Intrinsic and extrinsic size effects in shape memory zirconia

Size-effects upon the martensitic transformation can depend on sample microstructure (intrinsic) or sample size (extrinsic). While both types are well-studied in shape memory alloys (SMAs), these effects are largely not established as yet in ceramic shape memory counterparts. Large-scale polycrystalline shape memory ceramics (SMCs) are known to undergo transformation-induced cracking due to stress concentrations arising from the highly constrained martensitic transformation. By leveraging the intergranular disaggregation with thermal transformations captured with calorimetry experiments, we can examine both intrinsic and extrinsic size effects for a single set of samples with initial grain sizes ranging from ~0.6 to 9.5 µm as the samples evolve from a coherent pellet to a loose aggregate of single crystal particles. Key parameters, including transformation temperatures, thermal hysteresis and heats of transformation, are extracted from the beginning and end of the calorimetry experiment to establish first data on size-effect trends for shape memory zirconia.