Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Abstract

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20?000?h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.     

Effects of QD concentrations and thermal annealing on the performance of self-assembly QD-OLEDs

We have fabricated and investigated the effect of CdSe/ZnS quantum dot (QD) concentrations on the performance of self-assembly hybrid inorganic/organic light emitting diodes (QD-OLEDs). The uniform distribution of QDs with controllable density was achieved using the conventional spin-coating method. There was a QD threshold concentration for the emission of QDs in QD-OLED. Below that threshold concentration we did not observe the QD emission from the QD-OLED. The best performance of QD-OLED was found for the QD concentration of -9 x 10(11) cm(-2). The QD electroluminescence intensity was increased about three times after the annealing of QD-OLED at 80 degrees C for about 20 minutes in nitrogen atmosphere. The QD electroluminescence peak energy was remain same before and after the annealing of QD-OLED. The maximum external quantum efficiency was around 2.1%. The effect of process parameters and the QD emission mechanism are discussed.     

Effects of thermal exposure on cyclic deformation and fracture behavior of Ti600 titanium alloy

Effects of thermal exposure on cyclic deformation and fracture behavior of Ti600 alloy were investigated by laser scanning confocal microscope (LSCM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results demonstrated that both the nonthermal exposure (NTE) specimens and the thermal exposure (TE) specimens showed the cyclic softening, within a total strain amplitude range from ±0.45% to ±1.00%. During thermal exposure, since the harder α₂ (Ti₃Al) phase precipitated in the α_p (primary α) phase, the resistance of crack propagation of α_p phase could be increased by the precipitation of α₂ phase. Therefore, the fracture behavior of TE specimens is different with that of NTE specimens. For the NTE and TE specimens, the crack mainly passes through the α_p phase with “cutting” and “bypass”, respectively.     

Effects of SWCNTs on mechanical and thermal performance of epoxy at elevated temperatures

A property which limits the breadth of application of thermoset polymers and their composites is their relatively low maximum operating temperatures. This work investigates the potential application of both functionalized single-walled carbon nanotubes (f-SWCNTs) based on negative charging, and unfunctionalized SWCNTs (u-SWCNTs) to increase the mechanical and thermal performance of a high-temperature aerospace-grade epoxy with a glass transition temperature of approximately 270 °C. Thermal and mechanical properties of the baseline epoxy and nanocomposites containing a low content of SWCNTs (0.2 % by weight) were characterized through thermogravimetric analyses, tensile tests, and dynamic mechanical analyses. Tensile tests were performed both at room temperature and at 80 °C. Further, room temperature tensile tests were performed on untreated and heat-treated specimens. The heat treatment was performed at 300 °C, slightly above the resin glass transition temperature. Results demonstrate that f-SWCNTs are effective in improving the mechanical and thermal performance of the epoxy. No significant improvement was observed for u-SWCNT nanocomposites. For the nanocomposite with f-SWCNTs, the ultimate tensile strength and strain to failure at room temperature (80 °C) increased by 20 % (8 %) and 71 % (77 %), respectively, as compared to the baseline epoxy. The f-SWCNT nanocomposite, unlike other examined materials, exhibited a stress–strain necking behavior at 80 °C, an indication of increased ductility. After heat treatment, these properties further improved relative to the neat epoxy (160 % increase in ultimate tensile strength and 270 % increase in strain to failure). This work suggests the potential to utilize f-SWCNTs based on negative charging to enhance high-temperature thermoset performance.     

