Composition Modulation Induced Superelasticity Over a Wide Temperature Due to Precipitate Dissolution

Superelasticity (SE) in shape memory alloys (SMAs) can be attributed to the stress-induced reversible martensitic transformation (MT), which influences the operation temperature of SE. Recent studies have demonstrated that some processes could influence the transition temperatures and related properties of MT such as alloying design, introducing dislocation, thermomechanical treatments, nano-composition, to name a few. In this paper, we propose a method to extend the operating temperature range of SE by designing composition modulation by Ni₄Ti₃ precipitate dissolution in Ni-rich NiTi SMAs. Phase field calculations in Ni-rich NiTi alloys (Ni_50.8Ti_49.2 (at.%) and Ni_51.5Ti_48.5 (at.%)) are used to study the phase transition behavior and related mechanical properties. Calculations have shown that the Ni₄Ti₃ precipitate could dissolve under solution treatment for a short time, accompanied by residual Ni composition modulation. The residual composition modulation may lead to the continuous martensitic transition behavior under cooling. Further calculations under loading have shown the SE over a wide temperature range due to the continuous MT behavior. In addition, corresponding experiments confirm the precipitate dissolution process and related continuous MT. Our work may provide a simple way to alter the mechanical properties of SMAs by designing composition modulation through precipitate dissolution.