Influence of environment on mechanical behavior of alloy 718 at 650°C

Creep tests were carried out at 650degreesC on thin alloy 718 specimens (0.27 mm thick), under dry air or hydrogenated argon. Oxide scale were characterized using XPS analysis on material heat treated 1 hr under the same conditions. Under hydrogenated argon, there is no primary creep and the creep strain rate is lower than under air. XPS analysis points out the formation of a thin chromium and alloying elements oxide scale on the specimen heat treated at low oxygen partial pressure. On the specimen heat-treated at higher oxygen partial pressure, the oxide scale is much thicker and only its very surface could be analyzed. It contains chromium, iron and alloying elements but does not contain nickel. These elements support the oxidation assisted creep of alloy 718 process, which occurs during the first ages of loading, producing primary creep.