High temperature behaviour of yttrium implanted pure iron and extra low carbon steel

High temperature behaviour of yttrium implanted pure iron and extra low carbon steel were analyzed at T = 700°C and under oxygen partial pressure P_O2 = 0.04 Pa for 24 h to show the benefit of yttrium incorporation to the improvement of the implanted sample corrosion resistance at high temperature. Compositions and structures of yttrium implanted and unimplanted samples were investigated prior to high temperature oxidation studies by several analytical and structural techniques (RBS, RHEED and XRD) to observe the initial yttrium implantation depth profiles in the specimens. High temperature oxidation tests performed by thermogravimetry and by in-situ high temperature X-ray diffraction were carried out with the same experimental conditions on yttrium implanted and unimplanted samples to have reference analyses. The aim of this paper is to show the initial nucleation stage of the main compounds induced by oxidation at high temperature according to the initial sample treatment. The results obtained by in-situ high temperature X-ray diffraction will be compared to those by thermogravimetry to show the existing correlation between weight gain curves and structural studies. Our results allow to understand the improved corrosion resistance of yttrium implanted pure iron and extra low carbon steel at high temperature.     

Temperature dependence of fracture toughness in cleavage fracture of iron and iron alloys

In order to examine the temperature dependence of fracture toughness in cleavage fracture, exploratory work was carried out. Then effects of alloying elements and micro-structure on the low temperature fracture toughness were studied quantitatively in iron and iron alloys.The results indicate that (1) the relationship between fracture toughness G_ic and testing temperature T at low temperatures is G_ic = G_o exp (T/T_e), where G_e and T_o are the material constants; (2) G_o exhibits a stro dependence on solute carbon and nitrogen contents but is independent of micro-structure and other elements; (3) G_o values increases with increasing solute carbon and nitrogen contents; (4) T_o depends on the structure; (5) $\text{1}\text{T}{}_{\mathrm{o}}$ values increase with increasing nickel and manganese contents, to the contrary, decrease with increasing carbon, silicon and phosphorus contents; and (6) $\text{1}\text{T}{}_{\mathrm{o}}$ values increase with decreasing grain size.     

Comparison of methods for fracture toughness testing of thin low carbon steel plates

Abstract

The fracture toughness of double edge notched tension (DENT) specimens of a low carbon steel sheet was evaluated using experimental and numerical methods. The concepts of critical J integral J_c critical crack tip opening displacement δ_c, essential work of fracture We_e, and essential work of fracture and initiation we^init were compared. The numerical methods were based on finite strain, three-dimensional finite element simulations of the tensile straining of the DENT specimens. Good agreement was found between numerical and experimental J_c values. Fair agreement was also found between J_c and we^init. The essential work of fracture W_e was ～20% lower than J_c. This discrepancy is attributed to inaccuracy in the detection of cracking initiation. The Shih factor derived from the measured J_c and δ_c values closely corresponds with the plane stress prediction.     

The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron

The fracture toughness of a high chromium, reduced carbon white cast iron was measured using the K_Ic fracture toughness test. The toughness was found to increase with increasing heat treatment temperature for the temperature range of 1273–1423 K. Increases in the fracture toughness were due to crack deflection into the dendritic phase. Cracking in the dendrites was promoted by the presence of secondary carbides which formed during the high temperature heat treatment employed. The characteristic distance for brittle fracture as calculated by the Ritchie–Knott–Rice model correlated well with the centre to centre mean free path of the secondary carbides on the fracture plane.     

Deformation and fracture of sintered iron

Conclusion Metallographic-fractographic evaluation of the distribution of microdeformation processes in the volume of porous iron at the static tensile test demonstrates the influence of porosity on the concentration of deformation flows into the microvolume of connections. The degradation of deformation strengthened active volumes with the subsequent degradation of macrodeformation characteristics can occur in geometrically and structurally slightly developed connections.Published in Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 50–54, May–June, 1992.     

A fracture criterion for sharp V-notched samples

The objective of this paper is to show the advantages of the cohesivecrack model for predicting fracture of V-notched components. Criticalvalues of the generalized stress intensity factor can be obtained fromthe knowledge of the material softening function and the elastic parameters,avoiding a cumbersome experimental work. The results were checked successfullyagainst experimental ones, from other authors, in different materials: steel,aluminium, PMMA and PVC. A non dimensional formulation of the fracturecriterion for sharp V-notched components was obtained and a simpleapproximate expression derived for easy appication.     

Bending fracture in carbon nanotubes

A novel approach was adopted to incur bending fracture in carbon nanotubes (CNTs). Expanded graphite (EG) was made by intercalating and exfoliating natural graphite flakes. The?EG was deposited with nickel particles, from which CNTs were grown by chemical vapor deposition. The CNTs were tip-grown, and their roots were fixed on the EG flakes. The EG flakes were compressed, and many CNTs on the surface were fragmented due to the compression-induced bending. Two major modes of the bending fracture were observed: cone-shaped and shear-cut. High-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the crack growth within the graphene layers. The bending fracture is characterized by two-region crack growth. An opening crack first appears around the outer-tube due to the bending-induced tensile stress. The crack then branches to grow along an inclined direction toward the inner-tube due to the presence of the shear stress in between graphene layers. An inner-tube pullout with inclined side surface is formed. The onset and development of the crack in these two regions are discussed.     

