Papers by Keyword: Aging

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Authors: Lydia Giersberg, Benjamin Milkereit, Christoph Schick, Olaf Kessler
Abstract: Isothermal time-temperature-precipitation (TTP) diagrams deliver important material data like temperature and time ranges critical for precipitation. During the last years an in-situ calorimetric method to record continuous cooling precipitation diagrams has been developed to application level by our group. However, isothermal TTP-diagrams were still determined by ex-situ analyses only. In this work in-situ measurements of precipitation reactions were carried out during isothermal soaking. Therefore the whole heat treatment cycle was performed in a differential scanning calorimeter (DSC). Al-Mg-Si-alloys 6063 and 6005A were analysed. Solution annealing and overcritical quenching to several temperatures between 450 °C and 250 °C was followed by isothermal soaking. Based on the heat flow curves during isothermal soaking TTP-diagrams were determined. Further microstructure investigations by scanning electron microscopy and hardness tests after artificial ageing were performed. Both alloys show similar results. In the TTP-diagramms three so-called ”C-curves” could be observed. Every C-curve is expected to represent precipitation of a different phase. Hardness and microstructure investigations correspond with the data of the TTP-diagramms.
Authors: Abdelali Hayoune, Nacereddine Titouche
Abstract: The effects of cold deformation and low temperature aging on the microstructural stability of a peak aged (PA) Al 6061 alloy were investigated by means of DSC and microhardness measurements. During aging at a relatively low temperature (100 °C) of the PA material, a small increasing of the mechanical properties was detected, which was explained by the formation of atomic clusters, GP zones and β phase. The response to the aging treatment of the cold deformed materials depends on both the level of the cold deformation and the aging temperature. During aging at relatively low temperature (100 °C), in contrast to the 75 % deformed material that shows a small variation in their mechanical properties, the mechanical properties, of the 30 % deformed material, are almost constant. This was attributed to the higher driving force of the recovery reaction in the heavily deformed material. In the other hand, aging at relatively higher temperature (140 °C) of the heavily deformed material, leads to a fast softening due to an increasing of the recovery kinetics.
Authors: Clark Hyland, W. George Ferguson
Abstract: A method for assessing likelihood of brittle fracture in cyclically loaded steel assemblies subjected to inelastic strains is proposed. The method proposed is based upon relationships between monotonic and cyclic endurance of steel specimens proposed by Kuwamura and Takagi, and analysis of crack tip opening displacement (CTOD), Charpy V-Notch (CVN) and tensile results of pre-strained, fatigue pre-cracked and side-grooved specimens of constructional steel. The proposed method allows the influence of displacement ductility classification (as used in seismic design of structures), notch geometry, and cyclic strain amplitude history on crack initiation to be incorporated into a single design analysis approach. Small scale CTOD testing of steel materials with various levels of pre-strain may be used to identify stress intensity and crack tip displacement at crack initiation for use in the analysis. The integration of a fracture mechanics based approach to analysing stress intensity in conjunction with assembly plastic deformation characteristics derived from finite element modeling offers the promise of an improved approach to steel assembly design for cyclic plastic endurance and should result in more reliable structures and reduced need for large scale testing. This has particular relevance to the structural design of seismic resisting steelwork assemblies which are expected to develop dependable ductile behaviour under high strain variable amplitude cyclic actions.
Authors: Aurélie Clair, Marc Foucault, J. Marcos Salazar, Vincent Vignal, Eric Finot, Laurent Markey
Abstract: Studying the corrosion of the alloy 600, under water pressure, is of high importance to understand the ageing process of pressurized water reactors. Today, the impact of the oxide growth on the mechanical properties of nickel alloys is a challenge. The surface analysis and the quantification of the local deformation are key factors to deduce the surface damage of the substrate produced by corrosion. Here, we introduce a new methodology to determine the deformation distribution of the alloy 600 by using polycrystalline samples. The method is based on nanopads disposed on the surface samples, which allow a mapping, at the microscopic scale, of the spatial deformation. We applied to the samples a tensile loading in the plastic domain from 0 to 4%. The obtained asymmetric deformations distribution reflects the polycrystallinity of the system.
