X-ray diffraction of nitinol orthodontic arch wires

Area of Science:

• Materials Science
• Orthodontics
• Crystallography

Background:

• Superelasticity in nitinol alloy orthodontic wires is attributed to stress-induced austenite to martensite crystallographic transformation.
• Understanding this transformation is key to optimizing nitinol wire performance in orthodontics.

Purpose of the Study:

• To compare the superelastic (SE) mechanical behavior of nitinol wires with their stress-induced phase changes.
• To investigate the relationship between superelasticity and the austenite to martensite transformation in nitinol.

Main Methods:

• Eight rectangular nitinol arch wires were subjected to tensile strain from 0% to 10% using a mechanical testing machine.
• Load/extension data were analyzed to rank superelastic behavior.
• X-ray diffraction (XRD) was used to analyze phase transformations at 0% and 6% strain, with XRD patterns ranked for percent transformation and 110 peak width.

Main Results:

• Nitinol wires were predominantly austenite without strain, with some containing minor martensite.
• A 6% strain induced a significant austenite to martensite phase transformation in superelastic wires.
• Superelasticity rankings positively correlated with martensitic transformation (p < 0.05) and negatively correlated with XRD peak width (p < 0.01).

Conclusions:

• A range of superelastic behavior and martensitic transformation exists in commercially available nitinol wires.
• Increased XRD peak width, indicative of greater cold work, was associated with lower superelasticity.
• Controlling cold work and heat treatments during nitinol manufacturing is essential for consistent superelastic properties.