PhD Thesis Defense: Sharon Park

The future development of power generation, space exploration, and hypersonic flight systems all depend on the availability of materials that maintain strength at ultra-high temperatures (UHTs). Refractory metals and alloys possess unprecedented high temperature properties attributed to their extraordinarily high melting temperatures, making them attractive for these applications. Additive manufacturing, specifically laser powder bed fusion (LPBF), has opened new avenues for processing of near net-shape refractory components. Despite this, UHT data, particularly tensile properties, of these materials is limited, and in some cases, non-existent. Successful implementation of refractories is contingent on the elucidation and understanding of mechanisms that govern their UHT deformation behavior. This study was undertaken to address this need through UHT testing and characterization of: additively manufactured tantalum (Ta), wrought WC3009 (Nb-30Hf-9W), and additively manufactured C103 (Nb-10Hf-1Ti). Their properties were assessed by leveraging custom UHT testing systems available in the Hemker lab, which are capable of reaching temperatures in excess of 2000°C. The mechanical strength and elongation of LPBF Ta specimens decreased with increasing temperature. Anomalous strain hardening was uncovered at 400°C, which was attributed to dynamic strain aging associated with oxygen penetration. Internal oxidation was persistent at higher temperatures. At 800°C, the formation of oxide bands was accompanied by a significant reduction in strain to UTS, while specimens tested at 1000°C exhibited both oxide banding and dynamic recovery which greatly increased plasticity. TEM revealed the existence of dense dislocation cells, Orowan bowing, and other strengthening mechanisms, which highlighted influences of oxidation and recovery on high temperature properties of LPBF Ta. Wrought WC3009 was tested in tension from 1200 to 1800°C with strain rates of 10-4 to 10-2 s-1. In addition to strength drop-offs with increasing temperature, WC3009 exhibited significant strain rate sensitivity that contributed to variable stress-strain behavior, including identification of a brittle-to-ductile transition strain rate. Grain boundary sliding was identified as a dominant failure mechanism at UHTs and the calculated strain rate exponent of 0.5 was consistent with activation of superplastic-like mechanisms at high temperatures and slow strain rates. The presence of Hf oxides and carbides were noted in all post-mortem samples but they did not strongly influence strength or ductility. Dynamic recrystallization resulted in significant grain refinement at 1200°C and grain growth at 1800°C. LBPF C103 was tested from 23 to 1415°C in both as-built and stress-relieved conditions. All samples exhibited strength plateaus at intermediate temperatures followed by significant drop offs in strength when approaching UHTs. As-built C103 displayed a strength advantage over wrought until 1010°C, which was due to its higher dislocation density. The strength of as-built C103 matched wrought above 1010°C, which suggested an in-situ stress-relieving effect. Collectively, these studies helped identify the operative mechanisms behind the UHT performance of LPBF Ta, WC3009, and LPBF C103, providing insights to enhance commercial refractories and enable their increased use in UHT applications.