1,721,110 research outputs found
Selektive Korrosionsmechanismen zur Ableitung einer Wirksumme für Al-Mg-Si-Cu-Knetlegierung
No full textHeat-treatable wrought aluminum alloys of the Al-Mg-Si series play a key role in the automotive industry as car body panels. They combine the three important material characteristics of strength, ductility and corrosion resistance in the best possible way. The addition of copper to Al-Mg-Si alloys increases strengthening during pre-cipitation hardening by forming copper-containing quaternary phase modifications of Q’-AlxCuyMgzSiw. However, Al-Mg-Si-Cu wrought alloys tend to be more susceptible to intergranular corrosion (IGC). Since that the corrosion mechanism of wrought Al-Mg-Si(-Cu) alloys has not yet been fully outlined, the new copper-containing wrought alloy Al-1,3Mg-1,4Si-1,3Cu was investigated at specific manufacturing and vehicle heat treatment stages. To enable the corrosion resistance of wrought Al-Mg-Si(-Cu) alloys without carrying out extensive corrosion tests, as so-called effective sum formula WSAl-Mg-Si(-Cu) was developed for Al-Mg-Si(-Cu) alloys. It is based on the excess silicon formula, which was first mentioned in Gupta et al. Publication for the determina-tion of mechanical properties. By inserting the alloy contents in wt.%, the corrosion depth can be calculated easily. In present work, the silicon excess formula has been adapted for the first time. With the help of statistical calculations, the copper content could be included in the effective sum formula WSAl-Mg-Si(-Cu). The validation took place with experimental and simulative methods. For this purpose, the corrosion depths from eleven different technical wrought Al-Mg-Si(-Cu) alloys were correlated together with the alloy-specific results from the phase calculations of the phase simulation program JMatPro®. The corrosion behavior was verified by means of tests in accordance with DIN EN ISO 11997-1 Cycle B, DIN 55665 and ASTM G110. The results were analyzed from a fundamental scientific point of view and related to the microstructure in order to elucidate the corrosion mechanism. For microstructural analyses, scanning electron microscopy and scanning transmission electron mi-croscopy were used
Development and characterisation of surface pretreatments of aluminium AA6082-T6 for adhesion applications
No full textSurface pretreatments of aluminium alloys are known to be of great importance for stronger and environmentally stable adhesively bonded joints. The wet chemical surface pretreatments like anodising are the benchmark techniques for adhesively bonded aluminium structures but are very complex and environmentally hazardous as well as difficult to automate for reliable process control. Therefore relatively new, simple, and environment friendly dry pretreatments like laser processing are under rigorous scrutiny to replace the state of the art pretreatments. However, comparatively little is known on how laser generated surface structures and chemistries, which depend on laser-matter interactions, affect the adhesion and durability of adhesively bonded joints. This study investigates in detail the influence of the laser ablation regimes and hence material removal mechanisms on the morphologies, topographies and chemistries of laser formed surface structures and correlates the characteristics of these surface structures to the adhesion and durability of the adhesive bonded joints. The morphologies and topographies of surfaces are studied by scanning electron microscopy (SEM) and confocal laser scanning microscopy(LSM), and X-ray photo spectroscopy (XPS) analysis has been used to examine the surface chemistries of pretreated surfaces. The mechanical behaviour of the pretreated joints prior and after exposure to hydrothermal environment is assessed by single lap and double cantileverbeam (DCB) tests. An extensive analysis of fractured joints has been carried out using optical, SEM, focused ion beam (FIB), and transmission electron microscopes (TEM) to determine the locus of failure and interface characteristics as well as to further assess the morphologies and topographies of the pretreated surfaces. These analytical studies have shown that the laser ablation regime has almost no influence on surface chemistries, i.e., the surfaces consist mostly of aluminium oxide/hydroxide while the surface structures are affected significantly and consequently is the adhesion and durability of the adhesive bonded joints. The failure analysis has revealed that the laser formed micro- and nano-structures act as weak boundary layers (WBLs) on the laser pretreated metal substrates and adversely affect theadhesion and durability of the joints. However, WBLs generated in the melt-displacement regime are stronger than the ones generated in melt-ejection regime. These WBLs on laser pretreated surfaces can be reduced or eliminated when phosphoric acid anodising (PAA) follows the laser pretreatment. The resulting joint performance is comparable or better than that of standard anodising, along with reduction in harsh chemical used in standard PAA
Prediction of Oxidation Induced Life Time for FCC Materials at High Temperature Operation
