UC Santa Barbara

High temperature structural materials are critical for many applications that underpin modern civilization, including commercial aviation, spaceflight, and chemical processing. These materials must be able to function within extreme conditions, including severely corrosive and oxidative environments, for extended periods, and improvements in mechanical and environmental properties must be balanced with the ability to tolerate damage without catastrophic failure. The materials of choice for many extreme environments have been the Ni-based superalloys. Developed continuously from the mid-nineteenth century, and most recognizable for their role in the hot sections of jet turbine engines, Ni-based superalloys exhibit an excellent balance of properties, including high temperature creep and fatigue strength, intrinsic resistance to oxidation, and reasonable fracture toughness and cost. Their properties are derived from their two-phase microstructres, where a high volume fraction of coherent intermetallic L12 (γ′) precipitates are embedded in a solid solution strengthened FCC (γ) matrix. However, further improvements to the temperatures capabilities of Ni-based superalloys are unlikely due to their level of technological maturity. Increasing temperature demands for turbine engines and other applications motivate the search for new materials that can surpass the temperature limits of Ni-based superalloys. Nb-based alloys are one promising group of candidate materials, where the principal element Nb has a high melting temperature of 2477 ◦C, a density lower than Ni at ρ = 8.582 g/cm3, and excellent room temperature fabricability.

Of the Nb-based alloys that have been investigated, precipitation strengthened alloys are the closest to achieving the balance of properties required of high temperature materials for critical components. The most promising design strategy is strengthening by a large volume fraction of coherent intermetallic precipitates. This approach attempts to replicate the enormous success of the Ni-based superalloys, only within refractory alloys at higher temperatures. The refractory metals are body-centered cubic (BCC, β) materials at high temperature and therefore, alloys designed with this strategy are β + β′ alloys, where the β′ phase is an ordered derivative of the BCC crystal structure. To be successful, β + β′ alloys must contain coherent precipitates that are morphologically and thermodynamically stable at temperatures above 1200 ◦C.

The β′ phase is ideally the B2 (CsCl) phase. A B2 former of particular interest is Ru, a platinum group metal that forms a variety of B2 phases with the refractory metals. The binary Ru-B2 phases are thermodyanmically stable to much higher temperatures compared to the other candidate strengthening phases in the literature, which dissolve at temperatures below 1200 ◦C. For example, the binary BCC + B2 phase field in the Hf-Ru binary extends up to 1610 ◦C. While most of the Ru-based B2 phases have lattice parameters that are significantly smaller than the lattice parameter of pure Nb (aNb = 3.301 ̊A), Nb can theoretically be alloyed to be coherent with the largest of the B2 phases, HfRu (aHf Ru = 3.225 ̊A) and ZrRu (aZrRu = 3.253 ̊A). Therefore, Ru presents an opportunity to form both coherent and thermally stable precipitates.

n this dissertation, the first comprehensive investigations into the potential of Ru-B2 precipitates to strengthen Nb-based alloys are investigated. Due to their lower lattice misfit with Nb, alloys were designed to contain HfRu- and ZrRu-B2 phases. Initial investigations focused on equiatomic Hf-, Zr-, and Ru-containing alloys, where the B2 phases were stable to the melting point and alloys demonstrated complex solidification pathways. The insights gained from these preliminary investigations are used to successfully design solution and age hardenable alloys with controllable B2 solvus temperatures from1000-1900 ◦C. When the matrix compositions are tailored to reduce the lattice misfit between the B2 and BCC phases to below 1%, a homogeneous distribution of spherical precipitates is achieved. HfRu is found to be more stable than ZrRu, exhibiting higher B2 soluvs temperatures and fewer deleterious phases at equivalent Ru concentrations. An initial investigation of dislocation behavior in these systems is provided by post-mortem transmission electron microscopy of microcompression pillars, revealing the presence of paired dislocations and dislocation loops around precipitates. Preliminary investigations of high temperature mechanical properties show promise compared to current commercial refractory alloys. Implications for further development of Nb-based alloys by Ru-B2 precipitates are discussed, including strategies to mitigate deleterious phase formation and increase B2 volume fractions. While many fundamental questions remain regarding Nb-BCC + Ru-B2 systems, they offer a promising path forward for developing new high temperature materials.