原位在线监测多因素协同对玻璃纤维/环氧树脂复合材料热老化性能的影响

针对玻璃纤维增强聚合物(GFRP)复合材料作为火电烟囱内衬的服役老化问题,以玻璃纤维/环氧树脂(GF/EP)复合材料为研究对象,用正交试验法研究温度、偶联剂含量和热流老化时间等因素对GF/EP复合材料热损伤后的质量损失率、弯曲强度和剪切性能的影响。采用金相显微图像处理法测量计算GF/EP复合材料的孔隙率,使用自主设计并搭建的原位在线监测系统对GF/EP复合材料进行测试。结果表明,不同因素对GF/EP复合材料性能的影响程度不同。偶联剂含量的增加会有限改善GF/EP复合材料的质量损失率,而温度因素对复合材料弯曲强度的影响较大,复合材料本身存在的后固化行为会影响弯曲性能的变化趋势,随温度升高弯曲强度总体下降了11.8%。GF/EP复合材料的层间剪切强度与热老化时间密切相关,16 h相比8 h热流老化后的层间剪切强度均值提高了10.2%。      

环氧树脂粘合剂热氧老化行为研究

研究了环氧树脂粘合剂的热氧老化行为,并用热失重分析仪(TG)和傅立叶红外光谱仪(FTIR)分析了其热氧老化的机理.结果表明,环氧胶接接头的剪切强度随着老化时间的增加,呈现出先增加后下降的趋势,并且其下降幅度随老化温度的增加而增大;在较低温度条件下粘合剂的失重是由脱湿或试样中低分子物的挥发造成的,而在较高温度条件下,粘合剂的失重主要是由老化过程中粘合剂分解产生的低分子量挥发物造成的;空气中的氧气是影响粘合剂热降解的重要因素.     

Effects of nozzle shape on the interruption performance of thermal puffer-type gas circuit breakers

During the last decade the advanced interruption techniques, which use the arc energy itself to increase the pressure inside a chamber by PTFE nozzle ablation, have displaced the puffer circuit breakers due to reduced driving forces and better maintainability. In this paper, we have investigated thermal flow characteristics inside a thermal puffer-type gas circuit breaker (GCB) by solving the Navier-Stokes equations coupled with Maxwell's equations for considering all instability effects such as turbulence and Lorentz forces by transient arc plasmas. These relative inexpensive computer simulations might help the engineer researching and designing new advanced interrupters in order to downscale and uprate high-voltage gas-insulated switchgear (GIS) integral.     

Scanning electron microscopy studies on failure of filled polychloroprene

Scanning electron microscopy (SEM) studies of the effect of particulate fillers (silica and carbon black) and short fibres (silk) on the fracture surface morphology of polychloroprene vulcanizates failed under tension, tear, abrasion and flexing have been made. It has been observed that the type of failure testing and the nature of the filler cause drastic changes in the fractographs. An attempt has been made to correlate these changes with the strength of polychloroprene vulcanizates.     

聚氯丁二烯橡胶改性聚酰胺研究

以氯丁橡胶、尼龙66为主要原料,采用双面涂氟丁橡胶胶乳于尼龙布上的涂覆工艺,硫化后研制出一种复合材料,经测试这种复合材料具有比纯尼龙布拉伸强度高,撕裂强度高等特点.     

时效对2524铝合金热稳定性的影响

研究了2524铝合金自然时效和高温短时间时效组织的热稳定性.结果表明,2524铝合金自然时效试样的抗拉强度在热暴露过程中有较大的提高,而高温短时人工时效的试样强度几乎不变.2524铝合金自然时效态试样在热暴露中析出S'相的速度较快、数量较多,而高温短时人工时效态试样S'相的析出速度较慢、数量较少.2524铝合金高温短时间人工时效合金中形成的GPB区尺寸大于自然时效合金中的GPB区,而在热暴露过程中尺寸较大的GPB区比尺寸较小的GPB区较难回溶,因此高温短时人工时效获得的GPB组织比自然时效获得的GPB区稳定性更高.     

