可用方程Ri=A-Bt表示。     

Corrosion fatigue behavior of extruded magnesium alloy AZ80-T5 in a 5% NaCl environment

The corrosion fatigue behavior of extruded AZ80-T5 magnesium alloy has been investigated in three different environments: (1) a low humidity environment (35-40% relative humidity), (2) a high-humidity environment (80% relative humidity), and (3) a 5 wt.% NaCl environment. Fatigue tests were conducted under axial loading at a stress ratio of −1 and at a frequency of 20 Hz. It was found that in both the high-humidity environment and in the 5 wt.% NaCl environment the fatigue strength was reduced relative to the low humidity environment, especially in the NaCl environment: the reduction rates of fatigue limit under high humidity and NaCl environments were 18%, and 78%, respectively. The reduction of fatigue strength under the corrosive environments was attributed to the pit formation and growth. At low stress amplitudes, multiple pits were formed and coalesced to form a large pit under NaCl environment. A fatigue crack nucleated when the pit grew to the critical size.     

Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments

Fatigue crack propagation (FCP) was studied on wrought magnesium alloys, AZ31 and AZ61, in laboratory air, dry air and distilled water. In laboratory air, the FCP rate versus stress intensity factor plots consisted of two sections with different slopes, which was clearly recognized after allowing for crack closure. This was attributed to the transition in fracture mechanisms operated. In distilled water, FCP rates were nearly the same as in laboratory air, while in dry air, an order of magnitude slower than in laboratory air and distilled water. After allowing for crack closure, the environmental effects still existed and FCP rates were the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Fractography revealed that the fracture mechanisms operated in laboratory air and in distilled water were different, possibly hydrogen embrittlement and anodic dissolution, respectively.     

Formation by reactive magnetron sputtering of TiN coating on Ti-implanted magnesium alloy

Reactive magnetron sputtering method was used to deposit TiN coating on Ti-implanted magnesium alloy. FESEM, GIXRD and AES were used to investigate this surface system's micro-morphology, crystalline structure and chemical composition, respectively. It showed that TiN coating took on a compact surface morphology and existed as a columnar crystalline structure. The results of the electrochemical test and immersion test showed that the corrosion resistance of AZ31 was greatly improved in 3.5 wt.% NaCl solution by this kind of surface treatment technique. It was mainly due to the combined effect of the dense TiN coating, the Ti-implanted layer and the good adhesion between coating and substrate.     

Corrosion resistance and hydrophobicity of myristic acid modified Mg--Al LDH/Mg(OH)2 steam coating on magnesium alloy AZ31

A hydrophobic surface was successfully fabricated on the Mg–Al-layered double hydroxide (Mg–Al LDH)/Mg(OH)₂-coated AZ31 magnesium alloy via an in-situ steam coating (SC) process and a subsequent surface modification with environment-friendly myristic acid (MA). The microstructure, composition and hydrophobicity of SC/MA composite coating were investigated by XRD, SEM, EDS, FTIR, and contact angle (CA) measurement. The corrosion behavior of the hybrid coating was evaluated by potentiodynamic polarization, EIS and hydrogen evolution test in 3.5 wt.% NaCl solution. The results showed that the LDH coating had nano-flake microstructure, which remained unchanged after modification with MA. The CA of the MA-modified coating surface reached up to 129°±3.5°, and the corrosion current density of SC/MA-2 coating decreased about three orders of the magnitude compared to that of the substrate. It is proven that the modified surface has an effective anti-corrosion effect on AZ31 alloy. The formation mechanism and the corrosion mechanism of the coating were also discussed.     

Improved corrosion resistance of Mg alloy AZ31B induced by selective evaporation of Mg using large pulsed electron beam irradiation

Large pulsed electron beam (LPEB) irradiation was employed as a surface treatment of magnesium (Mg) alloy AZ31B to enhance its corrosion and wear resistance. Selective evaporation of Mg induced by LPEB irradiation at an energy density of 5 J/cm² for 40 cycles has led to the formation of an Al-enriched re-solidified layer with nano-grained structure consisting of Mg_3.1Al_0.9 metastable phase. The formation of such a re-solidified layer after LPEB irradiation has enabled a decrease in corrosion rate of Mg alloy AZ31B in 3.5% NaCl solution. Different equivalent electrical circuit models were proposed to account for the corrosion behavior of untreated Mg alloy AZ31B and those subjected to LPEB irradiation. A decrease in wear depth when compared to that of the untreated alloy suggests an increase in wear resistance of LPEB-irradiated Mg alloy AZ31B. Adhesive wear is the predominant mechanism of untreated Mg alloy AZ31B while abrasive wear mechanism dominates for LPEB-irradiated Mg alloy AZ31B.     

Corrosion fatigue behavior of extruded AZ80, AZ61, and AM60 magnesium alloys in distilled water

Rotary bending fatigue tests were conducted in laboratory air and distilled water using three extruded magnesium (Mg) alloys AZ80, AZ61, and AM60 with different chemical compositions. In laboratory air, the fatigue strengths at high stress levels were similar in all alloys because cracks initiated at Al-Mg intermetallic compounds, whereas AZ80 with the largest Al content exhibited the highest fatigue strength at low stress levels, which was attributed to the crack initiation due to cyclic slip deformation in the matrix microstructure. In distilled water, fatigue strengths were considerably decreased due to the formation of corrosion pits in all alloys, and the difference of fatigue strength at low stress levels among the alloys disappeared, indicating that the addition of Al that improved the fatigue strength in laboratory air was detrimental to corrosion fatigue. __________ Translated from Problemy Prochnosti, No. 1, pp. 141–145, January–February, 2008.     

AZ31镁合金的微弧氧化处理及其对粘接性能的影响

采用正交试验方法对镁合金硅酸盐体系的微弧氧化处理工艺进行优化,采用SEM、EDS、XPS和XRD对微弧氧化膜的微观成分与结构进行了表征,进而分析探讨了电解液成分及氧化参数对镁合金表面膜粘接性能的影响规律。研究结果表明,所研究的几种工艺参数对AZ31镁合金表面的粘接性能的影响程度从小到大排列为:KOH的浓度<频率相似文献