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Abstract: In this investigation, strips of an experimental Al-Mn-Cu-Be alloy were manufactured by high-speed single-roll and twin-roll casting to stimulate the formation of a quasicrystalline phase during solidification. The strips were characterised by light microscopy, scanning and transmission electron microscopy, microchemical analysis, and X-ray diffraction. Indentation testing was used to determine the mechanical responses of the strips in different areas. A smooth surface was achieved on both sides of the twin-roll-cast strip, while the free surface of the single-roll-cast strip was rough. The microstructures in both strips consisted of an Al-rich solid solution matrix embedding several intermetallic phases Θ-Al2Cu, Be4Al (Mn, Cu), Al15Mn3Be2 and icosahedral quasicrystalline phase (IQC). The microstructure of the single-roll-cast strip was more uniform than that of the twin-roll-cast strip. Coarse Al15Mn3Be2 particles appeared in both alloys, especially at the centre of the twinroll strip. These coarse particles adversely affected the strength and ductility. Nevertheless, both casting methods provided high-cooling rates, enabling the formation of metastable phases, such as quasicrystals. However, improvements in alloy composition and casting procedure are required to obtain enhanced microstructures and properties.
Keywords: single-roll casting, twin roll casting, microstructure, quasicrystal, hardness, aluminium, characterisation
Published in DKUM: 24.03.2025; Views: 0; Downloads: 0
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Abstract: A new silver-based alloy with 2 wt.% of lanthanum (La) was studied as a potential candidate for electric contact material. The alloy was prepared by rapid solidification, performed by the melt spinning technique. Microstructural examination of the rapidly solidified ribbons revealed very fine grains of αAg and intermetallic Ag5La particles, which appear in the volume of the grains, as well as on the grain boundaries. Rapid solidification enabled high microstructural refinement and provided a suitable starting microstructure for the subsequent internal oxidation, resulting in fine submicronsized La2O3 oxide nanoparticle formation throughout the volume of the silver matrix (αAg). The resulting nanostructured Ag-La2O3 microstructure was characterised by high-resolution FESEM and STEM, both equipped with EDX. High-temperature internal oxidation of the rapidly solidified ribbons essentially changed the microstructure. Mostly homogeneously dispersed nano-sized La2O3 were formed within the grains, as well as on the grain boundaries. Three mechanisms of internal oxidation were identified: (i) the oxidation of La from the solid solution; (ii) partial dissolution of finer Ag5La particles before the internal oxidation front and oxidation of La from the solid solution; and (iii) direct oxidation of coarser Ag5La intermetallic particles.
Keywords: Ag-La alloy, rapid solidification, metastable microstructure, internal oxidation, characterisation, formation mechanism
Published in DKUM: 20.03.2025; Views: 0; Downloads: 0
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Abstract: A complementary characterisation of the Al-alloy EN AW-5454 was carried out, intended for obtaining the laser hybrid welding parameters of subassemblies in the automotive industry. The investigation included a microstructural examination and the determination of the alloy’s properties using several analytical methods (HV5 hardness measurement, tensile test, Charpy impact toughness, fracture mechanics analysis). Samples were prepared in the longitudinal and transverse directions of a cold-rolled sheet of EN AW-5454 with thicknesses of 3.5 mm and 4 mm. The measured hardness on the thinner sheet was 5% higher than on the thicker sheet. The tensile and yield strength were nominal, while the elongations were smaller by 2.2–3.2% for the longitudinal samples and by 2.7–13.7% for the transverse samples. The smaller deviations from the nominal values are for the thinner sheet metal. A precise topographical analysis showed the brittle fractures of the samples. The Charpy impact toughness results on the thicker plate showed a 20% greater work needed to break it in the longitudinal direction than in the transverse direction. With the thinner sheet metal, 40% greater work was needed. SEM (scanning electron microscope) analysis has shown that the intermetallic Al6(Mn,Fe) particles in the longitudinal samples were mostly intact, with evidence of tough areas on the upper part of the fracture, indicating a better toughness than the specimens in the transverse direction. More crushed intermetallic particles were observed at the fractures of the transverse samples, and their distribution appeared to be more oriented in the direction of rolling. Fracture mechanics SENB (single edge notch bending) tests and their analysis showed that the resistance of the material to crack propagation in the longitudinal sample was about 50% greater than that in the transverse sample. SEM analysis of the fractures showed that the state of the intermetallic particles in the fracture mechanics testing and the fracture mechanism differed from the one in the Charpy fractures.
Keywords: Al-alloy EN AW-5454, characterisation, microstructure, properties
Published in DKUM: 02.12.2024; Views: 0; Downloads: 5
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Abstract: This study presents the comprehensive experimental investigation of the microstructure, mechanical and fatigue properties of a new high-strength aluminium alloy AA 6086, which was developed from a commercial aluminium alloy AA 6082. The new alloy possesses a higher content of Si, and, it also contains Cu and Zr. The alloy was characterised in the as-cast condition after homogenisation, extrusion, and T6 heat treatment. Light microscopy, scanning and transmission electron microscopy with energy dispersive spectrometry were used to analyse the microstructure and the fractography of broken specimens. The quasi-static and fatigue tests were performed on the MTS Landmark 100 kN servo-hydraulic test machine, controlled with a mechanical extensometer with a 25 mm gauge length. The quasi-static strength of the analysed aluminium alloy AA 6086 was found to be significantly higher if compared to some other AA 6xxx alloys, while the ductility was kept almost the same. The experimental results of the comprehensive fatigue tests in a Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) regime showed a good fatigue resistance, and represent a good basis for engineering design applications of the newly developed aluminium alloy AA 6086.
Keywords: aluminijeve zlitine, karakterizacija materiala, utrujanje, eksperimentalno testiranje, statistično ovrednotenje, Aluminium Alloy AA 6086, material characterisation, fatigue behaviour, experimental testing, statistical evaluation
Published in DKUM: 02.04.2024; Views: 281; Downloads: 29
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Abstract: The study presents an extensive mechanical and computational characterisation of novel cellular metamaterial with axisymmetric chiral structure (ACS) at different strain rates. The Direct Impact Hopkinson Bar (DIHB) testing device was used for impact testing up to 21 m/s striker speed, which was insufficient to reach the shock deformation regime. Thus, using computational simulations to estimate the structure behaviour at high strain rates was necessary. Experimental and computational results showed that all ACS structures exhibit a nominal stress–strain relationship typical for cellular materials. As the loading conditions shifted to a dynamic regime, the micro–inertia effect became increasingly pronounced, leading to a corresponding rise in structure stiffness. The Poisson's ratio in all ACS increases gradually, making them superior to traditional cellular materials, which experience a sudden increase in Poisson's ratio during loading. Additionally, the study found that the structures exhibited a rise in the auxetic effect with an increase in strain rate, highlighting the benefits of axisymmetric structures in high-loading regimes. Overall, the obtained results provide valuable insights into the mechanical properties of ACS under different loading regimes and will contribute to further design improvements and the fabrication of novel ACS metamaterials.
Keywords: axisymmetric chiral structure, auxetic, chiral unit cell, impact testing, dynamic characterisation, finite element simulations
Published in DKUM: 15.02.2024; Views: 345; Downloads: 39
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