Improved phase differentiation in EBSD data of oxidized stainless steel AISI 304 by indexing via full pattern matching and NPAR

Steel oxidation is a process linked to a wide range of oxidic and metallic phases with usually cubic or trigonal symmetry and their solid solutions. Applying the advanced post processing procedures Neighbour Pattern Averaging and Reindexing (NPAR) and full pattern matching to EBSD datasets of an austenitic AISI 304 steel oxidized at 800°C in air enables a significantly enhanced analysis regarding both representativity and phase discrimination. The presented results show that oxide ridges formed at the surface are mainly located on top of random high angle grain boundaries but not on Σ3 twin boundaries. They also enable to conclude that oxidation progresses into the austenitic alloy by incorporating oxygen ions to the existing austenite lattice (inward growth) which transforms to either spinel- or corundum-type lattices with basically matching closest packed planes. Metal ions are added to a growing outer oxide layer (outward growth) mainly consisting of corundum-type oxide. Applying parent grain reconstruction enables to show that most of the trigonal lattice detected in cross sections is apparently in an epitaxial relationship to the cubic substrate it originated from or grew onto. • Full pattern matching enables enhanced phase differentiation via EBSD. • EBSD-based separation of austenite and spinel-type oxide is accomplished. • Corundum- and spinel-type oxides are widely in an epitaxial relationship. • Inner oxidation occurs by incorporating oxygen into the existing crystal lattice. • Outer oxide growth is indicated to occur cubic and later transform to trigonal oxide.