Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873–923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.     

Evolution of dislocation patterns in a tricrystal model subjected to cyclic loading

When a polycrystalline aggregate is deformed beyond the elastic range, dislocations pile up at grain boundaries and make some patterned structures within the grains. If the external load is reversed, most dislocations at grain boundaries and in the patterned structure are supposed to disappear or change their arrangements, but the details are not yet well known. In this study, we examined such changes in dislocation structures by a crystal plasticity analysis. Models for the polycrystalline aggregates consist of three grains and the generation and degeneration of the geometrically necessary dislocations during cyclic loading are examined in detail. The results show that there are some groups of dislocations that do not dissipate but evolve upon reverse loading.     

Observation of dielectric relaxation modes in chiral smectic C liquid crystal mixture

The dielectric parameters in the frequency range 50 Hz to 1 MHz have been measured in a SmC* liquid crystal mixture in planar alignment. The relaxation frequency (f_r), distribution parameter (α) and dielectric strength (δε) have been evaluated and their corresponding values were found to be 206.62 Hz, 0.06 and 114.6 respectively. The Goldstone mode, domain mode, soft mode and a new relaxation mode have been observed.     

奥氏体不锈钢焊缝中的声学仿真模型与全聚焦成像检测

声波在奥氏体不锈钢焊缝中传播时声束弯曲,为超声成像带来了困难。基于Ogilvy焊缝模型,建立了奥氏体不锈钢焊缝非均匀各向异性声场仿真模型,采用Dijkstra路径搜索算法对各向异性条件下的声传播路径和声传播时间进行了数值模拟和分析。在此基础上,采用Verasonics超声相控阵成像系统,进行了奥氏体不锈钢焊缝的全聚焦成像实验,采集全矩阵回波数据,并结合理论模型计算的声传播路径和相应的传播时间,进行了成像结果修正。结果表明,与均匀介质模型的全聚焦成像结果相比,基于该文模型的焊缝全聚焦成像检测结果具有更高的缺陷定位精度和分辨率,验证了该方法的可行性,为奥氏体不锈钢焊缝成像检测提供了新的思路。     

Microstructure and corrosion behavior of austenitic stainless steel treated with laser

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO₂ laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of γ dendrite surrounded by a network of eutectic structures (γ+carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I_corr and E_corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.     

316L不锈钢在静态液态锂中的腐蚀行为研究

利用脉冲激光诱导击穿光谱仪(LIBS)结合SEM、EDX和维氏硬度(HV)等分析手段,分析了316L不锈钢在350℃的液态锂中静态腐蚀500h后的腐蚀特性。微观形貌分析表明：样品存在较明显的晶间腐蚀,其腐蚀层厚度约为2.7μm。硬度分析表明：腐蚀后表面硬度值约HV234.72,约是腐蚀前硬度值的0.78倍。LIBS组分分析表明：经液态锂腐蚀后的样品表面Ni、Cr等元素含量相对减少;在被腐蚀区域Li元素含量随激光击打深度的增加而减少,而Ni、Cr等元素含量与之近似呈成正比递增;母材区Li元素的渗透深度约3.5μm,约是焊缝区的0.6倍。     

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利用脉冲激光诱导击穿光谱仪(LIBS)结合SEM、EDX和维氏硬度(HV)等分析手段,分析了316L不锈钢在350℃的液态锂中静态腐蚀500h后的腐蚀特性.微观形貌分析表明:样品存在较明显的晶间腐蚀,其腐蚀层厚度约为2.7μm.硬度分析表明:腐蚀后表面硬度值约HV234.72,约是腐蚀前硬度值的0.78倍.LIBS组分分析表明:经液态锂腐蚀后的样品表面Ni、Cr等元素含量相对减少;在被腐蚀区域Li元素含量随激光击打深度的增加而减少,而Ni、Cr等元素含量与之近似呈成正比递增;母材区Li元素的渗透深度约3.5μm,约是焊缝区的0.6倍.     

