不同pH的碱性环境中16Mn钢及热影响区应力腐蚀行为

利用电化学技术和U形弯试样浸泡实验研究了16Mn钢及其模拟热影响区(HAZ)在不同pH的碱性硫化物和Cl-介质中的应力腐蚀开裂(SCC)行为与机理.结果表明:16Mn钢原始组织、粗晶组织(空冷组织)和硬化组织(淬火组织)在pH为11.8的碱性硫化物环境中均近似呈钝化状态,维钝电流密度依次降低;随着pH的降低,原始组织和粗晶组织的阳极过程逐渐由钝化态转变为活化态;HAZ中硬化组织、粗晶组织和原始组织在碱性硫化物环境下的SCC敏感性依次降低,其中硬化组织具有明显的SCC特征;随着pH的降低,SCC裂纹有从沿晶裂纹转变为穿晶和沿晶混合裂纹的趋势,并且裂纹的宽度增加.     

16Mn钢长轴裂纹缺陷成因分析

某16Mn钢长轴零件经超声波探伤发现存在缺陷,采用化学成分分析、低倍检验、金相检验、断口分析等方法对缺陷的类型和成因进行了分析。结果表明:该零件原材料中存在的非金属夹杂形成了裂纹源,在加工压力的作用下进一步扩展,最终形成了裂纹缺陷。     

超声热成像对含曲率TC4结构表面裂纹的检测仿真

目的 针对含曲率TC4结构的表面裂纹,在分析超声红外热成像检测原理的基础上,进行有限元仿真模拟,研究激励条件对检测效果的影响规律,得到最佳激励参数。方法 首先,设置不同激励幅值、激励频率、激励时间及激励位置等多组方案进行仿真计算。然后基于仿真优化出的最佳激励方案,对含表面裂纹的某航空发动机叶片进行检测,证明仿真结果的准确性。结果 在超声激励过程中,裂纹缺陷区域与非缺陷区域的温差逐渐上升,并在激励结束时达到最大表面温差。随着激励幅值的增加,最大表面温差逐渐上升,上升速率逐渐减小;随着激励频率的增加,最大表面温差逐渐上升,上升速率逐渐增大;随着激励时间的增加,最大表面温差逐渐上升,80 ms后逐渐趋于定值。在裂纹扩展方向上,超声激励施加在构件中央时,检测效果最佳;在垂直于裂纹扩展方向上,超声激励施加在裂纹正下方时,检测效果较差,激励源向两侧移动10~20 mm时,检测效果最佳。通过对含表面裂纹的某航空发动机叶片进行检测试验,可以清晰地看到裂纹信息,检测效果较好,表明仿真结果可靠。结论 研究成果有效地揭示了激励幅值、激励频率、激励时间及激励位置对检测效果的影响规律,为超声红外热成像技术检测含曲率结构表面裂纹的优化奠定了理论基础。     

累积叠轧TC4合金制备超细晶组织的研究

通过累积叠轧技术进行TC4合金超细晶组织的制备,考察了TC4合金的热变形特点以及叠轧工艺窗口,研究了叠轧工艺参数和热处理制度对叠轧板材界面结合和微观组织的影响。结果表明：TC4合金的应力-应变曲线表现为动态回复特征,热模拟条件下在加热温度(≥700℃)和变形速率(≤0.1s-1)下能够实现强烈塑性变形。最终TC4合金进行叠轧界面的防氧化处理后,并在加热温度为720℃、轧制速度小于0.5m/s时,获得良好的界面结合和板材质量。累积叠轧变形过程是α/β协同变形和剪切变形综合作用的结果,组织中存在拉长的条带组织以及大量的剪切带。随着叠轧层数的增加,条带组织的间距逐渐变小同时剪切带组织逐渐增加,在叠轧16层(变形量为92.3%)后条带间距为200nm~500nm之间。热处理过程中随着加热温度的增加,溶质扩散和再结晶过程促进了界面结合并最终与基体保持一致,同时叠轧16层的TC4板材在加热温度700℃、保温时间60min的热处理过程中能够实现完全再结晶,获得晶粒尺寸为300nm~600nm的超细晶组织。     

