Notranja oksidacija Cu-C in Ag-C kompozitov

The internal oxidation in copper-carbon and silver-carbon composites occurs when they are exposed to air or oxygen at high temperature. Solubility of carbon in copper or in silver is very low. The kinetics of oxidation at high temperature and activation energy were determined and the mechanism of internal oxidation was analysed. The kinetics of internal oxidation was determined for both cases and it is depended from the diffusion of oxygen following parabolic time dependence according to Wagner\u27s theory. The activation energy for Cu-C composite is 70.5 kJ/mol, and for Ag-C composite is 50.1 kJ/mol, what is in both cases close to the activation energy for the volume diffusion of oxygen in copper or in silver. In both cases gas products are formed during the internal oxidation of composites. In the internal oxidation zone pores, bubbles occur. The carbon oxidates directly with the oxygen from solid solution as long there is a contact, which breaks down with the presence of gas products. Then the oxidation occurs over the gas mixture of CO and CO2.Pri visokih temperaturah kompoziti bakra in srebra z ogljikom na zraku ali v kisiku reagirajo po mehanizmu notranje oksidacije. Topnost ogljika v trdnem bakru in trdnem srebru je zelo majhna. Analizirali smo kinetiko oksidacije kompozitov, določili aktivacijsko energijo in mehanizem notranje oksidacije. Kinetika oksidacije je pri obeh skupinah materialov odvisna od difuzije kisika in sledi parabolični odvisnosti od časa v skladu z Wagnerjevo teorijo. Aktivacijska energija procesa je za kompozit Cu-C enaka 70,5 kJ/mol, za kompozit Ag-C pa 50,1 kJ/mol, kar je blizu aktivacijski energiji za volumsko difuzijo kisika v trdnem bakru oziroma srebru. Pri oksidaciji kompozita nastajajo plinski produkti. Oksidacija ogljika poteka neposredno s kisikom iz trdne raztopine, ko pa se zaradi nastanka plinske faze stik prekine, pa preko plinske zmesi CO in CO2

The mechanics behind formation of secondary ledeburite during tool steel welding

U ovom prilogu opisujemo mikrostrukturne promjene u zoni toplinskog utjecaja (ZTU) alatnog čelika W. Nr. 1.2379, koji je bio navaren ili zavaren po postupku zavarivanja pod praškom s različitim parametrima zavarivanja. Mikrostrukturu zavara i navara analizirali smo pomoću optičnog i rasterskog (skenirajućeg) elektronskog mikroskopa. U analiziranju smo posebno utvrđivali mikrostrukturne promjene u okolini primarnih kromovih karbida u zoni toplinskog utjecaja i njihov utjecaj na kristalizaciju zavara. Utvrdili smo, da su temperature u zoni toplinskog utjecaja za vrijeme zavarivanja dovoljno visoke da se za vrijeme rastapanja primarnih karbida u okolnoj matici do te mjere povećala koncentracija karbidotvornih elemenata i ugljika, da je nastala tz. talina eutektičnog sastava, koja se stvrdnula u sekundarni eutektik (ledeburit), na kojemu se na granici ZTU/zavara pokrene kristalizacija zavara.In this article we describe the microstructural changes in the heat affected zone (HAZ) of the tool steel W. Nr. 1.2379, which was surfaced or welded by the submerged arc welding technique (SAW) with different welding parameters. Microstructure of the welds and of the surfacing welds was analysed by optical and scanning electron microscope. In this research, we particularly studied microstructural changes in the area of primary chromium carbides in the HAZ and their effect on the weld crystalization. We came to the conclusion that the temperature in the HAZ is high enough during the welding process that it caused primary carbides to dissolve and concentration of the carbide-forming elements and carbon increased in the surrounding austenite matrix area to eutectic composition, which remelts and solidified as secondary eutectic (ledeburite)

