![]() ![]() When the external stresses are placed on a NiTi wire, the austenite form is converted to martensite (stress-induced martensite) form, which can accommodate greater stress without increasing the strain (up to 8%). (As) austenite start temperature (Af) austenite finish temperature (Ms) martensite start temperature (Mf) martensite finish temperature This transformation requires “twinning,” which is a reversible atomic process that divides the lattice into two symmetric parts at an angle and allows reduction of strain during the transformation phase ( 3). These properties of the alloy occur as a result of transition from the austenite to martensite form. ![]() NiTi wires have gained a special commercial applications in dentistry because of their shape memory effect, and corrosion resistance. ![]() Other than the advantage of increased flexibility and shorter treatment time, NiTi wires also resulted in fewer procedural errors such as zipping, ledges, or transportation due to their superelasticity, compared with SS files ( 1, 2). NiTi files were able to overcome the issue of rigidity and low resistance to cyclic fatigue associated with stainless steel instruments. The introduction of nickel–titanium (NiTi) alloys in the late 1980s led to a revolution in endodontics as these files were shown to have considerable advantages over stainless steel (SS) files, especially in relation to the safety of instrumentation ( 1). This review discusses the different phase transformations and heat treatments that the NiTi instruments undergo. Contrary to this, martensitic alloys can easily be deformed due to phase transformation, and they can demonstrate the shape memory effect when heated. Instruments based on austenitic alloys possess superelastic properties due to the stress-induced martensitic transformation. NiTi alloys can be subclassified as the instruments mainly containing austenitic phase (conventional NiTi, M-wire, R-phase), and those containing martensitic phase (controlled memory wire, ProTaper Gold, and Vortex Blue). These heat-treated instruments also possess improved cyclic fatigue resistance when compared to conventional NiTi alloys. The main purpose of these treatments is to impart a more martensitic phase into the files at normal body temperature, so that the maximum advantage of flexibility can be obtained. These treatments include thermal, mechanical, electropolishing, and recently introduced electric discharge machining. Since the last decade, different proprietary processing procedures have been introduced to further improve the mechanical properties of NiTi alloys. Consequently, there has been considerable research conducted to investigate the mechanisms behind the occurrence of these procedural errors. However, despite of their superior mechanical properties, NiTi alloys still pose some risk of fracture. They have considerable advantages over the conventional stainless steel file in terms of mechanical properties. Ever since their introduction, nickel–titanium (NiTi) alloys have continued to revolutionize the field of endodontics. ![]()
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