One of the most important applications of shape memory alloy is in medicine, especially orthodontic archwires. In this category Ni–Ti orthodontic archwires is one of the oldest used materials. Biocompatibility, corrosion resistance, super elasticity, etc. are its outstanding properties. In spite of the importance of dependency of pre-strain on microstructure of Ni–Ti there are limited sources concentrated on the subject. For this reason the main purpose of the current study is determination of the effect of pre-strain on microstructure of Ni–Ti orthodontic archwires. In this regard, three-point bending was performed on the orthodontic archwire specimens to apply different amount of strain. The microstructures were compared with the un-strained wire using optical and scanning electron microscopes. The results showed that the stable phase depends strongly on the value of pre-strain. Increasing pre-strain causes to decrease martensite laths and leads the microstructure toward austenite phase. © 2007 Elsevier B.V. All rights reserved.

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