Phase transformations in yttrium –aluminium oxides in friction stir welded and recrystallised PM2000 alloys

Abstract

PM2000 is an Fe –Cr – Al oxide dispersion strengthened (ODS) alloy containing 0.5wt% of fine, uniformly dispersed, yttrium oxide particles in a ferritic matrix. The alloys are attractive candidates for high temperature applications since nano-dispersoids improve the creep resistance of the alloys at high temperatures. Friction stir welding (FSW) has been used successfully for the joining of PM2000 sheet without oxide particle agglomeration and significant change in the microstructure. However, it has been reported that the initial Y2O3 particles may sometimes oxidise the aluminium from the surrounding matrix to form mixed Y–Al oxides. Hence, in this study, we have been using extraction replication plus high-spatial resolution scanning transmission electron microscopy (STEM) to investigate phase transformations and oxidation of Y–Al during FSW processing and recrystallisation treatments (1380°C, 1 hour in laboratory air).High-resolution SuperSTEM images indicate that the Y2O3 can transform to Y3Al5O12 garnet (YAG) and YAlO3 perovskite (YAP) particles even in the consolidated PM2000. These dispersoids appear to be stable during the FSW process, but most of the Y2O3 or YAG particles transform into YAP particles after the high temperature recrystallisation treatment at 1380°C. In some cases partially transformed particles were observed and these may enable the details of the oxidation/transformation mechanisms to be elucidated.