Crack initiation and propagation induced by inclusions in a nickel-base P/M superalloy under fatigue load

1 IntroductionRecently the key components in advanced aircraftengines are widely made of nickel-base P/M super-alloy. However, non-metallic inclusions are intro-duced into this material during powder metallurgyprocessing. In industry, although many method…     

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.     

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。     

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.     

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。     

Grain-size effects in the quasi-static fracture resistance of a thermally embrittled RPV steel (quenched and tempered microstructures)

The elastic-plastic fracture toughness and crack extension behavior under the quasi-static loading regimen of several thermally embrittled conditions of a nuclear reactor pressure vessel (RPV) steel were assessed on the basis of microstructural parameters. It was discovered that the bainite packet size is the fracture properties controlling parameter of single-phase quenched and tempered microstructures. Results were found in close agreement to those obtained in a parallel study with dual-phase annealed microstructures derived from the same low alloy steel. Similarly, it was concluded that a Hall-Petch type relationship correlates J-fracture mechanics criteria to the grain size.     

Mechanical aspects of the rare-earth effect

In order to gain a better understanding of the reactive-element effect (REE), the improvement of the oxidation behavior of chromia- or alumina-scale-forming alloys by the addition of small amounts of elements with higher affinity to oxygen than the scale-forming element, it is necessary to clearly distinguish between isothermal oxidation and the behavior of the metal/oxide composite system during cooling. An approach is presented based on fracture-mechanical considerations which correlates critical differential strain between scale and substrate, fracture toughness of the metal/scale interface, scale thickness, defect size and interfacial amplitude. This approach allows a quantitative assessment of the REE for scale adhesion, and although the necessary experimental data are yet lacking, it describes the reported REE in a qualitatively correct manner.     

NiTi形状记忆合金螺钉的研制及医学应用

成功地研制了一种新型NiTi形状记忆合金螺钉并已临床应用了150例,经3年多考核,未发现骨不连及股骨头缺血坏死现象。