Consolidation of zirconium-based metallic glass powder by equal channel angular extrusion

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Robertson, Jonathan Mark
Texas A&M University
mechanical engineering.; Major mechanical engineering.
thesis / dissertation description
In this study, amorphous Zr₅₈.₅Nb₂.₈Cu₁₅.₆Ni₁₂.₈Al₁₀.₃ (Vitreloy 106a) gas-atomized powder was consolidated by equal channel angular extrusion (ECAE). Several copper cans were filled with the powder, vacuum encapsulated and subjected to one extrusion pass in the supercooled liquid region-above the glass transition temperature (Tg) and below the crystallization temperature (Tx). These temperatures were 395⁰C and 460⁰C, respectively. The extrusion temperature and extrusion speed were altered for each billet. The main objective of this study is to characterize the effects of extrusion temperature and the extrusion rate on the consolidate characteristics. Microstructure, thermal stability, and hardness evaluations were used as a first assessment of consolidation. Room temperature compression experiments were carried out on the best consolidates with a strain rate of 10⁴̄. The fracture surfaces were inspected to reveal whether the deformation mechanism was interparticle debonding or adiabatic shear banding. The consolidates in which the time-temperature-transformation boundary was not crossed during processing exhibit DSC patterns similar to initial powder with a slight decrease in T[x]. A compressive strength of about 1.65 GPa was recorded in the consolidates processed at 20, 30 and 40 Celsius degrees below T[x] which is close to what is reported for cast counterparts. The fracture surfaces exhibit vein patterns covering up to 90% of the fracture surface area in some samples, which are characteristic of glassy material fracture and indicate good interparticle bonding through ECAE processing as well as the avoidance of excessive crystallization during processing. The present results show that ECAE consolidation of metallic glass powder in the supercooled liquid region is possible even when a relatively high amount of oxygen is present in the initial powder (1280 ppmw), which is reported to lead to a decrease in the processing window. This opens a new opportunity to fabricate bulk metallic glasses with dimensions that are difficult to achieve by certain techniques.