Characterization of ferroelasticity in rare-earth orthophosphates by nanoindentation

Abstract

Superelasticity, or the rubber-like effect, describes the phenomenon of a material being able to ‘recover’ after an external strain, beyond the elastic limit, is removed. This behavior has been exhibited by shape memory alloys, as well as the emerging materials field of shape memory ceramics, and may be caused by the material undergoing deformation mechanisms such as phase transformation or twinning/detwinning – also known as ferroelasticity. Superelastic materials are known to be attractive candidates for shape memory materials as well as applications with actuating, sensing, and damping needs. Rare-earth orthophosphates are a group of ceramics that are known to exhibit incredible flexibility with lanthanide element substitution as well as high resistance to chemical and thermal degradation. These materials have also been shown to undergo both of the deformation mechanisms associated with superelasticity. Nanoindentation is a promising technique for studying superelasticity due to its ability to run large-scale experimental matrices and the fact that the technique is highly sensitive and therefore able to pick up even the smallest amounts of recovery. Pioneer indentation testing of several rare-earth orthophosphates near the monazite/xenotime boundary has shown indentation recovery as a slope change that occurs during the unloading portion of test. Similar behavior has been attributed to phase transformations in silicon and twinning in germanium. This thesis conducts an in-depth study to determine the conditions under which these anomalous indentation features occur. Indentation recovery was not restricted to the phase transforming xenotimes, but could also occur in the non-phase transforming monazites as well; thus it was concluded that the presence of an elbow in the indentation data was not a unique identifier of phase transformation in rare-earth orthophosphates. Furthermore, it was shown that the elastic modulus of each of these compositions approached the value predicted by simulations and hardness was consistently above 5 GPa, provided that the samples were processed to nearly full density. Multiple indentations completed on a polycrystalline specimen of monazite GdPO4 revealed frequent anomalous unloading behavior with a large degree of recovery; therefore, an excised indentation was analyzed to determine the cause of recovery using TEM. The presence of a twin along the (100) orientation, along with a series of stacking faults contained within the deformation site, provide evidence that the mechanism of recovery in GdPO4 is twinning-based ferroelasticity. In addition to the large instances of superelasticity, cyclic loading of single-crystal monazite-structured GdPO4 was shown to exhibit recovery ratios similar to that of the shape memory alloy NiTi at 0.9, and exceeded the amount of achievable dissipated energy (250 MJ/m3 for GdPO4 vs. 10-20 MJ/m3 for NiTi). Furthermore, this study provides the first evidence that twinning-based ferroelasticity may be used to achieve superelasticity in ceramics, making REPO4s potentially attractive candidates for actuation and damping applications.