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.
Effect of NbC Addition on the High-Temperature Oxidation Resistance of Co- and Ni-Based Chromium-Rich Alloys
Two {M–25Cr}-based alloys, with M = Co or Ni, containing 0.5C and 3.9Nb (wt%) to favor the crystallization of NbC carbides during solidification were prepared by casting and then subjected to oxidation in air at 1000, 1100 and 1200 °C for 46 h. The results were studied by comparison with recent similar ones obtained with Nb-free ternary alloys issued from the same preparation protocol and tested in oxidation in the same conditions. In both cases interdendritic script-like-shaped NbC carbides were obtained as expected, however, not as single carbides in the nickel alloy which also contain chromium carbides. Preliminary exposure to 1200 °C having shown a morphological evolution of the NbC, as-cast and preliminarily aged versions of both alloys were tested in oxidation. It appeared first that, whatever the aged state, the cobalt alloy behaved much worse than the nickel one, that the presence of Nb deteriorated the behavior in both cases and that aging the alloys, which resulted in NbC fragmentation, was also detrimental for the high-temperature oxidation behavior.
Suppressed Light Emission of the Reaction Zone in Detonation
THE detonation of high explosives in water can be photographed with an open camera and a flash light as the explosive gases do not emit light. The method has been used1 to investigate the spread of shock waves and explosive gases, and to investigate the initiation by shock waves2. We have found that it can be used also in certain gases such as butane and propane. The method is not suitable, on the other hand, for explosives with two levels of detonation velocity in the case of low velocity. A great part of the reaction then occurs after the reaction zone has passed and the expanding explosive gases are very luminous even in water.