Towards enabling shape memory ceramics : considerations for stress concentration optimisation through additive manufacturing

Abstract

The shape memory effect observed in ceramics is of particular interest for various applications, especially for those with extreme environments where ceramic materials offer unique advantages over metallic materials. The shape memory effect depends on the capability of the structure to withstand the critical stress required to initiate an underlying martensitic transformation. However, in bulk-scale applications the brittle nature of polycrystalline ceramics often leads to excess stress accumulation at the grain boundaries, ultimately leading to fracture. The pre-mature fracture arrests the capability of the material to initiate the martensitic transformation. In this work, it is proposed that an engineered microstructure will lead to amplified stress levels in the bulk of the material. The approach can suppress the occurrence of pre-mature brittle fracture and promote higher stresses that lead to phase transformation. A slurry-based technique is employed that uses a combination of additive manufacturing and tape casting to fabricate ceramic structures. The mesostructure that is developed (post thermal treatment) is a network of pores that act as stress concentrators due to the reduced cross-sectional area while undergoing compression. This thereby increases the probability of nucleation of the monoclinic phase, reduces the stress gradient between pores, and approaches oligocrystallinty, which is known to enhance transformation capabilities. Finite element analysis is used to further verify and confirm the effect of porosity on stress evolution during loading. Furthermore, thermally treating ceramic powders shows that a gradual transition of rounded smooth-edged particles to sharp faceted edges occurs prior to the growth of the particle size. The effect of these microstructural changes on the stress-induced phase transformation in compacted pellets is investigated. The effect of the combination of peak temperature and dwell time during thermal treatment on the total tetragonal phase recovery is explored to complete the shape memory effect cycle for these ceramic materials. Cumulatively, this work shows how the aid of stress concentrators, either with porosity in bulk structures or particle shape in powders, can control and increase the phase transformation ability in shape memory ceramic materials. However, significant further work remains for both the slurry based and powder-based techniques to optimise the stress-induced phase transformation and realise it in real-world applications. This optimisation may be ideal for newly evolving methods stemming from additive manufacturing methods for ceramics.