Damage evolution and failure mechanism of thermal barrier coatings under Vickers indentation by using acoustic emission technique

Acoustic emission(AE) technique was adopted to monitor the damage evolution of air plasma-sprayed(APS) yttria-partially-stabilized zirconia(YSZ) thermal barrier coatings(TBCs) during instrumented indentation testing, and then the failure mechanisms were investigated by cluster analysis and wavelet transform methods.The results of cluster analysis showed that there were three classes associated with distinct failure types for the 8YSZ coatings under Vickers indentation. Based on wavelet transform, these three clusters could be clearly distinguished from their dominant frequency bands, which were concentrated on levels A5(0–156.25 kHz), D5(156.25–312.5 kHz) and D4(312.5–625 kHz), respectively. Thus, the failure mechanism of 8YSZ coatings under Vickers indentation could be clarified by the distribution of different failure types in indentation depth. To sum up, as indentation load increases, the 8YSZ coatings can accommodate the indenter by elastic or little plastic deformation, microcracks propagation and then debonding at the splat boundaries. By comparing the distribution of AE signals induced by different failure types in indentation depth for samples with different thermal exposure time, it can be inferred that thermal exposure treatment can accelerate the degradation of APS 8YSZ TBCs.     

Improved dehydriding property of polyvinylpyrrolidone coated Mg-Ni hydrogen storage nano-composite prepared by hydriding combustion synthesis and wet mechanical milling

In this work, polyvinylpyrrolidone(PVP) coated Mg_(95)Ni_5 nano-composites were prepared by hydriding combustion synthesis(HCS) plus wet mechanical milling(WM) with tetrahydrofuran(THF) and donated as WM-x wt% PVP(x = 1, 3, 5 and 7) respectively. The phase compositions, microstructures and dehydriding property, as well as the co-effect of PVP and THF were investigated in detail. XRD results showed that the average crystal size of MgH_2 in the milled Mg_(95)Ni_5 decreased from 23 nm without PVP to 18 nm with 1 wt% PVP. The peak temperature of dehydrogenation of MgH_2 in the milled Mg_(95)Ni_5 decreased from 293.0 ℃ without THF to 250.4 ℃ with THF. The apparent activation energy for decomposition of MgH_2 in WM-7 wt% PVP was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of milled Mg_(95)Ni_5 without THF and PVP. PVP and THF can facilitate the refinement of particle size during mechanical milling process. Attributed to small particle sizes and synergistic effect of PVP and THF, the composites exhibit markedly improved dehydriding properties.     

Surface modification of Ti-45Al-8.5 Nb alloys by microarc oxidation to improve high-temperature oxidation resistance

Microarc oxidation(MAO)electrolysis plasma deposition was used to prepare Al_2O_3coatings on Ti-45Al-8.5 Nb alloys to improve high temperature oxidation resistance.The surface and cross-section morphologies before and after high-temperature oxidation,the chemical composition,and the phase identification of the coatings were investigated by scanning electron microscopy(SEM),electron probe microanalyses(EPMA),and X-ray diffraction(XRD).The results show that Al_2O_3coatings with a thickness of approximately 8μm can be obtained on the Ti-45Al-8.5 Nb alloys by MAO for 600 s.The samples with the Al_2O_3coatings exhibited better high-temperature oxidation resistance.A minimal weight gain of only 0.396 g/m~2after 100 h oxidation at 900°C was observed for the coatings formed with a deposition voltage of 400 V and using a duty cycle of 3%.The deposition mechanism of the Al_2O_3coatings and the effect of the MAO parameters are also described.     

Study on the hydrogen storage property of (TiZr0.1)xCr1.7-yFeyMn0.3 (1.05

Jigang Li  Li Xu  Xiaojing Jiang  Xingguo Li 《自然科学进展(英文版)》2018,28(4):470-477

Mechanical characterization and strain-rate sensitivity measurement of Ti- 7333 alloy based on nanoindentation and crystal plasticity modeling

The relationship between crystal orientations and meso-mechanical properties of β phase Ti-7333 titanium was investigated through the combination of nanoindentation experiments and simulation. The crystal plasticity finite element(CPFE) model for nanoindentation of single body-centered cubic crystal was established based on the experimental data. And the crystal plasticity constitutive law was implemented to simulate the nanoindentation process, obtaining satisfied results with an acceptable error. From the simulated pileup morphology patterns with different crystal orientations, it was found that the β phase experienced a symmetrical and orientation-related deformation process. Meanwhile, the strain-rate sensitivity(SRS) of β phase was investigated through nanoindentation tests based on continuous stiffness measurement(CSM) under different strain rates,varying between 0.2, 0.05 and 0.01 s~(-1). Two grains with different orientations exhibited similar SRS exponents,m, calculated from the experimental results.     

The hidden disintegration of cluster heterogeneity in Fe-based glass-forming alloy melt

The evolution of liquid metal at high temperature is known much less than their solid states. This is partially due to that the message concerning clusters, metastable phase or heterogeneity in liquid is usually too slight to be traced. Here, we shed some light on the nature of structural evolution of Fe-based glass-forming alloy during overheating process by the investigation of high-temperature melt viscometry and first principles simulations. It was found that a structural transition around 1400 ℃ occurred in the melts of initial homogeneous ingot, heterogeneous ingot and amorphous ribbons jointly, and was confirmed by the results from differential scanning calorimeter(DSC), and ab initio molecular dynamics(AIMD). Combining these results with Fe-Si-B ternary phase diagram and the melting characteristics of Fe-B compounds, it is safe to conclude that the disintegration of Fe_2B type clusters to Fe_3B-type clusters leads to the observed transition. These results offer a significant reference for the preparation and property control of Fe-based amorphous alloys.     

从生活到审美——兼论陶渊明之前田园诗赋的流变

描写田园生活的诗赋在《诗经》时代就已大量涌现,不过还只限于描写原生态的生活现象,此后虽有在题材内容、主题内涵及创作方法、艺术表现上发展,但在作品中比兴的作用并无多少改变。从陶渊明始,田园生活才真正上升到审美层面,成为表现社会及人生的主要题材,在艺术表现上则经历了思想感情、艺术风格、表现手法的转变。     

水相法合成巯基乙酸和L-半胱氨酸稳定的CdTe纳米棒

提出一种以水相法合成巯基乙酸和L-半胱氨酸稳定的CdTe纳米棒的简单路线.在pH=11.0的反应条件下,产物的荧光量子效率最高,达到30.67%.考察了反应条件对产物的荧光性能的影响及产物在不同pH溶液中的稳定性.     

Improving the Characteristic of the LaFeO_3 Alcohol Gas Sensor by Using Doping and Surface-Decorating Technique

采用掺杂和表面修饰技术制备ＬａＦｅＯ３乙醇气体传感器，明显地改善了传感器的特性．元件的灵敏度β≥１０（当乙醇气体浓度为２００×１０－６时），乙醇气体和汽油的分离度α为１４．３，在整个相对湿度变化范围内，元件阻值的相对变化在４％～６％之间，元件不通电存放７ｄ，其初期恢复时间为７０ｓ，不通电贮存２７０ｄ，其初期稳定时间小于３ｄ，在使用环境中连续工作９０ｄ其阻值相对变化率小于５％．     

高温干燥对杨木主要力学性能的影响

探讨了高温干燥对木材抗弯强度、抗弯弹性模量、顺纹抗压强度和横纹抗压强度的影响,结果表明,木材经高温干燥后的顺纹抗夺强度和横纹抗压强度大于气干材。木材的变形随干燥温度的增加而减小,强度随干燥温度的增加而增加。