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  • Grolig, Jan Gustav, 1986- (författare)
  • Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties
  • 2015
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potentialuses in decentralized and clean electricity and heat production. Several challengeswith respect to materials have to be overcome to achieve efficiencies and life-spansthat are sufficient for long-term applications.An important element of an SOFC stack is the interconnect component, which connectstwo adjacent fuel cell elements. Interconnects, which are commonly composedof ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized.This work aimed to investigate economic solutions for interconnect materials and tounderstand the underlying mechanisms of degradation and electrical conduction ofthese materials. Mainly two substrates, a commercially available steel (AISI 441) anda ferritic stainless steel that was optimized for an SOFC application (Sandvik SanergyHT) were combined with different barrier coatings and exposed to a cathode-sideatmosphere. A method was developed that allows for the electrical characterizationof promising material systems and model alloys, thereby facilitating a fundamentalunderstanding of the dominant electrical conduction processes linked to the oxidescales that grow on interconnects. The AISI 441 steel coated with reactive elementsand cobalt showed good corrosion and chromium evaporation profiles, while AISI 441coated with cerium and cobalt also had promising electrical properties. The SanergyHT steel was examined with coatings of copper and iron and copper and manganese,respectively. The corrosion and chromium evaporation profiles of Sanergy HT wereimproved by coating with copper and iron. The copper and iron-coated Sanergy HTshowed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia,which is the main constituent of oxide scales, was synthesized using differentmethods. The electrical properties of chromia were found to be sensitive to not onlyimpurities, but also heat treatment. Finally the electrical properties of cobalt- andcobalt cerium-coated Sanergy HT steels were investigated. It was revealed that theaddition of cerium improved the conductivity of the interconnect by both slowingdown chromia growth and preventing the outward diffusion of iron into the spinel.
  • Goel, Sneha, et al. (författare)
  • The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718
  • 2018
  • Ingår i: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications. - 2367-1181 .- 2367-1696. - 978-3-319-89479-9 ; s. 115-129
  • Konferensbidrag (refereegranskat)abstract
    • Alloy 718, usually referred to as Inconel 718, is a Ni-Fe based superalloy widely used in the aerospace industry by virtue of the excellent balance of mechanical properties and corrosion resistance that it affords at high temperature [1]. While Alloy 718 parts have been traditionally fabricated using conventional cast and wrought (C&W) techniques, there has been a growing interest in production of the complex high temperature service components through additive manufacturing (AM) processes due to the significant design freedom offered by the technology. Alloy 718 has been processed by laser (laser metal deposition, LMD; selective laser melting, SLM) and electron (Electron Beam Melting, EBM) beam based technologies [2]–[4]. AM, in general, involves high solidification rates and high thermal gradients, thus resulting in a finer microstructure compared to C&W products [5]. In EBM, a relatively high temperature is maintained throughout the process compared to SLM, thus resulting in lower residual stresses in as-manufactured parts [6]. However, as a relatively newer technology, the degree to which the EBM processing conditions (scanning strategies, preheat and melting parameters) affect the properties of EBM-built Alloy 718 material is less understood compared to the laser based techniques. Also, it has been challenging to date to build Alloy 718 parts that are free from issues such as defects, inhomogeneity, anisotropy, etc., thereby demanding post-treatment to achieve the required microstructure and intended part performance. Although post treatment of C&W Alloy 718 has been widely investigated over several decades [7]–[9], the inadequacy of conventional post-treatment protocols for EBM Alloy 718 [10]–[12], justifies further investigation of microstructural changes that occur in EBM-built Alloy 718 during post-treatment. A comprehensive study may eventually allow optimization of the combined build and post-treatment processes to obtain components with properties deployable in severe environments.
  • Jaladurgam, Nitesh Raj, 1993- (författare)
  • Heterogeneous deformation of multi-phase engineering materials - an in-situ neutron diffraction study
  • 2019
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Gas turbines are complex power generation systems used in aerospace or land-based-power stations. Materials such as Ni-base superalloys are involved in the combustion zone of these machines, which continuously experiences harsh environments with loading at high temperatures. Moreover, the continuous demand for increasing operating temperature to achieve higher efficiencies and reduced emission levels opens the scene to new heat resistant materials like the state-of-the-art high entropy alloys (HEAs), which require a thorough understanding of the structure-process-property relationships. The microstructures of these advanced multi-phase, multi-component alloys are complex, and the deformation is generally heterogeneous both with respect to the different phases and to the crystallographic orientation within each phase. Hence, it is important to understand their behavior and performance during processing and service. In-situ neutron diffraction is a unique technique to probe the deformation behaviour during service/processing-like conditions, including plastic deformation at various temperatures, in order to provide insights into the structure-property relations. In the first part of this work the deformation mechanisms of a newly developed Ni-base superalloy was investigated using in-situ neutron diffraction and electron microscopy at room temperature. In addition, elasto-plastic self-consistent (EPSC) crystal plasticity simulations are used to obtain insights into the operating deformation mechanisms. In the second part, the as-cast eutectic high entropy alloy AlCoCrFeNi2.1 was studied using in-situ neutron diffraction at temperatures from 77 to 673 K. These investigations provide unique insights into the complex heterogeneous deformation behavior of these high-performance multi-phase engineering materials.
