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 4 9 MODULATING THE PSEUDOELASTICRESPONSE OF NiTi USING ION IMPLANTATION Ni-ion irradiated NiTi is observed to be nearly 50%harder, retains 85%recoverable deformation, and has reduced hysteresis. Alejandro Hinojos,* Daniel Hong,* Longsheng Feng, X. Gao, Yunzhi Wang, FASM,* Michael J. Mills, FASM,* and Peter M. Anderson, FASM,* The Ohio State University, Columbus, Ohio Chao Yang and Janelle P. Wharry, Purdue University, West Lafayette, Indiana Khalid Hattar,* Sandia National Laboratories, Albuquerque, New Mexico Nan Li, Los Alamos National Laboratory, New Mexico Jeremy E. Schaffer,* Fort Wayne Metals Research Products LLC, Fort Wayne, Indiana This work explores whether ion beam modification can be used to modulate the austenite to martensite phase transformation in Nickel-Titanium (NiTi), thereby achieving novel or localized transformation properties in near-surface regions. This could provide alternatives to laser shot peening or other surface treatment methods and possibly expand applications in biomedical, aerospace, and other fields[1-5]. Irradiation induces defects and internal stress that can serve as nucleation and/or pinning sites for the phase transformation. Thus, it can augment more conventional approaches, including alloying[6-11], severe mechanical work[12-14], grain size reduction[15], and precipitation of coherent precipitates[15,16]. A range of outcomes is possible in principle, including a shift of the critical stress or temperature for onset of the transformation, linearization, reduction of hysteresis, stabilization, and extent of transformation strain. Because prior studies show irradiation amorphization of NiTi-based intermetallics[17-24], this work employs lower doses of irradiation (< 0.1DPA, displacements per atom) to retain a large volume fraction of ordered phase, and then uses nanoindentation and structural analysis to probe submicron and nanoscale features that are not accessible with conventional or micron-scale pillar testing[25-29]. Nanoindentation has the capacity to clearly identify the austenite-martensite phase transformation[30] as well as tension-compression asymmetry and anisotropy[31-33]. APPROACH A combined experimental-computational approach was used to study the hypothesis that irradiation defects can modulate the phase transformation. First, cross-sectioned samples of drawn Ti-Ni50.5%at 3.2 mm diameter wire (Ft. Wayne Metals) with a <111> axial texture were metallographically prepared and irradiated into the top axial surface with 30 MeV Ni6+ ions to achieve a fluence of 5 x 1013 cm-2 using the Tandem Accelerator Facility at the Center for Integrated Nanotechnology at Sandia National Labs[34]. SRIM software[35] was used to specify this fluence to achieve < 0.1 DPA, the critical threshold for amorphization[19]. Next, Berkovich indentation to 250 nm depth on polished side facets was performed with a Hysitron TI950 Tribo- Indenter at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory[36], to sample submicron volumes Vind ≈ 0.125 μm3 at positions z = 0 to 8 µm below the implanted surface. For reference, ~70 indents in unirradiated drawn Ti-Ni50.5%at wire were performed perpendicular to the <111> axial texture. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for structural characterization. Two simulation methods were used to explore the influence of possible irradiation defects on the martensitic phase transformation. At the submicron scale, phase field (PF) modeling following Zhu et al.[36] and at the atomistic scale molecular dynamics (MD) simulations were used to study stress-induced transformation in 170 nm and 21 nm cubes, respectively, with and without irradiation type defects. The PF simulations applied a compressive stress of 700 MPa along the <100> B2 direction and solved the time-dependent Ginzburg-Landau equation[37] on a 128 × 128 × 128 grid. The MD simulations applied a tensile strain of 0.1 at a rate of 0.001 ps-1 and used LAMMPS[38] and Ko and coworkers’[39] modified embedded atom method (MEAM) potential, which accurately captures the B2-B19′ transformation, and OVITO[40] for phase identification and common neighbor analysis. RESULTS The indentation results (Fig. 1) show that at z ≈ 3.6 μm below the implantation surface, the irradiated material exhibits an indentation load P250nm = 8.5 mN, ~46% larger than for unirradiated material, yet the recoverable displace- ment δrec upon unloading is comparable to unirradiated material. The cycle 2 indentation curves—where the ma- terial is reindented at the same site—show that the irradi- 3 FEATURE *Member of ASM International
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