Carbon segregation and inclusions effects on surface fracture morphology of a 12% chromium stainless steel

Martensitic 12% chromium stainless steel is generally used for the manufacture of water vapour turbine blades. This material, under these environmental conditions, develops fatigue corrosion with failure as a result of the segregation of certain constituent elements such as phosphorus (P) and sulphur (S),^[1–3] or the presence of some types of inclusions.^[2–4] To be able to understand and explain these phenomena, in situ characterization of the fractured surfaces were performed for two types of samples: steel 1 as manufactured turbine material whose fracture mode is intergranular and steel 2 issued from last stage turbine blades after 100 000 h service at 40 °C whose fracture mode is transgranular. The techniques used for characterization were scanning electron microscopy (SEM) coupled with the x-ray analysis by energy dispersive spectroscopy (EDS), and auger electron spectroscopy (AES). The Auger results enabled the understanding of the brittle to ductile transition for the material by showing the simultaneous diffusion of carbon from grain boundaries (GB) to grains (G) and chromium from G to GB. Furthermore, the heavy segregation of phosphorus at the GBs could explain the intergranular crack rupture traces observed in steel 2. SEM observations coupled with EDS analysis showed the presence of different types of non-metallic inclusions such as silicon-based complex inclusions and manganese sulfides (MnS). The presence of silicon-based complex inclusions at GB could explain the intergranular fracture mode previously reported. The characterization of the fracture appearance suggests also that MnS inclusions can act as nucleation sites for secondary microcracks at the GB level that were observed after service.     

Enhanced oxidation of the 9%Cr steel P91 in water vapour containing environments

The short term (∼100 h) oxidation behaviour of the 9%Cr steel P91 was studied at 650 °C in N₂-O₂-H₂O gas mixtures containing a relatively low oxygen level of 1%. The oxidation kinetics were measured thermogravimetrically and the oxide scale growth mechanisms were studied using H₂¹⁸O-tracer with subsequent analyses of oxide scale composition and tracer distribution by MCs⁺-SIMS depth profiling. The corrosion products were additionally characterised by light optical microscopy, SEM-EDX and XRD. It was found that the transition from protective, Cr-rich oxide formation into non-protective mixed oxide scales is governed by the ratio H₂O^(g)/O₂ ratio rather than the absolute level of H₂O^(g). The results of the tracer studies in combination with the data obtained from experiments involving in situ gas changes clearly illustrated that under the prevailing conditions the penetration of water vapour molecules triggers the enhanced oxidation and sustains the high growth rates of the poorly protective Fe-rich oxide scale formed in atmospheres with high H₂O^(g)/O₂ ratios. The experimental observations can be explained if one assumes the scale growth to be governed by a competitive adsorption of oxygen and water vapour molecules on external and internal surfaces of the oxide scales in combination with the formation of a volatile Fe-hydroxide during transient oxidation. The formation of the non-protective Fe-rich oxide scales is suppressed in atmospheres with low H₂O^(g)/O₂ -ratios, and the healing of any such scale is promoted.     

原位反应自发渗透法TiC／AZ91D镁基复合材料及AZ91D镁合金的拉伸变形与断裂行为

利用原位反应自发渗透技术合成了47．5％碳化钛TiC（体积分数，下同）增强AZ91D镁基复合材料，对比研究了该复合材料与铸态镁合金AZ91D基体的室温与高温拉伸变形行为，观察了拉伸断口微观组织形貌，并分析了这两种材料的断裂特征。结果表明，TiC／Mg复合材料具有良好的高温力学性能，在拉伸变形速率为0．001s^-1以及温度为723K，时其拉伸强度可达91．1MPa，而此时相同变形条件下的铸态AZ91D镁合金拉伸断裂强度只有41．1MPa，增幅达120％。而在室温下，镁基复合材料的拉伸断裂强度仅高出基体铸态镁合金23．4％。镁基复合材料的断裂应变较低，高低温时均表现为脆性断裂；而镁合金则由室温下的脆性断裂向高温下的韧性断裂过渡。     

