Effects of Sc Addition and Direct Aging Treatment on Microstructure and Hardness of AlSi10Mg Alloy

Cu-mold centrifugal cast processing is employed as a rapid solidification method for producing samples with and without Sc. The Al–Si–Mg and Al–Si–Mg–Sc alloy samples are exposed to direct aging treatments varying temperature and time to verify the microstructural changes. Both rapidly solidified samples and as-aged samples are characterized by a number of methods, including optical microscopy, SEM–EDS, transmission Electron Microscopy (TEM)–EDS, TEM–HAADF, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Vickers hardness. At first, the results point to a strong precipitate-related hardening effect formed as a result of the Sc addition to the alloy. All samples containing Sc show a higher hardness value when compared to their respective treated samples without Sc. Second, when comparing the Al–10Si–Mg–0.4Sc alloy samples among themselves after being treated at different conditions, high temperatures, and excessive treatment times are recognized as detrimental to the hardness. This is due to the growth of larger Sc-bearing precipitates of approximately 1 μm in size under such conditions, having lower efficiency in pinning dislocations during loading. The best aging condition is 255 °C for 60 min, which produces a very fine dispersion of Mg and Sc intermetallics (200 nm in size) with a peak hardness of 110 HV.     

Development of a three-phase power-flow calculation method for distribution systems with automatic handling of arbitrary winding connections of transformers

Single-phase loads and photovoltaic generation cause three-phase imbalance in distribution systems, and prospective growth of normal chargers of electric vehicles may even increase the imbalance. The analysis of such unbalanced systems requires the three-phase power-flow calculation. Since existing methods require the admittance matrices of three-phase transformers, they must be derived for all possible winding connection patterns in advance to its computer-code implementation. This paper proposes a three-phase power-flow calculation method which formulates circuit equations using the modified nodal analysis, making it possible to automatically handle any winding connection. The power-flow constraints are then embedded into the circuit equations using a fixed-point iteration. Newton-Raphson, backward/forward sweep and fixed-point iteration methods are the existing three categories of solution methods. Newton-Raphson methods may show convergence problems due to the high R/X ratios of distribution lines. Backward/forward sweep methods cannot be used, because loops are temporarily formed in a distribution system during circuit switching. These justify the use of a fixed-point iteration. In this paper, the proposed method is validated by practical examples.     

Differential Sintering and Self-Stress Effects on YSZ Ionic Conductivity

Thermomechanical stress simulations are combined with experimental tests to assess the effects of rigid inclusions on the sintering of 8 mol% yttria-stabilized zirconia (8YSZ) green compacts and the phenomena of restricted and differential sintering on microstructure development and electrical properties are investigated. Rigid inclusions of sintered ceramic particles with different shapes (spherical and jagged) and compositions (alumina, 3YSZ, and 8YSZ) are added in different volume fractions (1, 5, and 15 vol%) to 8YSZ commercial powders, which are formed by isostatic pressing and sintered by conventional method. Restricted and differential sintering effects are observed in the development of the microstructure varying in function of volume fraction, shape, structural composition, and thermomechanical properties of the inclusions, resulting in different combinations of tensile and compressive strain states in the matrix, and varying electrical behaviors. The addition of 1 vol% of 8YSZ irregular rigid inclusions leads to an increase of 36% in total electrical conductivity and a 33% increase in power density under solid oxide fuel cells operation conditions compared to samples without inclusions.     

In-Situ X-ray Diffraction Analysis of Metastable Austenite Containing Steels Under Mechanical Loading at a Wide Strain Rate Range

Metallurgical and Materials Transactions A - This paper presents and discusses the methodology and technical aspects of mechanical tests carried out at a wide strain rate range with simultaneous...     

Synthesis and properties of alkali-activated iron ore tailings blended with metakaolin

In this study, synthesis, microstructural characteristics, mechanical properties, and environmental compatibility of alkali-activated binders derived from iron ore tailings (IOT)—with partial replacement with metakaolin (MK)—were investigated. The binders were produced with a NaOH solution, IOT, and MK. A thermal cure at 100°C was used and the MK was applied as a partial replacement of IOT in three proportions (10, 20, and 30 wt%). The IOT-based specimens presented an average of 98.0 and 18.0 MPa at 7 days age of curing for compressive and flexural strength, respectively. The mechanical properties of the alkali-activated binders with MK decreased as the substitution ratio increased. By the microstructural analysis, it was found a zeolite-type phase in alkali-activated IOT, while in the binders blended with MK, three new mineral phases were identified. Furthermore, Fourier transform infrared, quantitative X-ray diffraction, and environmental analysis suggest that the Fe species present in IOT acted in the alkali-activation reaction.     

Thermo-oxidative aging of acrylonitrile-butadiene rubber gaskets with real geometry used in plate heat exchangers

This study investigates the effects of aging on the physical and mechanical properties of commercially available acrylonitrile-butadiene rubber (NBR) gaskets while maintaining their original geometry. Thermo-oxidative cycles with 10 and 70 mm in length specimens were conducted from 80 to 170°C up to 180 days. The samples were analyzed employing compression set (CS), hardness, indentation modulus, cross-link density, total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and thermogravimetry. The results showed that longer specimens presented better resistance to thermo-oxidative aging. Indentation results indicated regular oxygen permeability into the entire samples up to 110°C, while at higher temperatures, limited diffusion oxidation (DLO) effects promoted non-uniform aging. Time–temperature superposition (TTS) and Arrhenius methods were applied to predict the specimens' lifetime using CS as a failure criterion. Activation energies for 10 and 70 mm samples were 68.74 and 43.63 kJ mol⁻¹, respectively. Thus, the 70 mm specimen's lifetime was greater than 10 mm. For temperatures below ≈38°C, the response to the thermo-oxidative aging is independent of specimen length. Therefore, in determining the lifetime of gaskets with complex geometry, longer specimens are recommended to provide more reliable results than those suggested by the standards.