Effect of aging on fracture behaviour of cast stainless steel and weldments

Abstract

An investigation has been undertaken to determine the magnitude of any reduction in properties that may occur in cast duplex stainless steels and weldments during long term exposure to reactor operating conditions. Test panels were fabricated in CF3 stainless steel using a manual metal arc (MMA) process and 19.9.L consumables. The mechanical properties of the parent material and weldments were measured following accelerated aging at 375 and 400°C for up to 20 000 h. Following aging at temperatures up to 400°C, reductions in both the Charpy impact and J integral–crack growth resistance R (J–R) fracture toughness of CF3 cast austenitic steel and 19.9.L austenitic weld metal were observed. For conditions equivalent to the proposed end of life for UK pressurised water reactors, the J–R fracture toughness at 300°C of both cast steel and MMA weld metal was reduced by ~30% for crack extensions of ≥1 mm. Hence, it is important that these reductions in weld metal toughness are taken into account during the development of safety cases and structural integrity assessments for any component in the primary loop that contains MMA stainless steel weldments.

MST/1198     

Effect of aging treatment on fracture toughness in ZM61 magnesium alloy

The effect of aging treatment on fracture toughness in Mg–6Zn–1Mn (wt-%) was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, uniaxial tensile and fracture toughness tests, respectively. The results showed that the fracture toughness of Mg–6Zn–1Mn alloy can be enhanced by aging treatment. The fracture toughness and strength showed a reverse trend in single aged and double aged alloy. Synergetic effect of fine grains and precipitates improved the fracture toughness more sharply than aging treatment. The precipitate free zones and grain boundary precipitates made the largest contribution to the reduction of toughness. Under as extruded and aged conditions, the main origins of cracks were elastic incompatibility and plastic deformation.     

Effects of atoms on brittle fracture

This article aims to answer two related sets of questions. First: in principle, how large an effect can structure at the atomic scale have upon the fracture of two macroscopically identical samples? The answer to this question is that the effects can be very large. Perfectly sharp cracks can be pinned and stationary under loading conditions that put them far beyond the Griffith point. Crack paths need not obey the rule K _II=0. Crack speeds can vary from zero to the Rayleigh wave speed under identical loading conditions but depending upon microscopic rules. These conclusions are obtained from simple solvable models, and from techniques that make it possible to extrapolate reliably from small numerical calculations to the macroscopic limit. These techniques are described in some detail. Second: in practice, should any of these effects be visible in real laboratory samples? The answer to this second question is less clear. The qualitative phenomena exhibited by simple models are observed routinely in the fracture of brittle crystals. However, the correspondence between computations in perfect two-dimensional numerical samples at zero temperature and imperfect three-dimensional laboratory specimens at nonzero temperature is not simple. This paper reports on computations involving nonzero temperature, and irregular crack motion that indicate both strengths and weaknesses of two-dimensional microscopic modeling.     

Effect of filler on thermal aging of composites for next-generation power lines

The thermal aging of pultruded composite rods was investigated to determine the effects of filler on oxidation kinetics and degradation mechanisms. The unidirectional hybrid composite rods were comprised of a carbon-fiber core, a glass-fiber shell, and an epoxy matrix filled with clay particles. A reaction-diffusion model was implemented for each of the two hybrid sections to calculate the oxygen-concentration profile and the thickness of the oxidized layer (TOL) within the composite rods, and results were compared with measured oxidation kinetics. The TOL was measured for samples exposed isothermally in air and in vacuum at 200 °C for up to 13,104 h (1.5 year), and the measured values were similar to modeling predictions (within 10%). The domain validity for the reaction-diffusion model was determined from gravimetric experiments (weight-loss measurement), which showed that after prolonged thermal exposure, the degradation mechanism changed from thermal oxidation to thermal degradation. Thermogravimetric analysis (TGA) was performed to determine the thermal degradation and stability of the aged composite. In addition, the effect of thermal aging on glass transition temperature (T_g) and short beam shear (SBS) strength was determined for isothermal exposures at 180 °C and 200 °C.     

