Oxidation Behavior of an Al-Modified Silicide Coating on an Nb-Silicide-Based Ultrahigh-Temperature Alloy

The oxidation behavior of an Al-modified silicide coating prepared through a siliconization and then an aluminization pack cementation process on an Nb-silicide-based ultrahigh-temperature alloy was investigated in air at 1,250°C. The coating consisted of a 50-μm-thick (Nb,X)Si2 (X represents Ti, Cr, and Hf elements) outer layer with (Nb,Ti)3Si5Al2 distributed discontinuously in its superficial part and a 90-μm-thick (Nb,Ti)Al3 inner layer containing Si-rich precipitates. The oxidation of this coating was controlled primarily by the preferential oxidation of Al with the formation of α-Al2O3 scale. The (Nb,Ti)Al3 inner layer could act as an Al reservoir for forming scale and maintaining the existence of (Nb,Ti)3Si5Al2 phase in the zone just beneath the scale. The parabolic oxidation rate constant of this coating was about 1.72 μm2/h, lower than that of the bare alloy by about 4 orders of magnitude.

Oxidation of Single-Crystal γ′-Ni3A1 at Low Oxygen Partial Pressure

ABSTRACTSingle crystals of γ′-Ni3Al((001)-oriented) were oxidized at 1223 K under an oxygen partial pressure of ∼4 ×10−19 atm for times ranging from 0.1 to 50 hours. Microstructural development of the oxide scale and subscale metal was studied by electron microscopy. A special technique permitted the reproducible and efficient preparation of TEM cross section specimens. Initially, a fine-grained γ-Al2O3 scale formed with a preferred orientation. Depletion of Al from the γ′-Ni3Al resultedin a Ni-Al solid solution zone between the oxide scale and the intermetallic. After 20 hours oxidation, a discontinuous α-A12O3 layer between the γ-A12O3 and the metal was observed. The α-A12O3 layer exhibited a much larger grain size than that of the γ-A12O3 and was continuous after 50 hours oxidation. Formation of the α-A12O3 layer correlated with a decreasing parabolic oxidation rate constant kp, as measured by thermogravimetric analysis (TGA).

Solidification And Strength Characteristics Of Ni-Cr-Ti And CO-CR-MO Eutectics

The results of an exploratory study of eutectics based on Ni-25Cr–15Ti (wt %)and Co-10Cr-32Mo compositions will be presented. These alloys were selected from a total of ten different eutectic systems, all having melting points above 1200°C, a specific gravity less than 9,000 kg/m3, and a parabolic oxidation rate below 10−7 g2 · cm−4 · s−1 at 1150°C as reported by Haour, Mollard, Lux, and Wright on work done at Battelle-Geneva Research Centre. The Ni-25Cr-15Ti eutectic appeared to have a high volume fraction of ductile chromium fibers in a brittle Ni3Ti matrix. This alloy although the strongest of the six alloys that have been tested from the group reported by Battelle is still not competitive with advanced superalloys. Its tensile strength at 1093°C was 227 MPa2 and its elongation was 14%. The Co-10Cr-32Mo eutectic had the highest melting point (1340°C) of the alloys screened by Battelle. It appeared to solidify in a basically bladelike or lamellar structure, consisting of the intermetallic μ phase in a metallic Co (Cr) matrix. The strongest of the Co-Cr-Mo eutectics tested had a tensile strength of 164 Mpa and an elongation of 15% at 1093°C. The correlation observed between the strengths of the Ni-Cr-Ti and Co-Cr-Mo alloys, compositional variations, and their microstructures will be discussed.

Transition from internal to external oxidation in indium–silver alloys

The kinetics of oxidation of In–Ag alloys of 5,10, and 15 at. % indium have been studied on a vacuum microbalance. The 15 at. % indium alloy oxidizes externally and the 5 at. % alloy internally. A plot of logarithm of the parabolic oxidation rate, kp, versus reciprocal of the absolute temperature for 10 at. % indium alloy gives two intersecting straight lines corresponding to the energies of activation of 23 and 39.6 kcal/mole for the oxidation below and above 600 °C respectively. These are comparable to the energies of activation of 23 kcal/mole for the internal oxidation of 5 at. % indium alloy and 40 kcal/mole for the external oxidation of 15 at. % indium alloy. The rate–controlling step in the external oxidation of 15 at. % indium alloy is the diffusion of indium through the alloy. Photomicrographs of the cross sections of the oxidized foils of these alloys confirm the conclusions derived from the kinetic data.