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.