title: Microstructure of cryogenically treated martensitic shape memory nickel-titanium alloy.
abstract: BACKGROUND
  Recent introduction of shape memory (SM) nickel-titanium (NiTi) alloy into endodontics is a major breakthrough. Although the flexibility of these instruments was enhanced, fracture of rotary endodontic instruments during instrumentation is an important challenge for the operator. Implementation of supplementary manufacturing methods that would improve the fatigue life of the instrument is desirable.
  
  
  OBJECTIVE
  The purpose of this study was to investigate the role of dry cryogenic treatment (CT) conditions on the microstructure of martensitic SM NiTi alloy.
  
  
  METHODS
  Experiments were conducted on Ni-51 wt% Ti-49 wt% SM alloy. Five cylindrical specimens and five sheet specimens were subjected to different CT conditions: Deep CT (DCT) 24 group: -185°C; 24 h, DCT 6 group: -185°C; 6 h, shallow CT (SCT) 24 group: -80°C, 24 h, SCT 6 group: -80°C, 6 h and control group. Microstructure of surface was observed on cylindrical specimens with an optical microscope and scanning electron microscope at different magnifications. Subsurface structure was analyzed on sheet specimens using X-ray diffraction (XRD).
  
  
  RESULTS
  Microstructures of all SM NiTi specimens had equiaxed grains (approximately 25 μm) with well-defined boundaries and precipitates. XRD patterns of cryogenically treated specimens revealed accentuation of austenite and martensite peaks. The volume of martensite and its crystallite size was relatively more in DCT 24 specimen.
  
  
  CONCLUSIONS
  DCT with 24 h soaking period increases the martensite content of the SM NiTi alloy without altering the grain size.
authors.affiliation: Department of Restorative Dental Sciences, College of Dentistry, Jazan Universtiy, Jazan, Kingdom of Saudi Arabia.
authors.forename: Thilla Sekar
authors.initials: TS
authors.lastname: Vinothkumar
authors.affiliation: Department of Conservative Dentistry and Endodontics, Faculty of Dental Sciences, Sri Ramachandra University, Chennai, Tamil Nadu, India.
authors.forename: Deivanayagam
authors.initials: D
authors.lastname: Kandaswamy
authors.affiliation: Department of Mechanical Engineering, Velammal Engineering College, Chennai, Tamil Nadu, India.
authors.forename: Gopalakrishnan
authors.initials: G
authors.lastname: Prabhakaran
authors.affiliation: Department of Production Technology, MIT Campus, Anna University, Chennai, Tamil Nadu, India.
authors.forename: Arunachalam
authors.initials: A
authors.lastname: Rajadurai
keywords: Cryogenic treatment; R-phase; microstructure; nickel-titanium; shape memory