Microstructure and Mechanical Properties of Friction Stir Welded AlMg4.5Mn Alloy

A comprehensive research of Friction Stir Welding of 4 mm thick sheet aluminium alloy (AlMg4.5Mn) for forming was done. A vast variety of process parameters was tested according to the plan of experiments at constant 2° tilt angle. Specially designed tensile test specimens were sectioned perpendicularly to the welding direction. The microstructure was prepared for the observation on a light microscope under the polarized light source. Vickers micro-hardness was measured. The results show the influence of FSW process parameters on the formation of the microstructure and mechanical properties

Effect of heat treatment on corrosion properties of CuAlNi shape memory alloy

The effect of heat treatment on corrosion properties of CuAlNi shape memory alloy was investigated in 0.9% NaCl solution at pH 7.4 and 37 °C by open circuit potential measurements, polarisation techniques, and electrochemical impedance spectroscopy. Investigations were performed on CuAlNi alloy samples in as-cast state and after heat treatment procedure containing annealing at 850, 885 and 920 °C followed by water quenching. Electrochemical impedance measurement results indicate that heat treatment of CuAlNi alloy leads to the increase in charge transfer resistance and surface layer resistance and the decrease in values of capacitance of the double and surface layers, indicating higher corrosion resistance compared with the as-cast CuAlNi alloy. The increase in polarisation resistance and the decrease in corrosion current density of heat-treated CuAlNi alloy also suggest beneficial influence of heat treatment on corrosion resistance of CuAlNi alloy. Optical microscopy, SEM/EDX and XRD analysis of samples surface after polarisation measurements show the occurrence of pitting corrosion on the electrode surfaces, with the existence of CuCl2, AlCl3 and Cu2Cl(OH)3 compounds as the surface corrosion products

Effect of heat treatment on corrosion properties of CuAlNi shape memory alloy

The effect of heat treatment on corrosion properties of CuAlNi shape memory alloy was investigated in 0.9% NaCl solution at pH 7.4 and 37 °C by open circuit potential measurements, polarisation techniques, and electrochemical impedance spectroscopy. Investigations were performed on CuAlNi alloy samples in as-cast state and after heat treatment procedure containing annealing at 850, 885 and 920 °C followed by water quenching. Electrochemical impedance measurement results indicate that heat treatment of CuAlNi alloy leads to the increase in charge transfer resistance and surface layer resistance and the decrease in values of capacitance of the double and surface layers, indicating higher corrosion resistance compared with the as-cast CuAlNi alloy. The increase in polarisation resistance and the decrease in corrosion current density of heat-treated CuAlNi alloy also suggest beneficial influence of heat treatment on corrosion resistance of CuAlNi alloy. Optical microscopy, SEM/EDX and XRD analysis of samples surface after polarisation measurements show the occurrence of pitting corrosion on the electrode surfaces, with the existence of CuCl2, AlCl3 and Cu2Cl(OH)3 compounds as the surface corrosion products

Corrosion behavior of CuAlMn and CuAlMnNi alloy in 0.9% NaCl solution

Corrosion behavior of CuAlMn and CuAlMnNi alloy ribbons, produced by melt spinning method, were investigated by electrochemical methods such as open circuit potential measurement, linear and potentiodynamic polarization method. Investigations were performed in deaerated 0.9% NaCl solution (T = 37 °C pH = 7.4). Results of electrochemical investigations have shown that CuAlMnNi alloy have higher values of polarization resistance and smaller values of corrosion current density, but in higher anodic potentials region anodic current density for CuAlMn is lower than for CuAlMnNi alloy which indicates higher dissolution of CuAlMnNi alloy. After polarization measurements CuAlMn and CuAlMnNi ribbon surfaces were investigated with light microscope and with SEM/EDS analysis and results have shown that CuAlMnNi alloy is prone to pitting corrosion, while the surface of CuAlMn alloy is partially covered with corrosion product without existence of pits

