Shape memory alloys (SMAs) have found widespread use in superelastic components and as compact, lightweight actuators. However, current use cases require SMAs to be joined to structural alloys (e.g., A17075, Inconel 718, 316L). At present, this is done mechanically (e.g., bolting or crimping) adding significant weight and volume, which reduces the benefit of using an SMA. Attempts at metallurgical bonds, which would not incur the weight/volume penalties of mechanical joints, have fallen short, often producing joints with deleterious phases and poor joint efficiencies (defined as σ joint /σ material ). This is largely because the elemental composition and phase structure of SMAs are highly dissimilar to nearly all structural alloys. As a result, most of the possible composition gradients connecting the SMA to the structural alloy led to the formation of undesirable phases, typically brittle intermetallics. Thus, for a metallurgical joint to be successful, it must have a final composition gradient that results in favorable (strong, ductile) phases, while avoiding unfavorable (weak, brittle) ones. This joint requires two features: (1) a target final gradient that is known (or predicted) to be favorable; and (2) a technique for guiding the joining process to such a targeted final gradient. This work focuses on developing the former.

Nitinol, a near-equiatomic nickel-titanium alloy, is one of the most widely used shape memory alloys (SMAs) due to its excellent mechanical behavior, corrosion resistance, and large recoverable strain limit (~8%). For these reasons, nitinol has been fielded in many aerospace applications, including self-deploying actuators, folding aircraft wings, and rover tires. However, these applications require joining nitinol to support structures which are often a different alloy class. Traditional welding methods, when applied to nitinol, lead to the formation of brittle intermetallic phases, reduced strength within the joint region, and decreased functional performance (such as recoverable strain and heterogeneous transformation temperatures). Thus, dissimilar joining of nitinol remains a grand challenge in the ultimate use of these materials, and advances in joining technologies will greatly expand the potential applications of SMAs.