Inhibition of scale growth on 20Cr–25Ni–Nb stabilized stainless steel by yttrium ion implantation revealed by analytical electron microscopy

Abstract

Yttrium ion implantation significantly improves the oxidation behaviour of the 20Cr–25Ni–Nb stabilised stainless steel in carbon dioxide at 750–900°C by the incorporation of this reactive element within the scale. The exact location and chemical state of the yttrium ion were established by transmission and scanning transmission electron microscopy. The implanted yttrium atoms are incorporated as Y₂O₃ grains and as ionic segregants along the oxide grain boundaries within the outer spinel layer. of the scale next to the inner Cr₂O₃ layer. As a consequence of the yttrium segregation, the cation diffusion along the oxide grain boundaries, which is responsible for continuing scale growth, becomes energetically less favourable. Scale growth is inhibited to an extent according to the proportion of grain boundaries so affected. The improved spallation resistance could derive from the yttria grains and yttrium segregants inhibiting scale failure, the prelude to decohesion, by increasing the energy for through scale crack initiation and propagation.

MST/932