Microstructural investigation of the effects of oxygen exposure on NARloy-Z

NARloy-Z is a Cu-base alloy used to fabricate the liner of the main combustion chamber (MCC) for the space shuttle main engine (SSME). This study was undertaken to assess the effects of unusual or transient conditions (i.e., high temperature and exposure to high levels of oxygen) on the robustness of the MCC design, providing a data base to better quantify environmental conditions. The results indicated that, upon exposure to severe environmental conditions, wrought NARloy-Z develops extensive microstructural variations which will reduce the service life of an MCC. Oxygen diffusion into the material closely followed predictions based on theoretical considerations. Above 760 °C, the presence of oxygen dissociates Cu(AgZr) intermetallics, forms ZrO2, and embrittles the alloy. Above 593 °C, Ag precipitates coarsen in the matrix, becoming incoherent and degrading material strength.