Phase transformations in NITI alloys during cutting processes

Nickel-titanium (NiTi) alloys, commonly known as nitinol, occupy a special place among shape memory materials due to their unique combination of high strength, corrosion resistance, biocompatibility, and the ability to undergo reversible phase transformations. These properties make nitinol widely used in medical engineering (stents, orthodontic wires, implants), as well as in aerospace, energy, and robotics industries. The functional behaviour of nitinol is based on the reversible martensitic transformation between the austenitic B2 phase and the martensitic B19′ phase, which provides both the shape memory effect and superelasticity. The temperatures and kinetics of this transformation depend on the alloy composition, thermal history, loading conditions, and microstructural state of the material. This article presents a review of current research on phase transformations in nickel-titanium (NiTi) alloys and their influence on the microstructure and functional properties of the material during cutting processes. The binary NiTi phase diagram, the main crystalline modifications (B2, B19′, and R phases), and the mechanisms of superelasticity and shape memory effect are discussed. Experimental data on the influence of turning and milling parameters on the phase state and structural changes in the near-surface layers of NiTi alloys are summarized. The complex thermomechanical conditions in the cutting zone that can initiate stress-induced martensitic transformations, R-phase formation, and partial reverse transformation of martensite into austenite are described. Promising directions for further research are identified, focusing on a comprehensive analysis of phase transformations occurring in the work material during cutting and the development of machining regimes that ensure the preservation of the functional properties of nitinol.