Effects of vacuum thermal cycling on mechanical and physical properties of high performance carbon/bismaleimide composite

The aim of this article was to investigate the effects of vacuum thermal cycling on mechanical and physical properties of high performance carbon/bismaleimide (BMI) composites used in aerospace. The changes in dynamic mechanical properties and thermal stability were characterized by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA), respectively. The changes in linear coefficient of thermal expansion (CTE) were measured in directions perpendicular and parallel to the fiber direction, respectively. The outgassing behavior of the composites were examined. The evolution of surface morphology and surface roughness were observed by atomic force microscopy (AFM). Changes in mechanical properties including transverse tensile strength, flexural strength and interlaminar shear strength (ILSS) were measured. The results indicated that the vacuum thermal cycling could improve the crosslinking degree and the thermal stability of resin matrix to a certain extent, and induce matrix outgassing and thermal stress, thereby leading to the mass loss and the interfacial debonding of the composite. The degradation in transverse tensile strength was caused by joint effects of the matrix outgassing and the interfacial debonding, while the changes in flexural strength and ILSS were affected by a competing effect between the crosslinking degree of resin matrix and the fiber-matrix debonding.     

老化对改性沥青微观结构及疲劳性能的影响

为了研究老化对改性沥青微观结构及疲劳性能的影响,通过沥青常规指标测试试验、动态剪切流变（DSR）试验和AFM测试试验,从宏细观角度分析了老化前后苯乙烯-丁二烯-苯乙烯三嵌段共聚物（SBS）改性沥青和胶粉改性沥青的针入度、延度和软化点及疲劳性能、微观结构的变化情况。结果表明：随着老化程度的加深,两种改性沥青的稠度、硬度增加,高温性能得到改善,而温度敏感性变低,低温抗裂性能变差。综合分析针入度比、延度比和软化点比得出胶粉改性沥青的抗老化性能优于SBS改性沥青;随温度的降低、频率的增大和老化程度的加深,两种改性沥青的抗疲劳性能变差;微观结构观测结果显示,SBS改性沥青具有"蜂型结构",老化后"蜂型结构"的体积增大、高度增加、数量减少;胶粉改性沥青没有"蜂型结构",胶粉颗粒与沥青在共混共融过程中发生溶胀、脱硫和降解等行为会影响沥青中"蜂型结构"的形成;老化前后胶粉改性沥青表面形貌粗糙度和高度变化不大;胶粉改性沥青的抗疲劳、抗老化和高温性能总体优于SBS改性沥青。     

Effects of thermal annealing on the performance of Al/ZnO nanorods/Pt structure ultraviolet photodetector

ZnO nanorod arrays were fabricated on ZnO coated glass substrate by hydrothermal method. Schottky barrier ultraviolet photodetectors (PDs) were obtained by sputtering Pt electrode and evaporating Al electrode on the top of ZnO nanorod arrays with thermal treatment. It is illustrated that Schottky contacts at the electrode/ZnO NRs interface were formed at the annealing temperature of 300 °C and above. When annealing temperature was up to 300 °C, the performance of the PDs was improved with the great decrease of response and recovery times. At the forward bias of 2 V, the Schottky contact PDs showed the biggest responsivity and the best detectivity at the annealing temperature of 300 °C. For annealing temperature at 300 °C and above, the responsivity decreases with increasing annealing temperature and the ratio of detectivity (D₂₅₄^* to D₅₄₆^*) was calculated as high as 10³ for all PDs annealed at 300 °C and above.     

Effect of aging on high temperature brittle intergranular fracture in austenitic stainless steels

Abstract

The present paper describes the effect of aging on crack growth at 550–750°C in a series of 316 and 347 based stainless steels. Crack initiation parameters and crack growth rates have been measured, and detailed fractography and microstructural characterisation carried out. The study shows that the high temperature brittle intergranular fracture mechanism operates in these alloys, as expected from incidences of cracking in austenitic stainless steels used in power plant. High temperature brittle intergranular fracture leads to lower crack tip opening displacements at initiation, and slightly higher crack growth rates than ductile intergranular failure. Susceptibility to high temperature brittle intergranular fracture is enhanced by aging. This increased susceptibility is explained in terms of the increased hardness, the reduction in dissolved C, and grain boundary precipitation. The effects of temperature, composition, and loading mode on the behaviour of the aged alloys are determined.

MST/3100