(Georgia Institute of Technology, 2018-05-29)
Bryant, Alex W.
The research performed in this work was aimed at investigating pressure-induced phase changes in a Ce-based metallic glass (MG) through the use of laser-driven shock experiments and atomic resolution structural characterization. MGs exhibit very high strength, have intrinsically low density, and plastically deform by shear banding. MGs are also metastable and can undergo phase changes upon heating and/or application of high pressure into higher density configurations. The atomic structure changes concomitant with these phase transitions occurring during high pressure shock compression are not well understood, which provides the motivation for the present work. Thermal analysis of Ce3Al MG melt-spun ribbons was first performed to characterize the crystallization response and structure. Ce3Al MG was found to strongly resist growth of crystallites but easily nucleate. Thermal crystallization occurs via a two-stage primary path wherein a metastable phase forms and converts fully into the hexagonal-intermetallic α-Ce3Al. The Avrami number and dimensionality constants indicate the crystallization occurs via plate-like growth, resulting in thermally crystallized grains on the order of 6 nm and a density ~4% greater than the reference α-Ce3Al. Shock compression experiments performed using the Nd:YAG 3 J laser and velocity interferometry allowed for in operando measurements of particle velocity coupled with sample recovery for structural analysis. The results provide a clear indication of the Hugoniot Elastic Limit (at ~1.8 GPa) as evidenced by the presence of a two wave structure in the velocity profile. At shock pressures exceeding the elastic limit, plastic deformation of the Ce3Al MG occurs via structural transformation to the crystalline state forming α-Ce3Al with nanocrystalline grain sizes, higher densities, and plate-like growth. The trends suggest that shock compression causes break-up of grains, higher densities due to Ce 4f delocalization, and increased preferred orientation. Shock compression experiments were also performed using the 50 J Omega laser facility at the Laboratory for Laser Energetics. A stack of samples was shock-compressed with pressures progressively decreasing across the stack thickness, resulting in two regimes of recovered samples. Highly deformed and partly damaged samples close to the shock front showed varying degrees of long-range order, medium-range order, and short-range order with distance away from the shock front. Visually undeformed samples showed decreased bond lengths for the nearest-neighbors, second nearest-neighbors, and fourth nearest-neighbors but increased bond lengths for the third nearest-neighbors, with associated densification of ~2-6% in all layers. These changes in the undeformed samples are indicative of polyamorphism. The visually undeformed samples also reveal an increase in magnitude of structural change with increased distance away from the shock-front, up to a maximum beyond which increasing distance decreases the magnitude of the bond length shifts. This trend is indicative of competing effects for densification and dilation, associated with the extreme and complex states generated. The mechanism and characteristics of the shock induced crystallized Ce3Al MG are different from the hydrostatic pressure-induced crystallization of Ce3Al MG (which occurs via a coordinated and instantaneous rearrangement of all atoms into the FCC-Ce3Al phase) and thermal crystallization into α-Ce3Al (which occurs via diffusional nucleation and growth). Shock-induced crystallization during shock compression occurs in a nucleation-like collective rearrangement with limited kinetic allowance for growth, resulting in larger needlelike crystallites than could nucleate through thermal processes. The dilatory effects and increased driving forces caused by shear bands and shock-induced heating result in larger grain sizes and longer lattice parameters. Increases in shock pressures appear to create larger driving forces for the formation of lower energy plate-like morphologies and higher densities while simultaneously breaking crystallites into smaller sizes.