Oxidation or Nitridation Behavior of Pure Chromium and Chromium Alloys Containing 10 mass%Ni or Fe in Atmospheric Heating
The oxidation or nitridation behavior of pure chromium and chromium alloys containing 10 mass%Ni or 10 mass%Fe in atmospheric heating was investigated in the temperature range of 1073 to 1373 K. The formation behavior of the outer oxide phase of Cr2O3 and its growth kinetics in both chromium alloys was very similar to that of pure chromium. The nitride layer with the Cr2N phase was formed beneath the oxide phase at temperatures above 1273 K in pure chromium. Marked internal nitridation with the formation of the Cr2N phase was observed in the Cr-10 mass%Fe alloy, while the formation of Cr2N was negligible in the Cr-10 mass%Ni alloy. Neither Ni nor Fe were detected inside the Cr2O3 phase, and was enriched in the subsurface layer between the oxide layer and the base alloy in a respective alloy. The Ni-enriched layer in Cr-10 mass%Ni alloy had a relatively narrow width, but a very high concentration of Ni compared with the Fe-enriched layer formed in the Cr-10 mass%Fe alloy. [doi:10.2320/matertrans.47.2540]
Effect of oxide grain structure on the high-temperature oxidation of Cr
Cr was oxidized in 1 aim O2 at 980, 1090, and 1200°C. ElectropolishedCr and some orientations of etched Cr oxidize rapidly and develop compressive stress in the growing Cr2O3; other orientations oxidize slowly, apparently free of stress. SEM examination of fracture sections shows that the thick oxide is polycrystalline whereas the thin oxide on etched Cr is monocrystalline. It is deduced that the monocrystalline oxide grows by lattice diffusion of cations outward, and the polycrystalline layer by the two-way transport of cation diffusion outward and anion diffusion inward along oxide grain boundaries. The consequent formation of oxide within the body of the polycrystalline layer generates compressive stress and leads to wrinkling by plastic deformation. The activation energy for oxidation of Cr by cation lattice transport is 58 kcal/mole. Polycrystalline Cr2O3 forms on Fe-26Cr alloy, whether electropolished or etched; oxidation is accordingly rapid and accompanied by compressive stress.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.