Structural Evolution of Ni-Ti Alloy Wires Produced by Hot and Cold Rotary Forging

Ni-Ti shape memory alloys (SMA), have interesting functional properties such as shape memory effect and superelasticity that enable their use in different segments. These functional characteristics are obtained through the thermomechanical processing (hot and cold). The hot deformation may promote the intended metallurgical transformations and the microstructural changes are improved by final cold deformation. These processes influence the final mechanical properties of the materials and, by consequence, their applications. This work focused on a Ni-rich Ni-Ti alloy, which may be used in the orthodontic archwires since the alloys used for this purpose need to show superelastic characteristics at room and oral temperature. It is sought by the mechanical and thermal treatments that the material displays an austenite finish temperature below room temperature.In this work, the characteristics of the thermomechanical processing are studied using samples representative of the different processing steps. For each processing step, the effect of the process parameters on the phase transformation temperature, superelasticity and shape memory effects was assessed and correlated to its microstructure. The structural analysis of each sample was performed by different techniques, which allowed the identification of the thermomechanical processing evolution. It was noticed that the austenite finish temperature was close to room temperature for all the steps. For all the samples, an austenite matrix at room temperature was observed. Different heat treatments were applied to identify the most suitable changes to be proposed along the rotary forging steps. Thermomechanical treatments were performed to understand and verify the structural evolution (by X-ray diffraction, using synchrotron radiation) and the mechanical behavior during the hot and cold deformations. These treatments allowed us to observe and discuss restoration phenomena, such as dynamic recovery and recrystallization.In addition, orthodontic archwires were studied in a reverse engineering approach to identify their structural characteristics and the corresponding functional behavior. The characterization of commercial functionally graded NiTi orthodontic archwire was performed and the introduction of graded functionality in conventional archwires was analyzed.This study aimed to contribute to the development of processing strategies that will give rise to more consistently uniform characteristics of Ni-Ti shape memory alloys and a minimization of the failures occurring during processing