Role of hydrogen in stress corrosion cracking of austenitic stainless steels

Effects of hydrogen charging on the stress corrosion cracking (SCC) behavior of 304 and 310 stainless steels under sustained load were investigated in boiling 42% MgCl₂ solution. The cracking was accelerated by the incorporation of hydrogen into the steel without altering the crack growth mechanism. The fact that the active dissolution is almost unaffected by the hydrogen charging and tensile stress indicates that the phenomenon of hydrogen-promoted SCC is unlikely a result of hydrogen-facilitated active dissolution. In contrast, hydrogen significantly promotes anodic dissolution in the potential range where the active-to-passive transition occurs. The electrochemical noise detected in the SCC process implies that the crack propagation process is discontinuous and hydrogen charging can raise the frequency of film breakdown at the crack tip. These observations suggest that the hydrogen-promoted SCC may result from the hydrogen-induced passivity degradation.     

孪晶对Be材料冲击加-卸载动力学影响的数值模拟研究

在高压、高应变率加载条件下,孪晶变形对材料的塑性变形具有重要的贡献,而目前孪晶对金属材料的动态屈服强度、冲击响应等的影响还没有被充分揭示.为此,本文考虑孪晶变形和晶粒碎化,针对铍(Be)材料在高应变率加载下的动态力学响应发展了含孪晶的热弹-黏塑性晶体塑性模型.经过和实验结果的对比,发现该模型可以更准确地预测Be材料在动态加载下,尤其是高压动态加载下的屈服强度.进一步,基于该塑性模型研究了Be材料在冲击加载下的准弹性卸载行为,结果表明剪切波速随着压力和剪应变的变化而发生变化是材料产生准弹性卸载现象的主要原因.此外,研究了冲击波卸载过程中Be材料孪晶的演化过程,发现Be材料卸载过程中也伴随着孪晶的产生.     

静态腐蚀条件下铁素体/马氏体钢和奥氏体不锈钢与液态铅铋合金相容性研究进展

液态铅铋合金（LBE）是铅冷快中子反应堆（LFR）和加速器驱动次临界系统(ADS)的主要冷却剂材料。反应堆用结构材料（如铁素体/马氏体钢、奥氏体不锈钢等）在液态LBE环境下存在液态金属腐蚀（LMC）和应力腐蚀的问题,这些问题给钢结构材料的安全服役带来隐患。阐述了钢材铅铋腐蚀类型及机理,归纳了材料设计与处理(元素成分、热处理、加工制造和表面处理)和腐蚀条件（氧质量分数、腐蚀温度和腐蚀时间）对钢材铅铋腐蚀行为的影响机制;澄清了LBE环境下的应力腐蚀与金属脆化机制,总结了内外因素（材料种类、表面缺陷、热处理、氧质量分数、腐蚀温度和拉伸速率）对钢材拉伸性能的影响,并展望了未来铅铋反应堆结构材料的研究方向。建议面向未来的铅铋堆用钢应优化材料设计和处理方式（提高Si、Al等元素的含量、表面镀膜和热处理）同时控制LBE中环境参数（温度、氧质量分数和腐蚀时间）以提高钢材的耐铅铋腐蚀性能。     

Crystal plasticity and multiscale modelling of superalloy creep

A crystal plasticity approach for superalloy creep has been presented which employs a finite element-based representative volume element (RVE) methodology. The γ channels are assumed to undergo crystal slip and the γ′ particles to deform elastically. A range of superalloys has been studied. Thermocalc computations provide the γ′ volume fraction and an automated scheme for generating the resulting RVE has been developed. It has been shown that primary creep response in a wide range of superalloys over high stress, low temperature regimes is represented excellently by the model, by determination of just an activation energy and an alloying element density. It has been hypothesised that the transition from primary to secondary creep results from the development of geometrically necessary dislocations within the γ channels at the γ′ interfaces. Without the need of further material parameters, it has been shown that secondary creep rates over a broad range of stress and temperature can be accurately predicted, hence supporting the hypothesis. An empirical relationship has been established between the alloying element density and the atomic weight percentages of the alloying elements, using a range of superalloy data. It is hypothesised that a role of the alloying elements within the γ channels is to act as inhibitors of ribbon dislocation motion, hence leading to the large range of macro-level primary and secondary creep responses observed in the alloys with variations in constituent alloys. The empirical relationship established, when combined with the crystal RVE methodology, then allows the prediction of superalloy creep rates from knowledge of alloying constituents.     

Structural properties of austenitic stainless steel in the initial state and after rf plasma nitriding

Mössbauer spectroscopy, TEM, electrical resistivity and magnetic measurements are used for investigation of structural changes of X5CrNi189 austenitic stainless steel sheet induced by rf plasma nitriding carried out at 400°C for 24 h. The initial structure formed by small grains of austenite and α′-martensite changes after nitriding into expanded austenite close to the surface and austenite with minor contents of ferrite/CrN in the bulk.     

