Internal oxidation of a nickel-based superalloy TMS-75 at 900 degrees C

Bill, Oleary Moshe
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Nickel-base superalloys have a wide range of use as high temperature materials of high mechanical integrity and good oxidation resistance. In most cases where a protective external adherent alumina scale does not develop, internal oxidation is prevalent. A study was undertaken to investigate the internal oxidation of an experimental, single crystal, nickelbased superalloy TMS-75 in air for 100h at 900oC. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) were employed to characterize the internal oxidation of this alloy. Internal oxidation was observed in the presence of a non-protective (Ni,Co)O outer scale. Immediately beneath and in contact with this scale was a thin layer of relatively fine grained spinel oxide inner scale. The internal oxidation or reaction zone was located below these two scales. This zone, of around 20 micrometres in thickness, was found to have had lost its original two phase γ-γ´ microstructure, this being replaced with a relatively course grained structure containing facetted precipitates. The precipitates were identified as being hexagonal AlN and rhombohedral -Al2O3. A study was made of the orientation relationships between the different precipitates and matrix. None of the precipitates were found to exhibit an epitaxial relationship with the surrounding matrix material. The zone axes of the matrix and precipitates were typically misaligned from a minimum of 1.5o up to about 5o. The orientation relationships found between the AlN precipitates and the Ni-based solid solution matrix were: 1) [013]M // [1230 ]P with (200)M // ( 0002 )P and ( 031)M // ( 2110 )P 2) [110]M // [ 0110 ]P with ( 220 )M // (1012)P within 2o 3) [ 011]M // [ 0110 ]P with ( 200 )M // (0002 )P and (311)M // (1010)P while the orientation relationships found between the -Al2O3 precipitates and the Ni-based solid solution matrix were: 1) [114 ]M // [ 2112 ]P with (131)M // (1011)P within 2o 2) [112 ]M // [1011]P with (153 )M // (1121)P