  • Goel, Sneha, et al. (författare)
  • Effect of post-treatments under hot isostatic pressure on microstructural characteristics of EBM-built Alloy 718
  • 2019
  • Ingår i: Additive Manufacturing. - 2214-8604. ; 28, s. 727-737
  • Tidskriftsartikel (refereegranskat)abstract
    • Electron beam melting (EBM) has emerged as an important additive manufacturing technique. In this study, Alloy 718 produced by EBM was investigated in as-built and post-treated conditions for microstructural characteristics and hardness. The post-treatments investigated were hot isostatic pressing (HIP) and combined HIP + heat treatment (HIP + HT) carried out as a single cycle inside the HIP vessel. Both the post-treatments resulted in significant decrease in defects inevitably present in the as-built material. The columnar grain structure of the as-built material was found to be maintained after post-treatment, with some sporadic localized grain coarsening noted. Although HIP led to complete dissolution of δ and γ′′ phase, stable NbC and TiN (occasionally present) particles were observed in the post-treated specimens. Significant precipitation of γ′′ phase was observed after HIP + HT, which was attributed to the two-step aging heat treatment carried out during HIP + HT. The presence of γ′′ phase or otherwise was correlated to the hardness of the material. While the HIP treatment resulted in drop in hardness, HIP + HT led to ‘recovery’ of the hardness to values exceeding those exhibited by the as-built material.
  • Hryha, Eduard, 1980-, et al. (författare)
  • Carbon Control in PM Sintering: Industrial Applications and Experience so Far
  • 2012
  • Ingår i: Proc. of EURO PM2012 Congress & Exhibition, 16-19 September 2012, Basel, Switzerland. - 978-1-899072-23-1 ; 1, s. 265-270
  • Konferensbidrag (refereegranskat)abstract
    • Challenges in controlling carbon potential during sintering of steel powder have been discussed in many experimental and theoretical studies. The main issues lie within the complex thermodynamics and kinetics of processing atmosphere chemistry in continuous sintering furnaces. Although many models have tried to address the problem, many of these have rarely come to reality and become an industrial practice.The purpose of this article is to summarize these discussions and investigate the interaction of the atmosphere constituents with the sintered compact within a sintering furnace. Considering an industrial practice perspective, the paper ensures the PM Industry with a fresh new look into the understanding of the furnace operations and provides recommendations to improve the control of the furnace conditions. As an example, existing furnace installation utilizing Linde SINTERFLEX technology allows monitor and/or control the furnace atmosphere. Article describes reduction of oxides and carbon potentials to enable optimization of the production parameters.
  • Hryha, Eduard, 1980-, et al. (författare)
  • Process Control System for Delubrication of PM Steels
  • 2012
  • Ingår i: Acta Metallurgica Slovaca. - 1338-1156. ; 18:2-3, s. 60-68
  • Tidskriftsartikel (refereegranskat)abstract
    • Lubricants are important additive in powder metallurgy (PM) steels as they are needed to improve powder compressibility and reduce tool wear. Removal of the lubricant results in production of number of gas species that can have harmful effect on the base material. Current paper describes process gas monitoring system for delubrication control that is built on industrially available gas sensors. Experimental results indicate that accurate delubrication process control can be performed by utilizing only CO2 and O2 sensors. It is recommended to perform delubrication below 500°C in dry inert atmospheres in the case of modern ethylene bis stearamide based lubricants.
  • Hörnqvist, Magnus, 1979-, et al. (författare)
  • High-temperature crack growth in a Ni-base superalloy during sustained load
  • 2014
  • Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - 0921-5093. ; 609, s. 131-140
  • Tidskriftsartikel (refereegranskat)abstract
    • he high-temperature sustained load crack growth behaviour of a Ni-base superalloy was investigated using a combination of mechanical testing in controlled atmosphere, fractographical and microanalytical investigations, and finite element modelling. The results show that the local crack front geometry is un- even on two scales – jaggedness on the scale of 100 μm was observed in all specimens, whereas mm- scale waviness could occasionally be observed. The jaggedness can be explained by a percolation-type crack growth along weaker grain boundaries, whereas the large-scale waviness is presumably due to larger regions of the material having specific grain texture with high crack growth resistance. The un-even crack front is shown to potentially have considerable effects on the loading conditions at the crack tip, whereas ligaments of un-cracked material in the crack wake are deemed to have less effect on the crack tip loading due to their low area fraction. The ligaments fail intergranularly in the wake as the crack grows in the present case, as opposed to by creep fracture as previously proposed. Finally, the plastically deformed regions about the crack and crack tip are shown not to exhibit any elevated oxygen levels, implying that the damage in these regions is purely mechanical.
