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.
High Temperature Oxidation of HfC-Containing Chromium-Rich Iron-Based Alloys
Three HfC-containing iron-based alloys were successfully prepared in the foundry. The alloys, all containing 25 wt% Cr, 0.25 or 0.50 wt% C and 3.7 to 5.6 wt% Hf, are characterized by a dendritic matrix and the presence of interdendritic script-like HfC carbides. These alloys were subjected to thermogravimetry test in air at 1,200 °C for 46 h. Such exposure at this high temperature did not significantly modify the characteristics of the bulk HfC carbides. In contrast the presence of hafnium led to non-parabolic oxidation kinetics and fast mass gain rates, by comparison with similar Hf-free ternary alloys. The scales formed all over the samples’ surfaces were never composed exclusively of chromia and iron oxides or (iron, chromium) oxides were present in significant quantities. Some particles of hafnium oxides were also present in the external scales. A subsurface zone in which the HfC carbides were in situ replaced by hafnium oxides developed from the oxidation front. Catastrophic oxidation started in several locations and threatened to expand all around the samples beyond the stage duration. The high temperature oxidation behavior of these HfC-containing iron-based alloys appeared better than their cobalt-based counterparts, but worse than their nickel-based counterparts, for the same conditions of oxidation test.
Growth and Characterization of β-RDX Single-Crystal Particles
1,3,5-Trinitrohexahydro-s-triazine (RDX) single-crystal particles crystallized in the β phase have been successfully grown by sublimation–recrystallization. Synchrotron X-ray diffraction results confirm that the recrystallized products are β-RDX single crystals. Compared to the traditional solution deposition methods, the sublimation method used in this work breaks through the mass limitations during the growth of β-RDX. The distribution of α- and β-RDX on different substrates reveals that hydrophilic substrates are prone to grow β-RDX single crystals and that hydrophobic substrates prefer α-RDX single crystals. The sublimation rate dominates the growth of pure β-RDX. The mechanisms of the phase transition from β-RDX to α-RDX during the process of sublimation–recrystallization have been studied. The current results can promote the potential application of β-RDX in the propellant, explosives, and military fields.