Transformation plasticity in TbPO4 and (Gd,Dy)PO4 orthophosphates during indentation of polycrystalline specimens

Phase transformations and deformation mechanisms were characterized in polycrystalline rare-earth orthophosphates by SEM and TEM after indentation. Xenotime-phase pellets with TbPO4 and (GdX,Dy1−X)PO4 compositions close to the monazite stability field were softer than DyPO4 compositions far from the monazite stability field, or monazite-phase GdPO4 compositions. Transformation to monazite was observed by TEM beneath indents in TbPO4. Grains with alternating (0 1 0) anhydrite and {0 1 0} xenotime lamellae were adjacent to areas that transformed to monazite. Pressure and/or shear stress in xenotime may cause three phase transformations: xenotime → monazite, xenotime → anhydrite, and anhydrite → monazite. These transformations were not observed in indented DyPO4 xenotime. Large displacement rebounds in some indentation load–displacement curves suggest that at least one of these transformations is ferroelastic. The phase transformations and material softening are consistent with previous observations made for sheared (Gd0.4,Dy0.6)PO4 xenotime fiber coatings.