No full textWith an increasing application of high temperature alloys, especially Ni-based superalloys in automobile and other industrial fields, the ability to predict components‟ lifetime becomes a predominant demand from both safety and energy consumption aspects. In the present investigation, an attempt was made to develop a generalized oxidation lifetime model for chromia-forming FCC alloys that can be incorporated into alloy data sheets and easily understood and employed by component designers. The model captures the most important damaging oxidation effects relevant for component design: wall thickness loss, scale spallation and the occurrence of “breakaway” oxidation. The material used for development of the concept was the wrought NiCrW base alloy 230. For deriving modeling input parameters and for verification of the model approach, specimens of this alloy with different thicknesses were exposed cyclically for different times at temperatures in the range 950 - 1050°C in static laboratory air. The studies concentrated on thin specimens (thickness 0.2 - 0.5 mm) to obtain data for critical subscale depletion processes resulting in “breakaway” oxidation within reasonably achievable test times up to 3000 h. The oxidation kinetics and oxidation induced subscale microstructural changes from the long term tests were combined with results from thermogravimetric analyses (TGA), scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) spectroscopy and electron backscatter diffraction (EBSD), as well as glow discharge optical emission spectroscopy (GDOES). Oxidation induced microstructural changes as function of specimen thickness, time and temperature were estimated and modeled using the software packages Thermocalc and DICTRA. The modeling of the oxide scale spalling and re-formation was based on the cyclic oxidation spallation program (COSP), which was published previously. The program was modified to adapt the approach to the present experimental observations. A new model was developed to describe accelerated oxidation occurring after longer exposure times in case of the thinnest specimens. The calculated oxidation kinetics was correlated with the Cr reservoir equation, by means of which the relation between the consumption and the remained concentration of the scale forming element (Cr) in the alloys is established as a function of temperature and specimen thickness. The results obtained by the reservoir approach were compared with calculations of Cr concentration profiles using a finite difference model. Based on this approach, a generalized lifetime diagram is proposed in which wall thickness loss as function of time, specimen thickness and temperature as well as times to reaching a critical chromium subscale depletion are presented. The same approach was subsequently applied to the nickel base alloys X and NiCr 8020 as well as the austenitic steel Nicrofer 2020. Both Ni base alloys showed shorter times to critical subscale depletion than alloy 230; alloy X mainly due to higher scale growth rates, alloy NiCr 8020 due to poorer scale adherence and a lower initial Cr content. The austenitic steel showed the shortest lifetime mainly due to Mn and Ti induced high growth rates of the chromia scale and resulting low Cr interface concentrations
Mechanobiological effects of 3D-printed hydrogel-calcium phosphate composite materials on multipotent human mesenchymal stromal cells
No full textRecrystallization mechanisms in wrought magnesium alloys containing rare-earth elements
No full textModifying the chemical composition of magnesium alloys with special emphasis on rare earth (RE) elements has shown numerous indications of improved properties of this class of alloys, such as possibility of grain refinement, better formability at low temperatures, and enhanced strength and creep resistance at elevated temperatures. In particular, has been their ability in facilitating formation of less-common and unconventional sheet and extrusion textures. However, such texture modification in RE elements containing Mg alloys has been associated primarily with changes in the recrystallization texture, as opposed to changes in the deformation texture. What has been and still remains subject to greater scrutiny and investigation are the primary underlying physical mechanisms that are associated with RE-influence over recrystallization and grain growth behavior, whereby such significant texture modification is realized. The present work investigated in detail the influence of dilute additions of RE-elements upon deformation, recrystallization and grain growth behavior under varying processing conditions involving large strain hot rolling, plane strain compression and uniaxial tension tests. Comparisons were drawn between solid solution Mg-1Gd alloy and precipitate based Mg-1Ce alloy. Recrystallization inside shear bands, deformation twins and near particles was analyzed with context to texture formation features in the presence of rare-earths. By means of in-depth characterization ranging from atomic to bulk scales, the overall role of RE elements on the microstructure and texture development was understood. Shear banding in magnesium was enhanced in the presence of RE element additions. With respect to deformation twinning, addition of solute Gd triggered activation of {101 ̅1} compression and {101 ̅1}–{101 ̅2} double twins in conjunction with {101 ̅2} tension twinning. While dynamic recrystallization was severely retarded during the large strain rolling treatment, deformation under plane strain compression witnessed slip assisted dynamic recovery inside twins. Solute rare-earths indicated a significantly larger potency of texture modification during annealing, as opposed to precipitate based Mg-RE alloys. Texture modification during recrystallization and grain growth was attributed to selective growth behavior stemming from segregation of RE atoms at the grain boundaries, whereby off-basal orientations assumed a growth advantage over their basal counterparts. The results indicated that not all RE elements behave