High temperature creep and tensile properties of alumina formed on Fecralloy foils doped with yttrium

The three batches of Fecralloy foils, which differ from each other in contents of yttrium, that is, 10, 280 and 560 ppm, respectively, were chosen as the thermally grown oxide (TGO), alumina (α-Al2O3) forming substrate. The creep tests were performed with the Fecralloy foils, which have the α-Al2O3 TGO of 0–4 μm thickness, on the surfaces. The creep rates decreased as the TGO thickness increased. The yttrium content above 280 ppm delayed the creep rate of the Fecralloy substrate. The higher creep rate than that of the stand-alone polycrystalline alumina and the dependency of the creep rate on TGO thickness agrees with the hypothesis that the high temperature creep of the TGO is a consequence of the inter-grain growth of the TGO. The yttrium content of lower than 560 ppm did not affect on the creep rate of TGO. Tensile tests were performed with the same alloys, which have the α-Al2O3 TGO of 0–3 μm thickness. The tensile strength of the substrate itself increased with Y content by ∼20%. The tensile stress of the α-Al2O3 TGO decreased with Y-content but it is almost constant, regardless of the TGO thickness. The peak stresses were found at the strain range of ɛ = 0.7–1.5%, regardless of the TGO thickness, and the batches, thereafter, the parallel cracks perpendicular to the loading direction, formed on the surface. The obtained stress–strain curves of TGO fluctuated. It showed a common feature, that is, a sharp stress drop after the initial yield point at σY = 40–85 MPa, but the stress increased again until the peak points, σUTS = 50–110 MPa.