Understanding the Elevated Temperature Properties of Niobium-Based Alloys Relevant to Aerospace Applications

There is renewed interest in refractory alloys that possess higher service temperatures than incumbent Ni-based superalloys (e.g., ⪆1100°C). The focus of this thesis can be divided into two distinct sections, with the first section providing a review of the high-temperature constitutive responses of Nb-alloys measured over a wide range of temperatures (≈860°C < T < ≈1760°C) and strain rates (≈10-9 s-1< ε ̇ < ≈10-1 s-1). Nevertheless, the extant data is sparse and informed materials selection decisions require constitutive expressions to interpolate and reliably extrapolate. The Larson-Miller parameter approach to describe creep-life provides a conservative estimate of material response at the highest temperatures and lowest strain rates, whereas the Sellars-Tegart model describes both steady-state creep and high-temperature tensile test data with a single, universal equation. A minimum flow stress based on the combination of these two models is proposed for design considerations to address the overprediction of strength that can arise from applying one or the other independently. This effort highlights the fact that refractory alloys exhibit strain rate sensitive flow strengths in the temperature range of interest for applications. The roles of alloying, thermomechanical processing, and impurity levels are discussed, and highlight the fact that these advanced Nb-alloys evidence Class 1 (Class A) solute drag controlled creep behavior, except the carbide precipitation strengthened alloy, D-43. In addition, the high-temperature strengths are confirmed to be strongly correlated with alloy melting point.

The second section of this thesis provides available thermophysical property data for a number of Nb-alloys and demonstrates their use within a performance index such that informed materials selection decisions can be made. Comparisons with Ni-superalloys and other refractory-metal based alloys give context for the provided design data. Physically based models are provided that describe the temperature dependencies of the Young’s modulus, coefficient of thermal expansion and density, thermal conductivity, and specific heat capacity. The results highlight some critical uncertainties and gaps in existing experimental data in the literature. New data are provided for two Hf-containing alloys. Elastic modulus, thermal expansion, thermal conductivity, and heat capacity are presented for one of the only currently available commercial Nb-alloys, C103 (Nb-10Hf-1Ti wt%), and new thermal conductivity data is provided for the higher strength Nb-alloy, WC-3009 (Nb-30Hf-9W wt%), which has yet to be fully commercialized. A performance index for ranking materials for use in lightweight panel-shaped applications subjected to sharp thermal transients or steep thermal gradients is employed to demonstrate the utility of the data. The results highlight the relative value of current alloy C103, comparisons to WC-3009, as well as the promise of specific Nb-W-Zr alloys.