in a similar fashion and the choice of RE element is critical for the desired texture modification.Adding non-RE elements (Zn and Zr) amplified the texture modification effects in the solute Mg-1Gd alloy, while Mg-1Ce, in the additional presence of non-RE elements, started exhibiting modified off-basal textures. The enhancement in texture modification was attributed to a larger solute drag in the Gd containing alloy and an intricate interaction of particle nucleation and Zener pinning effects in the Ce containing alloy. The resultant impact on the room temperature ductility was significant with the quaternary alloys revealing total elongation values as high as 34 -40%. Atom probe tomography investigations performed on two solute based RE-alloys viz. Mg-1Gd and Mg-1Dy, further highlighted that the aforementioned influence of the choice of RE element also holds valid when considering two elements showing similar interaction with matrix i.e. both are in solid solution. It was shown that the segregation tendencies are not just contingent to solubility limits of RE elements but strongly depend upon other factors such as clustering/ordering and phase formation energies. It was suggested that the greater tendency of clustering/ordering shown by Gd facilitates much stronger segregation to the grain boundaries, whereby the impact on texture evolution mechanisms during annealing is substantial
Investigations on Hydrogen Embrittlement of Advanced High Strength Steels for Automotive Applications
No full textThe utilization of Advanced High Strength Steels (AHSS) in automotive applications is motivated by a push to reduce the weight of cars in order to minimize CO2 emissions. The potential risk of Hydrogen Embrittlement (HE) limits the widespread application of AHSS, making it one of the hottest topics in materials engineering and the automobile industry.The current work focused on the hydrogen-induced cracking of 1180 MPa steels for car body constructions. Various mechanical tests and forming tests to investigate internal hydrogen assisted cracking as well as hydrogen environment assisted cracking were conducted, concerning the automobile production processes and conditions during the life of a car. Tests of component-similar samples were carried out to compare laboratory results with those from the situation close to reality. The susceptibilities of various steels to HE were characterized and compared. The test methods were evaluated in the terms of differentiation and simplicity. Additionally, the material- and process-immanent factors on HE-susceptibility were elucidated and discussed.The results obtained in this work reveal that the immersion test with U-bend samples and the incremental loading test are appropriate to evaluate the HE-susceptibility for the material release process. Factors such as the presence of retained austenite, the high content of carbon equivalent and the coarse grain size increase the HE-susceptibility of AHSS. However, the addition of carbide-immanent elements Ti and Nb, and the alloying elements related to pitting corrosion resistance like Cr, Mo, N reduces the material HE-susceptibility. In the production processes the welding point and punched edges are sensitive to HE. The strain state during the forming process and the corrosive surroundings during the life of a car are also significant in the safe application of AHSS in car bodies. On the basis of these results, by controlling the factors influencing on HE-susceptibility, roughly divided into the three areas of material, hydrogen and stress, significant steps toward the safe usage of AHSS in component parts have been made
Direct hot rolled dual phase weathering steel
No full textNew alloying concepts are proposed to develop hot-rolled weathering-dual phase steels that combine good strength-ductility balance and enhanced atmospheric corrosion resistance. The proposed concepts comprise mainly the alloying elements that improve the atmospheric corrosion resistance (Cu, Ni, P and Cr) and those that facilitate the adjustment of dual phase microstructure during processing by hot rolling (C, Mn and Si). Moreover, Nb and combined Nb-Mo additions are utilized to enable the controlled-thermomechanical processing and to tailor the mechanical properties. The applicable processing window and cooling strategy for microstructure adjustment are designed on laboratory scale by means of thermomechanical simulation experiments. The most successful processing parameters are transferred to the pilot scale. The microstructures of the pilot hot-rolled sheets are investigated and the mechanical properties are evaluated by means of tensile, hole expansion and bending tests. The atmospheric corrosion behavior in comparison to a reference-dual phase steel is studied by conducting an accelerated cyclic atmospheric corrosion test. The developed weathering-dual phase steels achieve tensile strengths in the order of 730-940 MPa and average total elongation of 11-14%. Moreover, they corrode at an average corrosion rate of about 0.26 µm/week (under accelerated corrosion testing conditions), which is comparable to the corrosion rate of weathering steel (0.22 µm/week) and around 40% lower than that of the reference-dual phase steel (0.42 µm/week)
Der Einfluss von Wasserdampf auf den Sauerstofftransport in keramischen Hochtemperaturmembranen
