Abstract
Ion implantation can be used to tune the mechanical, physical, and chemical properties of crystalline materials via introducing nanoscale defects. In particular, injected helium atoms are insoluble in solids and can spontaneously precipitate into helium nanobubbles or platelets. Helium precipitation was mainly utilized for introducing large-size helium bubbles to create porous nanostructures with enhanced mechanical and functional performance. However, when decreasing the diameter of helium precipitates to several nanometers by controlling helium diffusion, internal helium pressures can be readily increased up to the gigapascal level, making helium precipitates substantially different from conventional nanovoids. Such high pressure can profoundly modify interatomic distances and electronic orbitals near helium precipitates that yield exotic properties. To this end, in this thesis, pressurized helium nanobubbles and platelets have been controllably introduced into two advanced crystalline materials, FeNiCoCr high-entropy alloy and diamond, for material properties modification.A prototype FeNiCoCr high-entropy alloy was chosen because it has strong bubble formation resistance and extensive mechanical and functional applications. FeNiCoCr high-entropy alloys were implanted with 275 keV 4He+ ions to a fluence of 5.14 × 1016 ions cm-2 at 773 and 873 K for introducing pressurized helium nanobubbles. The morphology of helium nanobubbles was examined by Fresnel contrast transmission electron microscopy. Helium densities and pressures inside individual nanobubbles were quantitively measured by low-voltage monochromatic electron energy loss spectroscopy. Radiation-induced segregation of Ni and Co with the depletion of Fe and Cr around helium nanobubbles was also revealed by atom probe tomography. The helium-associated mechanism of radiation-induced segregation was proposed, and the influence of radiation-induced segregation on stacking fault energy fluctuation was investigated.
Subsequently, the effects of highly pressurized helium nanobubbles on the deformation mechanism and mechanical properties of FeNiCoCr high-entropy alloy were systematically assessed by in situ nanomechanical testing inside a transmission electron microscope. In situ tensile tests demonstrated that highly pressurized helium nanobubbles could not only increase the strength by serving as dislocation obstacles but also enhance the strain hardening capacity and accommodate considerable plasticity via facilitating the multiplication and interaction of interwoven stacking faults. Such stacking-fault-mediated deformation in the present FeNiCoCr high-entropy alloy is essentially different from deformation behaviors observed in its counterparts subjected to conventional thermal-mechanical processing. Notably, in situ compressive tests showed that highly pressurized helium nanobubbles increased the yield strength of FeNiCoCr high-entropy alloy nanopillar by ~50% with almost no reduction in plasticity.
Moreover, diamond was selected owing to the extremely high shear modulus and promising applications as the ultra-wide bandgap semiconductor. Single-crystal type Ib diamond samples were implanted with 275 keV 4He+ ions to a fluence of 6.4 × 1016 ions cm-2 at temperatures ranging from 1073 to 1573 K for introducing pressurized helium platelets. Irradiation-induced amorphization and graphitization of diamond lattices were suppressed at high temperatures, which was critical to the formation of helium platelets. Helium platelets and associated self-interstitial defects were resolved in diamond by atomic-resolution differential phase contrast microscopy. It was found that helium atoms were self-assembled into two dimensions between {100} lattice planes of diamond. Helium pressures inside platelets were further estimated by an elastic continuum approach.
Finally, strain mapping of the implanted diamond was performed by geometric phase analysis, indicating that pressurized helium platelets could achieve considerable strains to surrounding diamond lattices. Valence electron energy loss spectroscopy further suggested that the formation of pressurized helium platelets in diamond could give rise to bandgap narrowing. Besides, the reliable creation of helium-related and nitrogen-vacancy centers was observed in type Ib diamond by cathodoluminescence microscopy and spectroscopy. In situ tensile tests confirmed that helium platelets had less influence on the mechanical integrity of diamond.
Collectively, this thesis provides a novel strategy to tune the properties of crystalline materials via introducing pressurized helium precipitates. It is believed that this technique will not only contribute to the development of mechanically reliable micro/nanoscale high-entropy alloys for mechatronic applications, but also open avenues for “strain doping” of semiconductor materials to advance device performance.
| Date of Award | 27 Oct 2021 |
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| Original language | English |
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| Supervisor | Ji-jung KAI (Supervisor) |