粒径对庚烷液雾爆炸场影响的数值模拟

为了研究液雾索特直径(D_(32))对爆炸参数的影响,以庚烷为对象,控制液雾总浓度不变(80 g·m~(-3)),改变液雾粒径,数值模拟了爆炸容器内庚烷(C_7H_(16))液雾粒径对爆炸参数的影响。结果表明:庚烷液雾场索特直径D_(32)为0~18.1μm时,最大爆炸压力p_(max)随D_(32)增加逐渐减小,最大值为1.01 MPa,最小值为0.9015 MPa。最大爆炸压力上升速率随D_(32)增大整体呈下降趋势,在0处达到最大值0.37571 MPa·ms~(-1),在18.1μm处达到最小值0.18439 MPa·ms~(-1),但在6.81~12μm发生反向突变,在10.1μm处达到极大值0.34217 MPa·ms~(-1),随后恢复下降趋势。液雾的径向火焰传播速度在径向距离15 cm附近突然增加,在16 cm之后剧烈下降。最大火焰速度随液雾场D_(32)不同而变化,D_(32)=10.1,12.9,18.1,6.81,0μm条件下液雾场的最大火焰速度依次降低,最大值为5.714 m·s~(-1),其后依次为1.737,1.36,1.34,1.27 m·s~(-1)。     

银屑病患者肠道菌群多样性分析:单中心前瞻性研究

目的探讨银屑病患者与健康人群肠道菌群多样性的差异。方法收集2017年5月至2018年6月间在中国医学科学院皮肤病研究所住院治疗的银屑病患者(银屑病组)及同期本院健康体检人群(健康组)的新鲜粪便标本,分析受试者相关临床资料。提取肠道菌群DNA,采用16S r DNA基因扩增和Illumina平台双端2×300策略测序,基于Gold数据库按>97%的相似性聚类操作分类单元(operational taxonomic unit,OTU),对照Silva数据库进行物种注释及分类,各层级样本采用轶和检验分析物种差异; QIIME软件计算α多样性主要指数、β多样性分析,t检验分析指数差异,P<0. 05为差异有统计学意义。相关研究结果通过R及GraphPad Prism作图展示。结果符合入选和排除标准的11例银屑病患者及21例健康对照者入选本研究,两组研究对象的性别构成比、年龄和体质量指数无统计学差异(P均>0. 05)。DNA测序分析显示样本测序覆盖深度> 0. 99。银屑病组肠道菌群的OTU数量(278. 18±89. 75比722. 95±152. 81,t=10. 36,P<0. 01)、赵氏指数(433. 38±147. 47比1156. 08±292. 50,t=9. 291,P<0. 01)、香农指数(3. 56±0. 87比5. 73±0. 78,t=6. 972,P<0. 01)和辛普森指数(0. 79±0. 15比0. 94±0. 04,t=3. 287,P <0. 01)均显著低于健康组。RankAbundance曲线显示银屑病组肠道菌群均匀程度较低。PCoA分析(unweighted)显示银屑病组与健康组在第一主成分(24. 35%)可显著分离,Weighted Uni Frac分析可见银屑病组混杂在健康样本中无法区分,且与银屑病亚型无关。样本聚类分析显示,银屑病组肠道菌群与健康组有一定重叠性,银屑病样本特异性菌群少;在门层级,健康组中可检测到TM7,相对丰度为0. 000 066 9 (0. 000 033 4～0. 000 200 5),而银屑病组仅在个别样本中显示有微量存在,相对丰度为0(P<0. 05);在属层级,银屑病组双歧杆菌属[0. 000 033 4 (0. 000 016 7～0. 000 100 3)比0. 000 401 1 (0. 000 200 5～0. 001 337 0)]、布劳特氏菌属[0. 000 467 9 (0. 000 183 8～0. 000 434 5)比0. 002 206 0 (0. 000 935 9～0. 005 582 0)]、粪球菌属[0. 000 033 4 (0～0. 000 401 1)比0. 000 902 5 (0. 000 334 2～0. 005 315 0)]较健康组显著降低,健康组中的戴阿利斯特杆菌属和嗜血菌属仅在银屑病组个别样本中微量存在,克雷伯氏杆菌属在健康组和银屑病组个别样本中存在,但在银屑病组的相对丰度更低,接近0 (P均<0. 05)。个体样本高丰度菌群分析显示,银屑病组个别样本中多糖及短链脂肪酸代谢相关菌群的相对丰度降低。结论银屑病患者肠道菌群多样性低于健康人群。     

