Coalescence of wustite grains during iron oxidation in a hot-stage environmental SEM

The growth of wustite grains during the initial stages of iron oxidation at 1050°C and atm has been investigated in situ in a hot-stage environmental SEM (HSESEM). Adjacent wustite grains were seen to coalesce by annihilation of the grain boundary dislocations constituting original boundaries. Specifically, dissociation of a grain boundary into an array of dislocations followed by their annihilation either by glide or climb was observed. The grain boundaries exhibiting this coalescence are believed to be low-angle boundaries. A controlled extent of oxidation followed by rapid cooling in the HSESEM enabled the study of wustite grain boundaries. Dissociation of a grain boundary into dislocations without their annihilation was also observed in later stages of oxidation when the dislocation mobility was severely reduced.In this case, instead of grain coalescence, nucleation of a new grain with an orientation different from either of the two original grains took place.