A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 5 2 structure with fewer B19′ transformed lattice sites. The vacancy cluster case (Fig. 4d) shows even greater defect-mediated suppression of the transformation and at greater strain (Fig. 4e), the vacancy clusters appear to constrain the growth of B19′ martensite (compare insets, Figs. 4d and e). The MD results, like the PF counterparts, are consistent with the larger indentation load required for irradiated NiTi. The detailed defect structures are ultimately determined by a balance of free energy, which drives the maintenance of short- range order, and irradiation-induced ballistic disordering of lattice atoms[47]. CONCLUSIONS Ni ion beam modification of Ti-Ni50.5%at wire at <0.1 DPA is shown to increase the indentation load in implanted regions by as much as ~50%, yet retain ~85% of recoverable displacement while achieving ~60% reduction in hysteresis, greater stability, and more linear behavior. Electron microscopy reveals inhomogeneously distributed damage consisting of a mixture of amorphous and crystalline B2 phases in the implanted region. The increased hardness is supported by phase field and molecular dynamics simulations that predict defects at multiple scales to require increased load to achieve the B2-B19′ martensitic transformation. The results suggest the potential for Ni ion beam modification to achieve functional surface modification of NiTi. ~SMST For more information: Peter Anderson, professor, Department of Materials Science and Engineering, The Ohio State University, 140 W. 19th Ave., Columbus, OH 43210, 614.688.3050, anderson.1@osu.edu. Acknowledgments Work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), at The Ohio State University (OSU) under award #DE-SC0001258 (indentation analysis, characterization, and phase field simulations) and at Purdue University under award #DE-SC0020150 (molecular dynamics simulations). Material–Fort Wayne Metals (Fort Wayne, Ind.); Irradiation and nanoindentation experiments–Center for Integrated Nanotechnologies under User Proposals #2019BC0126 and 2121BC0097; Electron microscopy–OSU Center for Electron Microscopy and Analysis (OSU CEMAS). Computations–the Ohio Supercomputing Center under User Grants PAS0676, PAS0971. PMA also acknowledges support through the Independent Research/ Development Programwhile serving at the U.S. National Science Foundation. 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