Characteristics of Martensitic Transformation in Shape Memory Ceramic-Based Granular Packings and Composites

In the form of granular packings and polymer or metal matrix composites, shape memory ceramics can be scaled up for bulk applications despite their intrinsic brittleness. Because of the weaker mechanical constraint, the characteristics of martensitic transformation in the packings and composites can be drastically different from the monolithic shape memory ceramic polycrystals. In this presentation, I will provide an overview of our recent findings on the martensitic transformation characteristics of ceria-zirconia packings and composites based on in situ neutron diffraction and calorimetry characterization. As opposed to monolithic polycrystals wherein the mechanical constraint is strong, self-accommodation and autocatalysis are much less effective to facilitate busting-type transformation in packings or composites. In thermally-induced reverse martensitic transformation, the entire packing or composite transforms within a wide range of temperatures with conspicuous endothermic peaks suppressed in the calorimetry measurement. In stress-induced martensitic transformation, a slight increase of the external load can cause more particles to carry forces above the threshold for martensite nucleation, making the critical stress concept meaningless macroscopically. Given such ‘continuous modes’ of martensitic transformation that occurs under weaker mechanical constraint, the corresponding shape memory or superelastic devices may be designed to operate at a wide range of load and temperature for optimal performance. In addition to the fundamentals, I will discuss the routes to additive manufacturing of bulk-scale shape memory ceramic composites and architectured materials, which leverages the recent progress in additive friction stir deposition and stereolithography.