  • Jaladurgam, Nitesh Raj, 1993-, et al. (författare)
  • Microstructure-dependent deformation behaviour of a low γ′ volume fraction Ni-base superalloy studied by in-situ neutron diffraction
  • 2020
  • Ingår i: Acta Materialia. - 1359-6454. ; 183, s. 182-195
  • Tidskriftsartikel (refereegranskat)abstract
    • Ni-base superalloys are critical materials for numerous demanding applications in the energy and aerospace sectors. Their complex chemistry and microstructure require detailed understanding of the operating deformation mechanisms and interaction between the matrix and the hardening phase during plastic deformation. Here we use in-situ neutron diffraction to show that the dependence of the deformation mechanisms and load redistribution on $\gamma^\prime$ particle size in a Ni-base superalloy with a $\gamma^\prime$ volume fraction of around $20 \%$ can exhibit distinct differences compared to their high volume fraction counterparts. In particular, the load redistribution in the coarse microstructure occurs immediately upon yielding in the present case, whereas high $\gamma^\prime$ volume fractions have been observed to initially lead to shear mediated co-deformation before work hardening allows looping to dominate and cause load partitioning at higher stresses. The fine microstructure, on the other hand, behaved similar to high volume fraction alloys, exhibiting co-deformation of the phases due to particle shearing. A recently developed elasto-plastic self-consistent (EPSC) crystal plasticity model, specifically developed for the case of coherent multi-phase materials, could reproduce experimental data with good accuracy. Furthermore, the finite strain formulation of the EPSC model allowed deformation induced texture predictions. The correct trends were predicted by the simulations, but the rate of lattice rotation was slower than experimentally observed. The insights point towards necessary model developments and improvements in order to accurately predict e.g. texture evolution during processing and effect of texture and microstructure on component properties.
  • Joseph, Ceena, 1982-, et al. (författare)
  • Anisotropy of Room Temperature Ductility in Haynes 282 Forgings
  • 2014
  • Ingår i: Proceedings of the 8th International Symposium on Superalloy 718 and Derivatives. - 978-1-119-01680-9 ; s. 601-609
  • Konferensbidrag (refereegranskat)abstract
    • Haynes 282 is relatively new nickel base superalloy, offering excellent mechanical properties and attracting interest from the gas turbine and aerospace industry. One of the criteria for meeting specification requirement is the room temperature ductility.Forgings of Haynes 282 show large anisotropy in ductility, varying from approximately 25% in longitudinal transverse (LT) to 12% in the short transverse(ST) direction. In this paper LT and ST specimens from forgings were tested for its room temperature ductility.Fractographic and metallographic analysis show the presence of MC and Mc6C carbide banding. Failure in both ST and LT direction was ductile with intergranular fracture, but in the ST direction the failure was preferentially due to cracking of carbides. The orientation of carbides in preferred direction and their excessive segregation affecting ductility in ST is discussed in this work.
  • Joseph, Ceena, 1982-, et al. (författare)
  • Influence of heat treatment on the microstructure and tensile properties of Ni-base superalloy Haynes 282
  • 2017
  • Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - 0921-5093. ; 679, s. 520-530
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of heat treatment on the microstructure and mechanical properties of Ni-base superalloy Haynes 282 was investigated. Applying a standard two-step ageing (1010 °C/2 h +788 °C/8 h) to the as-received, mill annealed, material resulted in a the presence of discrete grain boundary carbides and finely dispersed intragranular γ′, with an average size of 43 nm. This condition showed excellent room temperature strength and ductility. The introduction of an additional solution treatment at 1120 °C resulted in grain growth, interconnected grain boundary carbides and coarse (100 nm) intragranular γ′. The coarser γ′ led to a significant reduction in the strength level, and the interconnected carbides resulted in quasi-brittle fracture with a 50% reduction in ductility. Reducing the temperature of the stabilization step to 996 °C during ageing of the mill annealed material produced a bi-modal γ′ distribution, and grain boundaries decorated by discrete carbides accompanied by γ′. This condition showed very similar strength and ductility levels as the standard ageing of mill-annealed material. This is promising since both grain boundary γ′ and a bi-modal intragranular γ′ distribution can be used to tailor the mechanical properties to suit specific applications. The yield strength of all three conditions could be accurately predicted by a unified precipitation strengthening model.
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