Effects of composition and particle size on the surface state and degassing behavior of nickel-based superalloy powders

Surface state and degassing behavior of nickel-based superalloy powders are essential to the melting and sintering properties of additive manufacturing (AM) and powder metallurgy (PM). Superalloy powders with various compositions and particle sizes were investigated by FESEM, XPS, AES, HAADF-STEM, and temperature programmed desorption mass spectrometry (TPD-MS). The surface analysis results demonstrate that IN625, IN718, and FGH96 superalloy powders are covered by homogeneous Ni oxide/hydroxide (NiO/Ni(OH)(2)) layer decorated with various oxides. NiO/Ni(OH)(2) layer thickness changes in the range of 5.7 nm to 9.8 nm with different powder compositions and particle sizes, while the influencing factors of thickness can be arranged in the following order (from highest to lowest) as IN625-IN718-FGH96 and (106-150 mu m)-(48-106 mu m)-(<48 mu m) respectively. The TPD-MS results indicate that the degassing amounts significantly increase with the decrease of NiO/Ni(OH)(2) layer thickness, in which the degassing behavior is partially controlled by molecular diffusion. Powders with more active metallic elements, i.e. Al and Ti, and smaller particle size possess relatively thinner oxide layer which result in lower diffusion barrier and higher degassing amounts. The investigation reveals the relationship between the surface state and degassing behavior of superalloy powders considering the differences in the composition and particle size, and gives guidance on nickel-based superalloy powder processing.