Microstructural changes in long-time thermally exposed Ni-base superalloy studied by SANS

The SANS experiment was a crucial part of a broader investigation of the long-time structural stability of the polycrystalline Ni-base superalloy and its effect on the subsequent creep characteristics. The results of the creep tests have shown that a long isothermal exposure at temperatures of 430 °C and 650 °C up to 25000 h changed the deformation ability and creep life of the exposed specimens. While structure analyses (TEM) showed no evidence of a morphological and/or dimensional γ′ change, the SANS technique was able to reveal significant changes of the morphology of γ′ precipitates already after the shortest applied thermal exposition, 2000 h. The resulting size distributions, distance distributions, mean sizes and mean distances between the precipitates made evident that both the size and the distance increase in the bulk of the alloy with increasing thermal exposure even at such relatively low temperatures.