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 0 ated material has smaller hysteresis and is more stable, with negligible plasticity, compared to the unirradiated material. Simulated unloading curves for an elastic-plastic material with an elastic modulus of austenite (90 GPa)[41] and a plastic flow strength that is adjusted to match the cycle 1 loading response are computed using a finite element model. The difference, δrecPT, between the simulated and experimental curves denotes the recoverable displacement attributable to the phase transformation. The irradiated material has ≈85% of the recoverable displacement of unirradiated material. The SEM electron backscatter diffraction (EBSD) inverse pole figure (IPF) map (Fig. 2a) reveals equiaxed grains and a dark band at ~3 μm below the implantation surface, as well as indentation sites in the vicinity of the band. The inability to properly index the band and indentation sites signify damage that could be caused by amorphous regions and other defects cited in prior work. The regions outside the band are still indexed as crystalline B2 phase but are likely to contain some amorphous damage. Additional evidence from enhanced contrast in backscattered electron (BSE)[42] imaging (not shown) supports the presence of a damage distribution consistent with implantation in other alloys[43-45]. The TEM bright field image (Fig. 2b) shows a cross-section near one of the indents. The faint traces of defects immediately beneath the indent are likely dislocations generated by indentation. The diffraction pattern inset of the <111>B2 reveals azimuthal elongation of the <110>B2 spots showing signs of crystal rotation[22] from plasticity or retained internal stresses[46]. Characterization using scanning TEM (STEM) bright field imaging (not shown) suggests that pre-existing defects near the damage band can be destroyed, setting the stage for investigation of dislocations in amorphous regions and whether amorphization in the B2 phase is continuous[19,22]. The phase field simulations (Figs. 3a-e) predict that a B2+amorphous composite with a morphology approximated by STEM observations (white = amorphous, black = B2) does not transform to martensite (variants v1v4 as indicated by colors) as readily as the B2-only case (Figs. 3f-j). The B2 phase in the composite is interconnected but the channels cannot be filled fully by self-accom- modating martensitic variants. The stark contrast between Figs. 3e and j clearly indicate that the autocatalysis of the martensitic transformation has been suppressed by the elastic but non-transforming amorphous phase. This is 5 6 4 FEATURE Fig. 2 — (a) EBSD IPF map normal to wire axis; (b) TEM bright field image, cross section of an indent located above the damage band in (a). Fig. 1 — Nanoindentation response in unirradiated (averaged over ~70 indentations) and irradiated (single indent location at z ≈ 3.6 μm below the implanted surface) drawn Ti-Ni50.5at% wire. (a) (b)
RkJQdWJsaXNoZXIy MTYyMzk3NQ==