Small fatigue crack growth characteristics and fracture surface morphology of low carbon steel in hydrogen gas

Owing to energy conservation and environmental concerns, hydrogen has been suggested as a next-generation energy source. However, hydrogen known to seep into a metal, degrade its strength, and accelerate fatigue crack growth rates. We have investigated the effects of hydrogen gas on the small fatigue crack growth characteristics of low carbon steel JIS S10C by conducting bending fatigue tests on a specimen with a small blind hole and placed in a low-pressure hydrogen environment. The fatigue crack growth rate in hydrogen was higher than that in nitrogen. The fracture surface of the specimen in hydrogen showed intergranular facets in the low- growth-rate range and a quasi-cleavage fracture surface with brittle striations in the high-growth-rate range. The specimen only showed a ductile fracture surface for nitrogen. The small-fatigue-crack growth rate for nitrogen is given by ${dl/dN\propto \Delta \varepsilon_{p}^{n}l}$ , where l, N, and ${\Delta \varepsilon_{p}}$ represent the crack length, number of repetitions, and plastic strain range, respectively. This equation was also satisfied for hydrogen, but only over a short strain range from ${\Delta \varepsilon_t = 0.25}$ to 0.37?% in which the fracture surface exhibited intergranular facets and a ductile morphology, but no quasi-cleavage fracture. The exponent n of the equation was 1.22 in nitrogen and 0.66 in hydrogen environment. The small-fatigue-crack growth law can be used for safe material designs in hydrogen environments.     

炭材料在低温型磷酸铁锂材料中的应用分析及展望

磷酸铁锂为正极的锂离子电池是目前电动汽车和储能领域应用最为广泛的电池体系之一,具有成本低廉、循环寿命长、安全性好等特点。但磷酸铁锂为正极的锂离子电池在低温下的容量和循环寿命衰减问题一直制约了其在寒冷地区的推广和应用。因此磷酸铁锂材料本身低温放电性能的提高,对于改善磷酸铁锂为正极的锂离子电池体系的低温放电特性具有重要意义。本文首先分析了磷酸铁锂为正极的锂离子电池的低温衰减机制,从炭材料作用的角度评述了低温型磷酸铁锂材料的研究进展,同时也关注了高倍率型磷酸铁锂材料。因磷酸铁锂的高倍率性能与低温特性具有很大的相似之处,两者对材料的要求基本接近,材料的设计原则和方法也基本相同。本文也重点分析了纳米炭材料,如碳纳米管和石墨烯等在低温型磷酸铁锂材料领域的应用。     

The fracture mechanism of carbon fibres

The physical properties of carbon fibres have been related to their structure by considering the influence of ribbon unbending during extension. An explanation of the fibres' anomalous strength characteristics, based on their structure and not on the presence of internal flaws, is presented.     

Carbon–carbon interactions in iron

Carbon atoms occupy interstitial sites in iron so that the configurations of any solid-solution at constant composition depend solely on the distribution of carbon atoms on the interstitial sub-lattice, this in turn being influenced by interactions between carbon atoms in close proximity. The carbon–carbon interaction energy, which influences the distribution of carbon atoms, is reviewed with a view to understanding the nature of the interaction and to highlight some recent developments in the subject. It appears that the C–C interaction energy for ferrite cannot be deduced from the thermodynamic data currently available, primarily because of the very low solubility of carbon in ferrite. On the other hand, there is ample evidence to support the view that the corresponding energy for austenite is consistent with a strong repulsion between near neighbour pairs of carbon atoms.     

Axial compression fracture in carbon fibres

Axial compression fracture of carbon fibres was studied by embedding single fibres in epoxy resin and compressing the specimens parallel to the fibre axis. By careful optical monitoring of the fibre surface the earliest stages of fracture were identified leading to estimates of the fibre axial compression failure strengths. Compression strength decreases markedly from about 2.2 GN m^?2 for moderately oriented fibres to <1 GN m^?2 for highest modulus filaments. The trend towards decreasing compression strength with increasing anisotropy is explained on the basis of an increasing fibre microfibrillar nature. However fracture morphology studies show that the unduly rapid strength decrease results from an increasing degree of fibre outer layer ordering which accompanies increasing axial anisotropy in carbon fibres since cracking occurs first on the more highly aligned filament surfaces. It is suggested that fibre compression fracture changes from a shear to a microbuckling or kinking mode with increasing fibre anisotropy, where the latter initiates in individual, well-aligned but uncoupled microfibrils. The similarity of fine axial compression fractures in oriented carbon fibres to those found in elastica loop experiments is noted as are the possible implications which the low strain-to-failure in compression of very high modulus fibres might have for practical composites.