Taming the Pseudoelastic Response of Nitinol Using Ion Implantation

Implantation of Ni50.5Ti49.5 wire with 30 MeV Ni6+ ions at doses (< 0.1 DPA) typically smaller than employed in the literature is shown to systematically alter the pseudoelastic response, with extrema in Berkovich nano indentation load (+50%), hysteresis (-60%), and recoverable displacement (-19%) occurring at 3.6 mu m below the implantation surface. These extraordinary values are attributed to 10 to 20 nm amorphous clusters that constrain the stress-induced B2-B19 ' phase transformation. This is substantiated by phase field simulations of crystalline-amorphous composites and molecular dynamics simulations of crystalline-vacancy cluster composites showing the spatial refinement of martensite caused by nm-scale defects. The results suggest that ion implantation may potentially expand the processing and performance space for NiTi, by creating amorphous defects at smaller length scales than dislocation substructures produced by conventional deformation processing.

Automated summary

Implantation of Ni50.5Ti49.5 wire with low doses of 30 MeV Ni6+ ions can systematically alter the pseudoelastic response, with extreme changes in nanoindentation load, hysteresis, and recoverable displacement occurring at a depth of 3.6 μm below the implantation surface. These changes are attributed to amorphous clusters that constrain the stress-induced B2-B19' phase transformation. Simulations support this by showing the refinement of martensite caused by nm-scale defects. These results suggest that ion implantation could expand the processing and performance capabilities of NiTi.