Transformation pathway engineering and microstructure design for high performance SMAs

Martensitic transformations (MTs), being strongly first-order, take place in an avalanche-like manner within a narrow temperature- or stress-range and are accompanied by dislocations, large hysteresis and strongly nonlinear pseudo-elasticity. This leads to many problems in applications, including functional fatigue and dimensional instability, low efficiency, and difficulty in precise position control of actuators in micro-electromechanical devices. In this presentation, we demonstrate new design principles of taming and tuning MTs in SMAs to achieve desired properties. On one hand, we show using computer simulations that, by suppressing autocatalysis and regulating the spatial extent of domain growth via random fields from defects and confinements from nano-scale concentration modulations, one could broadly smear out the strongly first-order nature and turn MTs into continuous transformations. Such continuous MTs are characterized by novel nanostructures and unprecedented properties, including nearly hysteresis-free and linear superelasticity, ultra-low and temperature-independent elastic modulus (Elinvar anomaly), and nearly zero thermal expansion (Invar anomaly), over a wide temperature range. On the other hand, we show by phase transformation graph (PTG) analysis that both the functionality and performance of SMAs are dictated by the topology of their PTGs. Using PTG topology as a guide, we discuss design strategies for high performance SMAs with much improved functional fatigue resistance. Most of our predictions have been confirmed by experimental characterization and testing. The findings have not only allowed us to solve some long-standing puzzles, but could also open a new avenue for judicious design of next generation of smart nano-structured SMAs for widespread technological applications.