Taming the Pseudoelastic Response of Nitinol Using Ion Implantation

A Hinojos and DN Hong and H Sriram and LS Feng and C Yang and JP Wharry and XS Gao and K Hattar and N Li and JE Schaffer and YZ Wang and MJ Mills and PM Anderson, SCRIPTA MATERIALIA, 226, 115261 (2023).

DOI: 10.1016/j.scriptamat.2022.115261

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.

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