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FATIGUE CHARACTERISTICS OF MODIFIED MAGNESIUM ALLOYS AFTER CORROSION DEGRADATION
Němcová, Aneta ; Očenášek, Vladivoj (referee) ; Hadzima, Branislav (referee) ; Pacal, Bohumil (advisor)
This doctoral thesis deals with the determination of the influence of plasma electrolytic oxidation (PEO) on fatigue behaviour of extruded AZ61 magnesium alloy in air and in the 3.5% NaCl solution. The coatings were formed in the silicate-phosphate electrolyte under pulsed current conditions at a frequency of 50 Hz. The influence of current density on coating formation was examined under current densities of 70, 130 and 200 mA cm-2 for different durations up to a maximum of 1800 s. 8 g dm-3 of KF were added to the electrolyte to study the influence of fluoride ions in plasma electrolytic oxidation. It is shown that fluoride ions inhibit localised oxidation in the initial stage of the process, associated with the secondary particles based on Al–Mn. The presence of fluoride also modified the sparking characteristics, decreased the rate of coating growth and changed the morphologies of the coatings. The influence of fluoride on the coating hardness, and the corrosion resistance of the alloy during exposure to salt spray, was negligible. Based on previous optimised PEO conditions, coatings formed under a current density of 130 mA cm-2 for 300 s in the electrolyte containing KF were chosen for fatigue testing. The high-cycle fatigue tests were carried out on cylindrical samples under a force controlled sinusoidal tension-tension cycle with asymmetry parameter R=0. The experimental data were fitted with Kohout & Věchet function. The fatigue limit of uncoated alloy in air for 107 cycles was determined at 145.4 MPa and the combination of PEO coating with chloride ions caused a reduction of ~55 %. Attention was paid to the fatigue crack initiation in different conditions of cyclic loading. The fracture surfaces underwent detailed fractography analysis including secondary crack observation on the gauge length. The contribution of Al–Mn particles were confirmed on the uncoated alloy in air and the presence of chloride ions were observed as another influential contributor to local corrosion attack. The cyclic loading caused spalling of the outer layer, and the multiple initiation was observed on PEO coated alloy, caused by cracks in the coating and stress transferring to the alloy.
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Physical and mechanical properties of lead-free solder joints
Harcuba, Petr ; Janeček, Miloš (advisor) ; Drozd, Zdeněk (referee) ; Očenášek, Vladivoj (referee)
Title: Physical and mechanical properties of lead-free solder joints Author: Petr Harcuba Department: Department of Physics of Materials Supervisor: prof. RNDr. Miloš Janeček, CSc., Department of Physics of Materials Abstract: Due to recent restriction of the classical Sn-Pb solder alloy, lead-free solders became an extensively investigated class of materials. This thesis focused on selected Sn-Cu based alloys. The influence of Cu content and minor additions of Ni and P on IMC layers morphology and reliability of simulated solder joints were investigated. The growth kinetics of IMC layers and mechanical properties of solder joints were studied after reaction of molten solders with Cu substrate at various times and temperatures, and after subsequent ageing at elevated temperatures. Detailed analysis of the evolution of IMC layers morphology was performed using EBSD. Higher Cu concentration in solders decreased the dissolution rate of Cu substrate. It was shown that the addition of 0.1 at.% of Ni significantly changed the morphology of IMC layers and improved mechanical properties of solder joints. Addition of P had only a minor effect on the IMC growth and the solder joint reliability. Keywords: Lead-free solders, intermetallic compound layers, growth kinetics, reliability of solder joints
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FATIGUE CHARACTERISTICS OF MODIFIED MAGNESIUM ALLOYS AFTER CORROSION DEGRADATION
Němcová, Aneta ; Očenášek, Vladivoj (referee) ; Hadzima, Branislav (referee) ; Pacal, Bohumil (advisor)
This doctoral thesis deals with the determination of the influence of plasma electrolytic oxidation (PEO) on fatigue behaviour of extruded AZ61 magnesium alloy in air and in the 3.5% NaCl solution. The coatings were formed in the silicate-phosphate electrolyte under pulsed current conditions at a frequency of 50 Hz. The influence of current density on coating formation was examined under current densities of 70, 130 and 200 mA cm-2 for different durations up to a maximum of 1800 s. 8 g dm-3 of KF were added to the electrolyte to study the influence of fluoride ions in plasma electrolytic oxidation. It is shown that fluoride ions inhibit localised oxidation in the initial stage of the process, associated with the secondary particles based on Al–Mn. The presence of fluoride also modified the sparking characteristics, decreased the rate of coating growth and changed the morphologies of the coatings. The influence of fluoride on the coating hardness, and the corrosion resistance of the alloy during exposure to salt spray, was negligible. Based on previous optimised PEO conditions, coatings formed under a current density of 130 mA cm-2 for 300 s in the electrolyte containing KF were chosen for fatigue testing. The high-cycle fatigue tests were carried out on cylindrical samples under a force controlled sinusoidal tension-tension cycle with asymmetry parameter R=0. The experimental data were fitted with Kohout & Věchet function. The fatigue limit of uncoated alloy in air for 107 cycles was determined at 145.4 MPa and the combination of PEO coating with chloride ions caused a reduction of ~55 %. Attention was paid to the fatigue crack initiation in different conditions of cyclic loading. The fracture surfaces underwent detailed fractography analysis including secondary crack observation on the gauge length. The contribution of Al–Mn particles were confirmed on the uncoated alloy in air and the presence of chloride ions were observed as another influential contributor to local corrosion attack. The cyclic loading caused spalling of the outer layer, and the multiple initiation was observed on PEO coated alloy, caused by cracks in the coating and stress transferring to the alloy.
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