Corrosion behavior of CuAlMn and CuAlMnNi alloy in 0.9% NaCl solution

Corrosion behavior of CuAlMn and CuAlMnNi alloy ribbons, produced by melt spinning method, were investigated by electrochemical methods such as open circuit potential measurement, linear and potentiodynamic polarization method. Investigations were performed in deaerated 0.9% NaCl solution (T = 37 °C pH = 7.4). Results of electrochemical investigations have shown that CuAlMnNi alloy have higher values of polarization resistance and smaller values of corrosion current density, but in higher anodic potentials region anodic current density for CuAlMn is lower than for CuAlMnNi alloy which indicates higher dissolution of CuAlMnNi alloy. After polarization measurements CuAlMn and CuAlMnNi ribbon surfaces were investigated with light microscope and with SEM/EDS analysis and results have shown that CuAlMnNi alloy is prone to pitting corrosion, while the surface of CuAlMn alloy is partially covered with corrosion product without existence of pits

The influence of pH and electrolyte temperature on corrosion behaviour of CuAlMnTi alloy ribbons in NaCl solution

The influence of pH and temperature of 0.9% NaCl solution on corrosion behavior of CuAlMnTi alloy ribbons, produced by rapid solidification using melt spinning method, were investigated by electrochemical methods. Open circuit potential measurement, linear and potentiodynamic polarization were employed during the investigation, and the measurements were conducted in the electrolyte temperature of 10, 24, 37 and 50 oC and in solution pH of 7.4, 5.4 and 3.4. In has been found that corrosion rate generally increase with increasing the temperature of the electrolyte while the influence of pH change on CuAlNiTi ribbon corrosion is little less pronounced. After polarization measurements CuAlNiTi ribbon surfaces were investigated with light microscope and with SEM/EDS analysis

The influence of pH and electrolyte temperature on corrosion behaviour of CuAlMnTi alloy ribbons in NaCl solution

The influence of pH and temperature of 0.9% NaCl solution on corrosion behavior of CuAlMnTi alloy ribbons, produced by rapid solidification using melt spinning method, were investigated by electrochemical methods. Open circuit potential measurement, linear and potentiodynamic polarization were employed during the investigation, and the measurements were conducted in the electrolyte temperature of 10, 24, 37 and 50 oC and in solution pH of 7.4, 5.4 and 3.4. In has been found that corrosion rate generally increase with increasing the temperature of the electrolyte while the influence of pH change on CuAlNiTi ribbon corrosion is little less pronounced. After polarization measurements CuAlNiTi ribbon surfaces were investigated with light microscope and with SEM/EDS analysis

Corrosion investigation of rapidly solidified Cu- Al-Ni alloy in NaCl solution

The corrosion behavior of the Cu-Al-Ni alloy ribbons obtained by rapid solidification was investigated in 0.9% NaCl solution (pH 7.4) at temperatures of 24, 37 and 50 ºC. Measurements have been performed by electrochemical methods such as monitoring the open circuit potential, linear and potentiodynamic polarization methods. The influence of chloride ions concentration on corrosion of Cu-Al-Ni ribbons has also been investigated in 0.1% and 1.5% NaCl solution. Results of the investigations have shown that an increase in chloride concentration and electrolyte temperature leads to increase the corrosion current density and decrease the polarization resistance values which mean higher corrosion attack on Cu-Al-Ni alloy. Microscopic images have shown significant pitting corrosion damages on the Cu-Al-Ni alloy surface. Increasing the electrolyte temperature increases the surface damages of the electrode due to more intense corrosion. SEM surface images of Cu-Al- Ni electrodes have confirmed that with the elevation of chloride concentration a more intense corrosion attack occurs. EDS surface analysis indicated dominant percentage of copper, chlorine and oxygen on the surface, indicating the formation of copper oxide and chloride as the major corrosion products on the surface. The presence of a small percentage of aluminum indicates its distribution in the form of aluminum oxide and chloride in the surface layer