Mechanisms and applications of radiation-induced amorphization and (re)crystallization

Abstract

Materials under irradiation are subject to dynamic force conditions that drive alloys into non-equilibrium configurations. In this presentation, radiation mechanisms under ion or electron beam and the applications will be discussed. One of the case studies is amorphization of Ca2Na2Nb5O16 crystalline nanosheets under electron beam with different energy [1]. Dielectric two-dimensional oxide nanosheets are attractive because of their thermal stability and high-k property. However, their atomic structure characterization has been limited since they are easily degraded by electron-beams. In this study, the effect of acceleration voltage of the e-beam on the damage mechanisms was investigated. Knock-on damage dominantly occurred at high voltages, leaving short-range order in the final amorphous structure. On the other hand, a series of chemical reactions predominantly occurred at low voltages, resulting in random elemental loss and a fully disordered amorphous structure. This radiolysis was facilitated by insulated nanosheets that contained a large number of dangling bonds after the chemical solution process. The radiolysis damage kinetics was faster than knock-on damage and induced more elemental loss. Using amorphous-to-crystalline transformation under electron beam, we suggest a fabrication of atomically manipulated nanostructures in amorphous LaAlO3 thin film grown on SrTiO3 substrate [2]. Epitaxial crystal growth from the interface can occur without direct e-beam irradiation at the interface due to accumulated charge around the beam position in the insulating materials. 2-dimensional electron gas, which acts as a current path delays the crystallization kinetics, thus delicate control of the crystallized pattern shape and size is available. As a result, successful pattern writing with a width of about 5 nm was performed. Lastly, ion beam radiation-induced dynamic recrystallization in CrFeCoNiCu high entropy alloy will be discussed [3]. Under ion beam, radiation-enhanced diffusion results from the supersaturation of vacancies and self-interstitial atoms produced by atomic displacement under irradiation, which accelerate solute diffusion and thus drive the system toward a new equilibrium state. Here, we utilized a precession electron diffraction technique in TEM to visualize recrystallization and polygonization locally occurred only at the sample surface. This orientation map enables the visualization of depth-dependent recrystallization at a glance and reveals the effects of irradiation dose and material entropy. Discontinuous dynamic recrystallization preferentially occurs near the surface, where the irradiation dose is low, but radiation-enhanced diffusion dominantly occurs leading to dislocation climb even at room temperature. Interestingly, in the high-entropy phase with a relatively low stacking fault energy, discontinuous dynamic recrystallizastion is suppressed due to the lower stored energy and grain boundary diffusivity compared low-entropy one.