Oxidation Characteristics of Nickel-Based Superalloy Powders Exposed at Ambient Condition

Abstract

The existence of adsorbed oxygen and oxides on the surface of initial powders has serious effects on the microstructure and mechanical properties of the powder metallurgy alloys. However, the powder surface is inevitably oxidized immediately after the powder preparation. In this work, the oxidation characteristics for the argon atomized powders of a Ni-based superalloy containing Cr, Co, W, Mo, Nb, Ti and Al after exposure at ambient condition for various time were investigated in detail. It is found that various gases can be absorbed on the powder surface, but most of them can be removed by low temperature (<151.5 °C) outgassing procedure. The thermodynamic calculation shows that the oxidation reaction occurs firstly with the alloying elements rather than Ni matrix, whether at room temperature or elevated temperature. The kinetic measurement indicates that the oxygen content on the powder surface approaches a saturation value after 24 h exposure and remains almost stable after 720 h. The oxygen content increases with the decrease of particle size after exposure. X-ray photoelectron spectroscopy characterized that, except the formed oxides, adsorbed oxygen also exists on the powder surface of the as-atomized initial fine powders with particle size <30 μm and the powders with size >18.7 μm after exposure, which may be caused by the internal stress and surface energy of the initial atomized powder. All alloying elements except Ti can form stable oxides directly on the powder surface. For the element of Ti, the metastable TiO forms on the initial powder surface after preparation and it transforms into stable TiO2 or Ti2O3 during exposure. The results provide a deep understanding of absorbed gases and oxide on the surface of powders under treatment and possible desorption approach.