Oxide Spallation during Post-Isothermal High Temperature Oxidation Cooling of Cr-rich Cast Alloys highly alloyed with Hf
Cobalt, nickel and iron-based alloys containing 25wt.%Cr and strengthened either by chromium carbides or by HfC were oxidized at 1000, 1100 and 1200°C to obtain external oxide scales. The spallation of these oxides during the post-oxidation cooling was studied by exploiting the cooling part of the thermogravimetry curves. The best resistant alloys against scale spallation were the iron-based alloys which did not lose oxide. The nickel-based alloys well behaved too. The worst alloys were the cobalt alloys, principally due to the complex composition of the oxide scales. Beside the base element, the thickness of the oxide was also identified as a major parameter. The presence of Hf with contents much higher than usual was also beneficial for the resistance against spallation.
Kinetics of high temperature oxidation of chromium rich HfC reinforced cobalt based alloys
Abstract Mostly known to improve the high temperature oxidation resistance of superalloys, hafnium may also form carbides. Several per cents of Hf allow developing a dense carbide network to mechanically strengthen alloys. Here, the high temperature oxidation behaviour of three HfC containing cobalt alloys was characterised at all steps of a thermogravimetry test: heating, isothermal stage and cooling, compared with two Co–Cr–C model alloys. The five alloys were heated in synthetic air, maintained at 1200°C during 50 h and then cooled. The mass gains were plotted versus time or according to (m×dm/dt = Kp–m×Kv) to specify the isothermal kinetic constants, or versus temperature to determine how oxidation acts during heating and oxide spallation occurs during cooling. Compared to the ternary alloys, the oxidation of the HfC reinforced alloys starts earlier but leads to lower mass gains during heating, the isothermal oxidation is faster and oxide spallation occurs later.
HIGH TEMPERATURE OXIDATION BEHAVIOR OF NiAl-28Cr-5Mo-1Hf ALLOY
The isothermal oxidation behavior of NiAl-28Cr-5Mo-1Hf alloy at 1000-1200 ℃ in air has been studied. The results show that isothermal oxidation kinetics of the NiAl-28Cr-5Mo-1Hf alloy obeys the parabolic law at all test temperatures. The microstructure and composition of the oxide layer have been studied by means of X-ray diffractometer, scanning electron microscope with an EDAX unit. The oxidation scale mainly consists of a-Al2O3, and there are also Cr2O3 and a little HfO2. The oxidation mechanism was discussed from the phase contitution.