Tuning the Martensitic Transformation Mode in Shape Memory Materials via Mesostructure and Microstructure Design

The shape memory and superelastic effects are based on the reversible martensitic transformation between two crystal structures, which can be triggered by temperature, stress, or electromagnetic fields. For monolithic, bulk shape memory materials, the martensitic transformation occurs when a threshold temperature or stress is reached; with strong mechanical constraint, autocatalytic transformation prevails, rendering complete transformation within a narrow temperature or stress window. This results in a macroscopically discontinuous transformation mode with conspicuous peaks in differential scanning calorimetry and plateaus in stress-strain curves. From a thermodynamic perspective, the martensitic transformation mode is governed by the uniformity of the driving force and the height of nucleation barrier, both of which are sensitively dependent on the mesoscale and microscale structural features^1-4. By mesostructure and microstructure design, a non-uniform distribution of the driving force can be achieved along with low nucleation barrier. As a result, the thermally or mechanically induced transformation window can be significantly enlarged, resulting in a macroscopically continuous transformation mode^2,3. Here, we investigate and demonstrate such phenomena utilizing shape memory ceramics in a variety of forms with mesoscopic and microscopic control, such as granular packings^2,3, micro-architectures¹, and metal matrix composites.