稀土含量对Ti6Al4V钛合金等离子渗氮层组织和摩擦学性能的影响

目的研究稀土含量对Ti6Al4V钛合金表面等离子体渗氮层结构和性能的影响。方法运用等离子表面改性技术对Ti6Al4V(TC4)钛合金进行等离子渗氮处理,渗氮过程中通入不同含量的稀土作为催渗剂,以获得钛合金表面强化层。利用金相显微镜和扫描电子显微镜(SEM)观察渗氮层组织,用X射线衍射仪(XRD)分析渗层相组成,用能谱仪(EDS)检测渗层的化学成分,用维氏显微硬度计测量渗层的显微硬度,用球-盘式摩擦磨损试验机和三维轮廓仪检测渗层的摩擦磨损性能。结果TC4钛合金表面等离子渗氮层结构包括表面化合物层(主要成分为δ-TiN)和扩散层(主要为N原子扩散形成的N-Ti固溶体),加入稀土可以促进N原子向基体的扩散,提高渗氮速度。渗层厚度增加,硬度和耐磨性能提高,扩散层使钛合金基体与化合物层之间的硬度梯度更加平缓。当稀土通入速率为60 mL/min时,渗层厚度可达155μm,表面硬度为1275HV0.05,摩擦系数降到0.27,磨损率明显降低。结论钛合金等离子渗氮过程中加入稀土可以有效提高渗速,改善渗氮层硬度,提高材料表面的耐磨性能。     

Microstructure and high-temperature mechanical properties of laser beam welded TC4/TA15 dissimilar titanium alloy joints

The microstructure evolution and high-temperature mechanical properties of laser beam welded TC4/TA15 dissimilar titanium alloy joints under different welding parameters were studied. The results show that the weld fusion zone of TC4/TA15 dissimilar welded joints consists of coarsened β columnar crystals that contain mainly acicular α′ martensite. The heat affected zone is composed of the initial α phase and the transformed β structure, and the width of heat affected zone on the TA15 side is narrower than that on the TC4 side. With increasing temperature, the yield strength and ultimate tensile strength of the TC4/TA15 dissimilar welded joints decrease and the highest plastic deformation is obtained at 800 °C. The tensile strength of the dissimilar joints with different welding parameters and base material satisfies the following relation (from high to low): TA15 base material > dissimilar joints > TC4 base material. The microhardness of a cross-section of the TC4/TA15 dissimilar joints reaches a maximum at the centre of the weld and is reduced globally after heat treatment, but the microhardness distribution is not changed. An elevated temperature tensile fracture of the dissimilar joints is located on the side of the TC4 base material. Necking occurs during the tensile tests and the fracture characteristics are typical when ductility is present in the material.     

Corrosion and wear properties of micro-arc oxidation treated Ti6Al4V alloy prepared by selective electron beam melting

In order to analyze the effect of voltage during micro-arc oxidation (MAO) on corrosion and wear properties of Ti6Al4V (TC4), the MAO technology was employed to treat TC4 samples fabricated by selective electron beam melting (SEBM) at the voltages of 400, 420 and 450 V. The results show that the metastable anatase phase gradually transforms to rutile phase with oxidation time and temperature increasing. The surface morphology of coating contains numerous micropores with uniform size distribution. Cracks and pores over 10 μm are found on MAO-TC4 sample with applied voltage of 450 V. The thickness of MAO coating is positively correlated with the voltage. The corrosion resistance and wear resistance are related to phase composition, micropore size distribution on the surface and film thickness. When the voltage is 420 V, the coating shows the smallest corrosion current density (0.960×10⁻⁷ A/cm²) and the largest resistance (7.17×10⁵ Ω·cm²). Under the same load condition, the coating exhibits larger friction coefficient and wear loss than the TC4 substrate. With the increase of voltage, the wear mechanism of the coating changes from abrasive wear to adhesive wear, and the adhesive wear is intensified at applied voltage of 450 V, with a maximum friction coefficient of 0.821.     

Dissemination strategies and the adoption of improved agricultural technologies: The case of improved cassava varieties in Ghana

Cassava (Manihot esculanta Crantz) is an important tropical root crop for food security and national economies in Africa. In Ghana, it is the most important staple food also used in breweries, bakery and confectionery industries. Several high-yielding and disease resistant improved cassava varieties (ICVs) have been released and are being promoted through diverse dissemination channels. Past studies investigate the role of information in the adoption of agricultural technologies without accounting for the diversity in the dissemination strategies. The paper explores the diversity of dissemination strategies in examining the adoption of improved cassava varieties using a sequential mixed-method approach which combines exploratory interviews and focus group discussions with a questionnaire-based quantitative survey and the Cragg's two-part model. The paper uses data collected from 608 farmers randomly selected from major cassava producing communities in the Ashanti and Brong-Ahafo regions in Ghana, West Africa. The results show statistically significant and identical impact of information through demonstrations, distribution of planting materials, farmer-to-farmer and media on the probability of ICV adoption. Other drivers of adoption are the membership of farmers based organizations (FBO), planting of two or more varieties and having large livestock size. However, ICV adoption and intensity was hampered by household size, distances to the nearest tarred road and market, and grey-skin color. These attributes need to be integral of dissemination campaigns for cassava variety adoption.