The internal oxidation (IO) of Ag - VC composites containing 5 vol.% of carbide was examined at three oxidation temperatures (350, 400, 600 °C). Two mechanisms were observed due to the formation of double oxides with relativelylow melting points. At temperatures below the lowest eutectic point in-situ, or diffusion less IO, was observed with the formation and growth of oxide layers surrounding the initial carbide. At temperatures above the eutectic point IO resulted in the formation of liquid oxide pools, which grew in size and developed into a network of oxides within the metal matrix. The kinetics confirmed the presence of two distinct mechanisms.
COBISS.SI-ID: 14376214
Rapidly solidified ribbons of Cu–Cr alloys with 2.27 and 4.20 at. % of chromium were produced using the meltspinning method. Alloys were analysed by electron microscopy for complete solubility of Cr in copper matrix. To avoid disturbing effects of Cr phase particles, the kinetics and the sequence of microstructural transformations during heating were analysed only the sample with 2.27 at.% of chromium with complete Cr solubility in the copper matrix. We then investigated the precipitation process for this alloy that was subsequently heated at a constant rate. The increased solid solubility obtained allowed the extensive precipitation of a Crrich phase. The kinetics and the sequence of microstructural changes that occurred during the heating were analysed using an in-situ measurement of the electrical resistance. The quenched microstructure was analysed at transition points using scanning and transmission electron microscopy. X-ray photoelectron spectroscopy, as a very surfacesensitive method, was applied to study the changes in the chemical composition of the surface for the Cu–Cr alloy ribbons in the temperature range 400–700 °C during an in-situ heat treatment in an ultrahigh vacuum. The results show a relatively rapid precipitation of chromium to the surface, which starts at 400 °C and is correlated with a change in the microstructure and the electrical resistance. The Cr precipitation is faster at higher temperatures and follows the parabolic law. The resistivity results for the supersaturated binary alloy were analyzed using the Ozawa method to give an activation energy for the precipitation of 196 ± 10 kJ/ mol.
COBISS.SI-ID: 26708263
This work studied the phases in the Al corner of the Al-Mn-Be phase diagram inthe as-cast state and heat-treated conditions. Metallographic investigations, X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy were used for identifying the phases. The Be contents in the identified phases were precisely determined using Auger electron spectroscopy. The results indicated that Al6Mn does not dissolve Be, whilst delta-Al4Mn dissolves up to 7 at.% Be. The average composition of the T phase, which is normally designated as Al15Mn3Be2, was 72 at.% Al, 19 at.% Mn, and 9 at.% Be. The phase with the nominal composition Be4AlMn contained more Al than Mn. The atomic ratio Al:Mn was between 1.3:1 and 2:1. The hexagonal Be-rich phase did not dissolve any Al and Mn. The icosahedral quasicrystalline (IQC) phase contained up to 45 at.% Be. The compositions of T phase, delta-Al4Mn, IQC, and Be4AlMn may vary, however, the ratio (Al + Be):Mn remained constant, and was close either to four or six indicating substitution of Al atoms with Be atoms in these phases.
COBISS.SI-ID: 16956694
In this work, the results of the transformation kinetics of austenite heatresistant stainless steel (AISI 310S) with 2.27 wt.% silicon were presented. The results of microstructural and fractography analysis, as well as the results of hardness after the isothermal heat treatment of the steel in the temperature range from 600 to 900 oC at different annealing times (from 1 to 956 h) are shown. It was found that the microstructure of steel after isothermal annealing consisted of austenite, carbide (M23C6) and rphase. The number of rphase precipitates increases with higher annealing times. In longer annealed steel samples the significant coarsening of precipitates was observed, as well as a large amount of rphase particles in the form of a chain and a network of plates. Precipitates were both observed at grain boundaries and within austenite grains. rphase precipitation involves two mechanisms: transformation c?Cr23C6?r and transformation c?a0 ?r. The hardness began to increase after 48 h of annealing at a temperature of 600 oC, while in the temperature range of 700–900 oC the hardness increased with annealing times higher than 8 h. The slope of hardness curve is caused by microstructural changes. The fracture surface mode was intercrystalline brittle. Relatively large dimples can be related to large particles of precipitates which occurred after long annealing times.
COBISS.SI-ID: 26633511
In fillet welds made of 18Cr–9Ni–7Mn austenitic stainless steel centerline cracks were established along the connection line of two solidification fronts in the weld. On the grain boundaries oxide and carbide films were identified. Hot cracks resulted from a combined activity of a large gap between plates and the resulting high tensile stresses occurring during cooling, free root surface of the weld in the large gap, and intergranular oxide and carbide films in the weld. Since the parent metal was cut with highly oxidating plasma and atmospherically corroded surfaces was not ground prior to welding, remelted surface oxides passed into the weld pool. The increased Cr content identified in intergranular oxide films results from Cr oxidation in the filler material.
COBISS.SI-ID: 1278303