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Microstructural studies of 35 degrees C Copper Ni-Ti orthodontic wire and TEM confirmation of low-temperature martensite transformation.

Brantley WA, Guo W, Clark WA, Iijima M

Section of Restorative and Prosthetic Dentistry, College of Dentistry, The Ohio State University, Columbus, OH 43210, USA.

BACKGROUND: Previous temperature-modulated differential scanning calorimetry (TMDSC) study of nickel-titanium orthodontic wires revealed a large exothermic low-temperature peak that was attributed to transformation within martensitic NiTi. PURPOSE: The purpose of this study was to use transmission electron microscopy (TEM) to verify this phase transformation in a clinically popular nickel-titanium wire, identify its mechanism and confirm other phase transformations found by TMDSC, and to provide detailed information about the microstructure of this wire. METHODS: The 35 degrees C Copper nickel-titanium wire (Ormco) with cross-section dimensions of 0.016in.x0.022in. used in the earlier TMDSC investigation was selected. Foils were prepared for TEM analyses by mechanical grinding, polishing, dimpling, ion milling and plasma cleaning. Standard bright-field and dark-field TEM images were obtained, along with convergent-beam electron diffraction patterns. A cryo-stage with the electron microscope (Phillips CM 200) permitted the specimen to be observed at -187, -45, and 50 degrees C, as well as at room temperature. Microstructures were also observed with an optical microscope and a scanning electron microscope. RESULTS: Room temperature microstructures had randomly oriented, elongated grains that were twinned. Electron diffraction patterns confirmed that phase transformations took place over temperature ranges previously found by TMDSC. TEM observations revealed a high dislocation density and fine-scale oxide particles, and that twinning is the mechanism for the low-temperature transformation in martensitic NiTi. CONCLUSIONS: TEM confirmed the low-temperature peak and other phase transformations observed by TMDSC, and revealed that twinning in martensite is the mechanism for the low-temperature peak. The high dislocation density and fine-scale oxide particles in the microstructure are the result of the wire manufacturing process.

Published 31 December 2007 in Dent Mater, 24(2): 204-10.
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