No full textCeramic high temperature oxygen transport membranes (OTMs) represent a promising alternative for the extraction of oxygen from ambient air compared to conventional high-energy intensive processes like cryogenic air separation. Especially materials with a mixed ionic and electronic conductivity (MIEC) are of current interest as O-ion conductors. The high number of vacancies and the high mobility for electrons at sufficient high temperature and partial pressure gradient, enables these materials to transport oxygen via the crystal lattice and achieve a 100% selectivity. Due to their high conductivities, mostly perovskites or a combination of fluorite- and spinel-phases are used as OTM-materials. Depending on the application, OTMs can be used either directly for the oxygen production or in a so-called membrane reactor where chemical reactions are controlled by selective oxidation of particular reactants. This work examines the so-called "oxyfuel-combustion", where fossil fuels like coal are combusted under pure oxygen atmosphere in a power plant, to reduce NOx-emissions and make the combustion more efficient. A OTM-module implemented in the power plant should provide the needed amount of oxygen. The flue gas of such a oxyfuel power plant consist, except for HO and traces of SO, of a pure CO-stream which can be directly used for Carbon Dioxide Capture and Storage. To reduce the high temperatures occurring in the oxyfuel-combustion and to flush the oxygen from the membrane module, usually recycled flue gas is used. Unfortunately some of the most promising OTMmaterials show degradations facing CO2 and CO from the flue gas. Therefore a new concept was invented, where water vapor is used to sweep the membrane instead of the aggressive flue gas. On this point the current work deals with lab-scale experiments on different membrane materials in a permeation measurement setup with humidied sweep gas. Several OTM-materials are investigated concerning their long-term stability and the degradation of the permeation performance while using different amounts of water vapor in the sweep. All tested materials show a decreasing permeation rate for oxygen with increasing water content. The reason for this declined performance can be explained by miscellaneous materials corrosion and degradation mechanisms, which are discussed in the results part
Alkali sulphates induced hot corrosion of a β-NiAl coating on a second-generation single-crystal Ni-based superalloy
No full textHigh-temperature components in gas turbines are frequently exposed to sulphur-containing gases and alkali sulphate deposits during service. The deposit-induced accelerated corrosion attack termed hot corrosion may result in rapid degradation and failure of the turbine components. Hot corrosion studies of the involved materials (Ni-based alloys and coatings) are thus of significant importance for understanding its mechanisms, finding the way to mitigate the attack, and extending the life span of the components. In the present study, alkali sulphates induced hot corrosion of a β-NiAl coating on a single crystal Ni-based superalloy was investigated. Thermodynamics of the Na2SO4-K2SO4-NiSO4 system is essential for understanding the reactions during the hot corrosion process. In the present work, the phase equilibria in the system NiSO4-K2SO4 were experimentally determined for the first time using differential thermal analysis (DTA), dilatometry (DiL), and X-Ray diffraction method (XRD). Based on the obtained experimental data, the NiSO4-K2SO4 system was thermodynamically assessed using the CALPHAD methodology. This dataset extends the previously developed general sulphate database including alkali- and alkaline-earth sulphates (CaSO4-MgSO4-Na2SO4-K2SO4) with the addition of NiSO4. Using the optimized dataset, the stability diagrams and the liquid content of the Na2SO4-K2SO4-NiO-SO3 system and sub-systems were calculated, to provide an insight into the corrosion mechanisms. The aggressiveness of Na2SO4, K2SO4, and binary Na2SO4-K2SO4 deposits was compared by performing a series of hot corrosion tests using a β-NiAl coating on a single crystal Ni-based superalloy. Apart from the hot corrosion experiments, various mixtures of alkali sulphates and Ni-oxide/sulphate powders were prepared and exposed under the same conditions as the coated specimens. Comparison of the composition and morphologies of reaction products between the coated superalloy and mixed powders allowed a better understanding of the involved corrosion phenomena. The corrosion attack rates and morphologies could be correlated with the deposit compositions and its different reactions with oxides. K2SO4 and Na2SO4-K2SO4 deposits are shown to be more aggressive than Na2SO4. This is attributed to the reduction of melting temperature of the deposit and the formation of an intermediate compound K2Ni2(SO4)3 in the presence of K2SO4.The hot corrosion kinetics of the aluminised Ni-based superalloy was studied in a wide temperature range of 600-1000 °C under the Na2SO4-20% K2SO4 deposit. The corrosion rate-temperature dependence for the studied system reveals two maxima at around 700 °C and 950 °C, corresponding to type II and type I hot corrosion, respectively. Typical hot corrosion morphology was found at 600 °C, where the liquid formation is thermodynamically unlikely, indicating the importance of solid-state reactions for the component lifetime in the low-temperature range. At 800 °C, a localized corrosion attack (pitting) was identified at the coating grain boundaries, correlated with Cr-rich precipitates, that might promote the attack by reacting with the deposit. At longer exposure times localized pitting leads to coating penetration and corrosion attack of the base material. At 1000 °C, a relatively dense protective alumina scale formed, and the corrosion rate dramatically decreased compared to that at 850-950 °C
- …
