New thermomechanically treated NiTi alloys - a review

Nickel titanium endodontic instruments have been reported to be more flexible with an increased torsional fracture resistance when compared to stainless steel instruments

J. Zupanc; N. Vahdat-Pajouh; E. Schäfer

2018

Scholarcy highlights

  • Nickel titanium endodontic instruments have been reported to be more flexible with an increased torsional fracture resistance when compared to stainless steel instruments
  • Beside variations in the design of NiTi instruments, manufacturers have introduced several proprietary manufacturing procedures including thermal, mechanical and surface treatment to improve the mechanical properties of NiTi alloys, in order to produce instruments with enhanced resistance to fracture and increased flexibility
  • Cyclic fatigue resistance decreases as the maximum tensile strain amplitude on the surface of the instruments increases, which occurs at the maximum bending point during shaping of a curved root canal
  • NiTi alloy used in endodontic instruments contain approximately 56 wt% nickel and 44 wt% titanium resulting in a nearly one-to-one atomic ratio. This equiatomic NiTi alloy can exist in two different temperature-dependent crystal structures named austenite and martensite phase and possesses typical characteristics which are superelasticity and shape memory effect
  • Thermomechanical treatment of NiTi alloy allows a change in the phase composition leading to the appearance of martensite or rhombohedrally distorted phase under clinical conditions
  • Similar to stress-induced martensite, the martensite reorientation enables deformation up to 8% strain without significant increase of stress
  • The use of austenitic alloy in pathfinding instruments may compensate for the decreased torque resistance caused by the smaller diameter of these files

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