1.4%C超高碳钢显微组织与疲劳性能的研究

对1．4％C超高碳钢试样热处理后的疲劳极限与断裂韧性进行测定，观察了显微组织与试样断口。结果表明，经950℃淬火700℃回火热处理后，又经860℃正火处理，试验用钢的组织结构超细化且碳化物弥散均匀分布，获得良好的综合强度与韧性及较高的疲劳极限，断裂强度较40CrNiMo调质态提高48％，屈服强度提高15％，延性指标相当，是一种优良的结构钢材料。     

工艺参数对微区沉淀法制备纳米MgAl2O4粉体的影响

采用高分子网络微区沉淀法制备纳米镁铝尖晶石（MgAl2O4）粉体，并采用TGA／DTA、XRD和TEM等分析测试技术对前驱体的热分解和所制得粉体的物相组成、形貌及颗粒尺寸进行表征，结果表明所制得的MgAl2O4粉体颗粒粒径小，结晶良好，成分单一且粒径大小均匀。而实验过程中工艺参数的改变会对生成粉体的粒径和形貌产生一定的影响，进而即研究了各种工艺参数的改变对该方法制备纳米MgAl2O4粉体的影响。     

STUDY ON THE TENSILE FATIGUE DAMAGE OF QUASI—ISOTROPIC COMPOSITE LAMINATES

Quasi-isotropic. laminates have isotropic elastic properties in all in-plane directions. Therefore, this kind of laminate is widely used for structural elements. The simplest stacking sequence of quasi-isotropic laminates is [0/-60/60]s. When the direction of applied axial load to [0/-60/60]s laminate is inclined at a 30-degree angle, we have the other quasi-isotropic laminate [30/-30/90]s under axial load. The failure mechanisms of these two laminates are, however, entirely different from each other because these two laminates have different distribution of the interlaminar stresses. It was confirmed by tensile fatigue tests that the [0/-60/60]s laminate does not show any visible fatigue damage, but the [30/-30/90]s laminate develops edge-delamination during cyclic loading. The analytical results were in good agreement with the experimental results.     

New inverse method for identification of constitutive parameters

A new inverse method by coupling iSIGHT and ABAQUS was proposed to determine the constitutive parameters of Al2O3sf/LY12 composite deforming at elevated temperature. It combined the merits of the f＇mite element simulation and optimization technique. The direct model was simulated with finite element code ABAQUS. The inverse problem associated with the identification of the constitutive parameters was expressed as a least square optimization problem. The direct simulation and the parameters optimization were implemented in iSIGHT integrated environment. The aim was to match the output of the direct simulation with the experiment data. The capability of the proposed inverse method was demonstrated through the identification of constitutive parameters of Al2O3sf/LY12 composite. The proposed new inverse method is also applicable to other parameters identification which is hardly determined through experiments or direct analytical method.     

人工智能在冷轧带肋钢筋工艺参数优化中的应用

论述了人工智能在冷轧带肋钢筋工艺参数优化中的应用,首先利用神经网络建立起冷轧带肋钢筋工艺参数与产品抗拉强度之间关系的数学模型,然后用遗传算法对工艺参数优化。按优化后的工艺参数在某拉丝加工厂进行了大批量生产试验,产品的合格率达到98%以上,从而为控制冷轧带肋钢筋工艺参数提供了一条行之有效的途径。     

THE INFLUENCE OF ENVIRONMENT ON FATIGUE AND FRACTURE OF IRON ALUMINIDES

ＴＨＥＩＮＦＬＵＥＮＣＥＯＦＥＮＶＩＲＯＮＭＥＮＴＯＮＦＡＴＩＧＵＥＡＮＤＦＲＡＣＴＵＲＥＯＦＩＲＯＮＡＬＵＭＩＮＩＤＥＳＳＴＯＬＯＦＦ，Ｎ．Ｓ．ａｎｄＡＬＶＥＮ，Ｄ．Ａ．（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＥｎｇｉｎｅｅｒｉｎｇＲｅｎｓｓｅ...     

预应变对16MnR钢断裂韧性的影响SCIEI

本文实验发现预应变对16MnR钢的断裂韧性具有显著的影响,断裂过程中裂尖区域材料的组织结构发生了变化,裂纹在二次珠光体组织上扩展。16MnR钢断裂韧性的大小取决于碳原子的扩散及二次珠光体组织生成的难易程度。