Conventional engineering design for structural materials relies on the implementation of large safety factors and inefficient overdesign. More modern approaches seek to develop metamaterials focusing on the topology. However, emphasis on material selection and metallurgy is needed to enable such optimized topological designs. A special class of metallic alloys, called shape memory alloys (SMAs), can recover a pre-deformed shape when heated. The most common SMAs are nickel-titanium (NiTi), which has a twinned martensite microstructure that when deformed, reorients to a detwinned martensitic structure (recoverable), rather than breaking atomic bonds through dislocation motion (permanent). Upon heating the SMA above its austenitic transformation temperature, the microstructure transforms to the austenite phase and recovers its pre-deformed, original shape. As the structure is cooled, it retains its macroscopic shape and converts back to the starting twinned martensitic microstructure.
The amount of recoverable deformation is critically dependent on the amount of detwinning achievable in the microstructure. Of course, this will differ depending on the material’s processing route. This study investigates multiple processing pathways with additive manufacturing to fabricate novel SMA structures that impart damage tolerance and dissipation, while the SMA imparts strength and the functional ability to recover damage when heated. SMAs can also exhibit a two-way shape memory effect (TWSME), where a macroscopic shape change is produced without any mechanical intervention, but instead from a change in temperature alone. The TWSME is not an intrinsic property of SMAs, like the one-way shape memory effect. Instead, the TWSME relies on the local strain field surrounding aligned Ni-rich precipitates in a NiTi matrix. The role of post-processing microstructural tuning of SMAs will also be discussed to achieve two different remembered shape conformations.