Robust Cellular Shape-Memory Ceramics via Gradient-Controlled Freeze Casting

Shape-memory ceramics offer promise for applications like actuation and energy damping, due to their unique properties of high specific strength, high ductility, and inertness in harsh environments. To date, shape-memory behavior in ceramics is limited to micro-/submicro-scale pillars and particles to circumvent the longstanding problem of transformation-induced fracture which occurs readily in bulk polycrystalline specimens. The challenge, therefore, lies in the realization of shape-memory properties in bulk ceramics, which requires careful design of 3D structures that locally mimic pillar structures. Herein, it is demonstrated that with a gradient-controlled freeze-casting approach, honeycomb-like cellular structures can be fabricated with thin and directionally aligned walls to facilitate martensitic transformation under compression without fracture. With this approach, robust bulk shape-memory ceramics are demonstrated in a highly porous structure under compressive stresses of 25 MPa and strains up to 7.5%.