A new generation of compact and highly efficient power production and propulsion technologies are critically needed in enabling NASAs long-term goals. Nuclear fission power technologies as part of project Prometheus are in development to meet this need. Proposed reactor concepts utilize a combination of refractory metals and stainless steels. One such refractory alloy, Niobium 1% Zirconium (Nb-1Zr), will be used because of its strength at high temperatures, neutron absorption properties, and resistance to corrosion by liquid alkali metals. One potential problem in using Nb-1Zr is that it undergoes rapid high temperature oxidation, even in low oxygen concentrations. Long-term oxidation of the niobium matrix can significantly deteriorate the mechanical properties of the alloy. This thesis reports on experimental studies of the high temperature interaction of 316 stainless steel (316 SS) and Nb-1Zr under prototypic space fission reactor operating conditions. Specifically, how the high temperature oxidation rate of Nb-1Zr changes when in contact with 316 SS at low external oxygen concentrations. The objective of the project is to determine if transport of gaseous contaminants, such as oxygen, will occur when Nb-1Zr is in contact with 316 SS, thereby increasing the oxidation rate and degrading material properties. Experiments were preformed in a realistic non-nuclear environment at the appropriate operating conditions. Thermal Gravimetric Analysis techniques were used to quantify results. Coupons of Nb-1Zr and Nb-1Zr in contact with 316 SS foil are subjected to flowing argon with oxygen concentrations between 4-15ppm and heated to a temperature of 500, 750, and 1000oC for 2 to 10 hours. Experiments were conducted at the Early Flight Fission Test Facility at NASA Marshall Space Flight Center. The experimental results indicate that a complex oxidation process, which depends greatly on temperature and oxygen concentration, occurs at the expected operating conditions. Non-linear regression techniques were applied to experimental data in order to derive correlations for the approximate oxidation rate of Nb-1Zr and Nb-1Zr in contact with 316 SS as a function of time, temperature, and oxygen concentration.

Out of the known refractory metals and alloys, molybdenum (Mo) and its alloys are very important due to their unique combination of properties which render them suitable for various applications. Owing to their good creep properties, minimum damage from neutron irradiation and good compatibility with the liquid alkali metals, molybdenum and its alloys are well suited candidates for structural components in the newly developed Compact High Temperature Reactor (CHTR). However, to fabricate components for structural application from molybdenum and its alloys, the processing response needs to be established. The present thesis is an attempt to address this issue in a more generic manner. The study have been specifically aimed to examine the hot deformation behaviour of molybdenum and two of its alloys (Mo-TZM and Mo-TZC) over a high temperature range, for obtaining stable microstructure with good mechanical properties. The thesis basically addresses the following (i) the thermos-mechanical response of the material with change in deformation conditions, and (ii) the evolution of microstructure during hot deformation, and identification of associated mechanisms. Chapter 1 of the thesis includes an introduction of the material system and alloys with a detailed survey of the literature on the deformation behaviour of refractory metals and alloys that are used as structural materials in nuclear reactors. More emphasis is given to molybdenum and two of its alloy Mo-TZM and Mo-TZC. Chapter 2 includes the detail of the experimental techniques and analysis procedures that are followed in the course of investigation. The hot deformation behaviour of molybdenum in temperature range 1400 - 1700°C and strain rate range 0.001 - 10.0s-1 is discussed in chapter 3. The stress - strain behaviour has been further analysed to obtain strain rate sensitivity maps. The micro-mechanisms operative in different deformation domain has been analysed extensively by Electron Back Scatter Diffraction (EBSD) technique. Different restoration processes which include dynamic recrystallization, recovery and grain growth have been identified in different domains of deformation conditions. Chapter 4 of this thesis is dedicated to the hot deformation behaviour of Mo-TZM alloy. Deformation behaviour was studied under identical conditions as molybdenum. Mo-TZM showed higher strain rate sensitivity and high temperature strength than molybdenum. Dynamic recovery is the most predominant mechanism in Mo-TZM alloy as revealed through the analysis of stress strain curve as well as EBSD based investigation. At higher temperature and strain rates dynamic recrystallization has also been observed. The effect of excess carbon which results in Mo-TZC alloy, deformation behaviour has been investigated in chapter 5. The analysis of stress – strain curves in this case indicates the predominance of dynamic recrystallization over a range of deformation conditions. The mechanism has been identified as particle simulated nucleation (PSN). The significant growth of the deformed grains is observed at the highest temperature of deformation. A comparison of deformation behaviour of alloying addition in molybdenum alloys has been discussed in chapter 6. The results of deformation behaviour of molybdenum and its alloys has been compared vis-a-vis with another similar class of alloys based on Niobium (Nb) and apparent similarities and differences in the deformation behaviour has also been discussed in chapter 6. Finally, the overall summary of the thesis has been presented in chapter 7.