Morphological and compositional analysis of passive film on austenitic stainless steel in nitric acid medium

Passive film properties of type 304L stainless steel in nitric acid medium are investigated in both ex situ and in situ conditions. Ex situ results revealed that variation in passive film morphology occurs depending upon the concentration and time of immersion. In situ surface morphological investigation showed formation of platelet like structures at lower concentrations (0.1 M, 0.5 M), and towards higher concentration (0.6 M, 1 M) the platelet like structures got agglomerated, homogenized and started depleting from the surface leading to opening up of oxide boundaries. Compositional analysis of the passive film revealed duplex nature at lower concentration consisting of hydroxide and oxide layer, and with increasing concentration oxide layer predominates over the surface.     

Investigation of defect structures in deuteron irradiated and deformed stainless steel 316 by positron annihilation

Abstract

Deuteron-irradiated and deformed stainless steel specimens were investigated by positron lifetime and Doppler broadening measurements. The evolution of the defect structures was studied as a function of the isochronal annealing temperature and for various degrees of deformation. A different behaviour was observed for deformed and irradiated stainless steel specimens. Evidence for vacancy clusters was found in the deuteron-irradiated steel. These clusters disappear after annealing around 900 K.     

Characterization of Laves phase in Crofer 22 H stainless steel

This study investigated the effect of annealing temperature on the precipitation behavior of Crofer^® 22 H at 600 °C, 700 °C, and 800 °C. The grain size distribution, precipitate phase identification, and microstructure were analyzed using electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS). The morphology of Laves phase (Fe,Cr,Si)₂(Nb,W) precipitates having the Cr₂Nb structure changed from strip-like to needle-shaped as the annealing temperature was increased. The precipitates of the Laves phase also shifted from the grain boundaries to the grain interiors when the temperature was increased. However, the average grain size (150 μm) of the ferritic matrix did not significantly change at 600 °C, 700 °C, and 800 °C for 10 h.     

铸造奥氏体不锈钢中铁素体与奥氏体位向关系及其对声衰减的影响

研究了铸造奥氏体不锈钢中铁素体与奥氏体位向关系及其对超声散射衰减的影响.利用电子背散射衍射技术表征了两相的晶体取向及其位向关系,基于真实的铁素体形貌建立了二维声传播各向异性模型并利用时域有限差分法进行了计算,分析了不同位向关系、铁素体形貌特征对声衰减系数的影响规律并进行了实验验证.结果表明：铸造奥氏体不锈钢奥氏体晶粒中散布着形状复杂的铁素体,典型铁素体形貌为条状和岛状;铁素体与奥氏体的位向关系以Kurdjumov-Sachs关系为主,少量满足Nishiyama-Wassermann关系.对声传播过程进行计算,发现两相位向关系和铁素体形貌协同作用影响超声波传播,在较高检测频率（15 MHz）下对散射衰减的影响不能忽略.结合“原位”实验对奥氏体<101>柱状晶粒的声衰减影响因素进行了定量分析,发现对于单一铸造奥氏体晶粒,晶粒内部取向不均匀性、奥氏体-铁素体位向关系以及奥氏体晶粒内铁素体形态都是超声散射衰减的主要原因.     

Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10~(-4)s~(-1)to 1 s~(-1), the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions(n_2and n_1) and the transition strain(εL) decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.     

Evolution and characterization of dynamically recrystallized microstructure in a titanium-modified austenitic stainless steel using ultrasonic and EBSD techniques

A C-scan ultrasonic imaging system was used to investigate the microstructural evolution during dynamic recrystallization (DRX) of a 15Cr–15Ni–2.2Mo–Ti modified austenitic stainless steel (alloy D9). Four specimens were forged at 1273 K to different strains in the range 0.1–0.5. Specimens with true strains of 0.2 or lower did not show any variation in the amplitude of the first back-wall echo. However, a visible variation in the C-scan image was observed at and above the 0.3 strain level. This variation was attributed to the evolution of fine grains. The formation of fine grains was related to DRX, as indicated by electron backscattered diffraction. This study also revealed the characteristics of the DRX or ‘necklace grains’, as opposed to the so-called parent grains or rest of the microstructure.