X-ray diffraction of nitinol orthodontic arch wires.

Superelastic mechanical behavior of nitinol alloy orthodontic wires is thought to be the result of a stress induced crystallographic transformation from austenite to martensite. The purpose of the study was to compare the SE mechanical behavior of nitinol wires to stress-induced phase changes. Eight nitinol arch wires having rectangular cross-sections were strained from 0% to 10% in tension with a mechanical testing machine. Load/extension plots were subjectively ranked for SE behavior. X-ray diffraction patterns were collected with and without 6% strain. Without strain, nitinol wires were found to be predominantly austenite with some wires containing a small amount of martensite. When strained 6%, superelastic wires demonstrated a phase transformation from austenite to martensite. XRD patterns were ranked for percent transformation and 110 peak width. Product rankings of the degree of superelasticity were positively correlated with the rank of martensitic transformation (p < 0.05). Superelasticity ranks were negatively correlated with XRD peak width ranks (p < 0.01). Increased peak width indicates greater cold work. A range of superelastic mechanical behavior and martensitic transformation is exhibited by wires currently on the market. Cold work and heat treatments are important variables to be controlled during the manufacture of nitinol products.