Dynamic deformation of titanium-based bulk metallic glass composites
Diaz, Rene Orlando
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This work sought to understand the role of the microstructure of titanium-based bulk metallic glass (BMG) and bulk metallic glass matrix composites (BMG-MCs) under dynamic deformation. BMG-MCs provide enhanced toughness and ductility in contrast to monolithic BMGs through in-situ formed crystalline dendrites. The BMG and BMG-MC system investigated in this work is the titanium-based "DVX" system consisting of Ti-Zr-V-Cu-Be with varying size, morphology, and distribution of the second phase dendrites. The effect of processing and the subsequent effect on dynamic properties is also addressed with the DV1 BMG-MC processed by two different methods -- semi-solid forging (DV1-SSF) and suction casting (DV1-SC) -- yielding different microstructures with the same composition. The focus of this work was to determine the influence of the glass-composite structure of titanium-based bulk metallic glass matrix composites with in-situ precipitated dendrites of varying composition, crystallinity, and morphology in the dynamic deformation response compared to monolithic titanium-based bulk metallic glasses. Precipitated second phase crystallites complicates the deformation and fracture mechanisms of the bulk material in contrast to that for monolithic bulk metallic glasses. The present study sought to provide a comprehensive assessment of the microstructural response on the dynamic yielding and spall response through controlled plate impact experiments. The experiments consisted of simultaneous impact of two samples with one being probed using VISAR interferometry and the other being recovered for post-mortem fractography and characterization. The dynamic properties observed focused primarily on the dynamic compressive yielding, referred to as the "Hugoniot Elastic Limit", and the dynamic tensile strength referred to as the "spall strength", were determined using VISAR interferometry from experiments performed at impact pressures from 6.0 -- 17.3 GPa. The spall strength and HEL were also determined as a function of strain rate from decompression, peak pressure, and subsequent recompression states after spallation. The decompressive strain rate sensitivity provides insight on the resistance to spall fracture and showed the DV1-SSF alloy, to have the highest resistance to spall fracture. The recompression characteristics after spallation were indicative of the role of microstructure on dynamic fracture characteristics. The recompressive strain rate sensitivity showed that the DV1-SSF results in the most ductile fracture response compared to the other DVX alloys. Post-mortem microstructural characterization done on the recovered samples provided a good correlation with the observed dynamic fracture characteristics seen during recompression. The dynamic fracture of the titanium-based bulk metallic glass was found to have the same macroscopic, microscopic, and nanoscale deformation mechanisms seen in zirconium-based BMGs in the form of simultaneous maximum in-plane shear stress failures from uninterrupted shear band formation, "cup"-"cone" fracture facets formed in the uniaxial-strain region, and presence of veiny shear band patterns consisting of alternating shear-transformation zones (STZs) and tension-transformation zones (TTZs). The dynamic fracture was seen to be directly dependent on the glass content, dendrite size measured using the mean linear intercept through the dendrite, interdendritic spacing measured through the mean free path through the matrix, interfacial surface area, two-dimensional matrix connectivity, and microhardness of the dendrite. The experimental microstructural parameters of glass volume fraction, surface area per unit volume, and mean linear intercept through the dendrite were in utilized to develop a stereologically driven empirical model to project the spall strength performance across varying strain rates. The stereological projection of spall strength performance revealed potential maximized spall strength performance for a metallic glass composite to be ~70-90% glass content with the surface area per unit volume and mean linear intercept through the dendrite peaking in the mid-range values.