Authors: Qing Yan Xu, Rui Chen, Yu Feng Shi, Bai Cheng Liu
Abstract: In the present investigation, a physically based numerical model was developed to predict the yield stress of Al-7Si-Mg cast alloy during processing. It covered the integrated unit step models of the physical metallurgy of solidification, solid-state of homogenization, and structural hardening of precipitation. The as-cast microstructure of Al-7Si-Mg alloy was calculated based on the cellular automaton method and the evolution of the precipitated phase during aging process was achieved by a precipitation kinetic model involved nucleation, growth and coarsening. The yield stress prediction was achieved by a strengthening model including the effects of as-cast microstructure, solution strengthening and precipitate hardening. The predictions of this model were verified by comparing with experimental measured yield stress which shows that this model is successfully applied to predict the yield stress evolution of Al-7Si-Mg cast alloy.
Authors: Ole Runar Myhr, Rune Østhus, Trond Furu
Abstract: The present paper describes a novel methodology for optimization of product properties and production costs in fabrication of aluminium alloys. The main idea is to represent each operation along the process chain by predictive tools, which include material-, mechanical-, cost-and logistics models. An optimisation tool is used to collect the simulation models into a common software environment, which allows fully automatic simulations to be carried out. When this coupling is established, the models are run in sequence using different types of optimisation strategies. The methodology has been applied for optimisation of strength, grain structure and costs of 6xxx series aluminium extrusions. The results indicate that the present methodology is sufficiently relevant and comprehensive to be used as a tool in fabrication of various aluminium products, for instance in optimisation of end-user properties and production costs of extruded, rolled or foundry based alloys.
Authors: Jackie Y. Cai, Bi Xu, Li Jing Wang, Yi Cao, Niall Finn, Zai Sheng Cai
Abstract: Hydrophobic and elastic aerogels derived from methyltrimethoxysilane (MTMS) precursor were prepared by ambient pressure drying under various processing conditions, and their morphology and physical and mechanical properties were characterised and evaluated. The results of this study demonstrated that for inherently hydrophobic gels like MTMS based gels, high temperature ageing is a practical and effective means to reduce volume shrinkage and produce low bulk density monolithic aerogels at ambient pressure, without the need of any solvent exchange and surface modification. By using this simple method, we were able to produce an aerogel with a bulk density as low as 0.064g/cm3. The results also revealed a significant difference from that previously reported in the literature.
Authors: Abdullatif Lacina Diaby, Lee Luong, Amer Yousef, Jonas Addai Mensah
Abstract: Refinery crude preheat train (CPHT) is prone to fouling and ageing effects due to the complexity of processed crude feedstock preheated prior to distillation. This has serious implications on the thermal and hydraulic performance of the CPHT. As a result, efficient performance of crude preheat trains is compromised and as such, optimal scheduling cleaning operations are required to restore performance. In this paper, we attempt to review the subject of fouling/ageing control and mitigation in crude preheat train network by optimal scheduling cleaning approach. Three prominent optimisation techniques/models namely Mathematical Models (Mixed integer linear programming (MILP) and Mixed integer non-linear programming (MINLP) models); Artificial Intelligence (AI) Models; and Heuristic Techniques used for achieving optimal cleaning are outlined.
Authors: Mohamed M. Aboras, Kai Yuan Theng, Andanastuti Muchtar, Che Husna Azhari, Norziha Yahaya
Abstract: The use of tetragonal zirconia as a dental restorative material has recently increased because of its unique mechanical and optical properties, as well as high biological compatibility with the oral cavity environment. However, the mechanical properties of zirconia can be severely degraded, which leads to the failure of dental restorations. This review focuses on the low-temperature degradation of dental zirconia and its effects on the properties of zirconia and on the oral environment. The purpose is to show the importance of this negative phenomenon and suggest guidelines for minimizing the aging of zirconia that is used as a dental restoration material.
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