| Shape Memory Alloys (SMAs) are 'smart' materials that can achieve high recoverable strains through a reversible phase transformation and have potential applications in many fields such as energy, actuation, and sensing. The transformation between austenite and martensite phases through a shear results in severe stress concentration at grain boundaries, rendering many polycrystalline SMAs inherently brittle. A Grain Boundary Engineering (GBE) method, alternative to traditional methods, has been developed to design dual-phase SMAs in which a ductile, strain accommodating, non-transforming second phase is precipitated in the material, primarily along grain boundaries. This GBE method aims to optimize the morphology and distribution of the second phase by tailoring the alloy composition and thermal processing methods and is shown to be applicable to several SMA systems. Co--Ni--Al is the primary model system selected for experimental studies. In nanoindentation tests using a Berkovich tip (~150 nm radius), enhanced strain recovery, superelastic recovery, and hardness are observed in austenite volume adjacent to its interface with precipitate as compared to other regions tested. A larger radius conospherical tip (~831 nm radius) produces less unrecoverable deformation, thereby enabling a larger portion of material to transform reversibly and is employed in nanoindentation tests in austenite beta adjacent to intergranular precipitate, austenite beta adjacent to bare grain boundaries, and in austenite beta interior. Austenite interfaces adjacent to precipitate have the lowest energy dissipation, the highest hardness, and the highest strain recovery compared to beta regions adjacent to bare grain boundaries or beta interior. The ductile second phase improves superelasticity by plastically accommodating transformation strain from stress-induced martensitic transformation in its adjacent volume, alleviating stress concentration and relieving constraint on transforming austenite. As a result, a greater portion of austenite transforms reversibly and higher strain recovery is observed in austenite adjacent to the second phase. These results enable design of dual-phase microstructures with enhanced ductility, high recoverable strain, and high hardness for several applications, such as mechanical actuation, damping, and those which apply high loads to materials. |
