Frank J. Ritzert

Evaluation of Candidate Materials for a High-Temperature Stirling Convertor Heater Head

The Department of Energy (DOE) and NASA have identified Stirling Radioisotope Generators (SRG) as a candidate power system for use on long‐duration, deep‐space science missions and Mars rovers. One of the developments planned for an upgraded version of the current SRG design is to achieve higher efficiency by increasing the overall operating temperature of the system. Currently, the SRG operates with a heater head temperature of 650 °C and is fabricated from the nickel base superalloy 718. This temperature is at the limit of Alloy 718’s capability, and any planned increase in temperature will be contingent on identifying a more capable material from which to fabricate the heater head. To this end, an assessment of material candidates was performed assuming a range of heater head temperatures. The chosen alternative material candidates will be discussed, along with the development efforts needed to ensure that these materials can meet the demanding system requirements of long‐duration operation in hostile ...

High Temperature Stability of Dissimilar Metal Joints in Fission Surface Power Systems

Future generations of power systems for spacecraft and lunar surface systems will likely require a strong dependence on nuclear power. The design of a space nuclear power plant involves integrating together major subsystems with varying material requirements. Refractory alloys are repeatedly considered for major structural components in space power reactor designs because refractory alloys retain their strength at higher temperatures than other classes of metals. The relatively higher mass and lower ductility of the refractory alloys make them less attractive for lower temperature subsystems in the power plant such as the power conversion system. The power conversion system would consist more likely of intermediate temperature Ni‐based superalloys. One of many unanswered questions about the use of refractory alloys in a space power plant is how to transition from the use of the structural refractory alloy to more traditional structural alloys. Because deleterious phases can form when complex alloys are jo...

Prototype Rhenium Component for Stirling Engine Power Conversion

The Stirling engine power conversion concept is a candidate to provide electrical power for deep space missions. A key element for qualifying potential flight hardware is the long‐term durability assessment for critical hot section components of the power converter. One such critical component is the power converter heater head, which is a high‐temperature pressure vessel that transfers heat to the working gas medium of the converter. Rhenium is a candidate material for the heater head application because of its high melting point (3453 K), high elastic modulus (420 GPa), high yield and ultimate tensile strengths at both ambient and elevated temperatures, excellent ductility, and exceptional creep properties. Rhenium is also attractive due to the potential of near‐net‐shape (NNS) manufacturing techniques that allow components to be produced using less material, which lowers the overall cost of the component. The objective of this research was to demonstrate the manufacturing method using rhenium for this ...

Preliminary Investigations of Joining Technologies for Attaching Refractory Metals to Ni-Based Superalloys

In this study, a range of joining technologies has been investigated for creating attachments between refractory metal and Ni‐based superalloys. Refractory materials of interest include Mo‐47%Re, T‐111, and Ta‐10%W. The Ni‐based superalloys include Hastelloy X and MarM 247. During joining with conventional processes, these materials have potential for a range of solidification and intermetallic formation‐related defects. For this study, three non‐conventional joining technologies were evaluated. These included inertia welding, electro‐spark deposition (ESD) welding, and magnetic pulse welding (MPW). The developed inertia welding practice closely paralleled that typically used for the refractory metals alloys. Metallographic investigations showed that forging during inertia welding occurred predominantly on the refractory metal side. It was also noted that at least some degree of forging on the Ni‐based superalloy side of the joint was necessary to achieve consistent bonding. Both refractory metals were readily weldable to the Hastelloy X material. When bonding to the MarM 247, results were inconsistent. This was related to the higher forging temperatures of the MarM 247, and subsequent reduced deformation on that material during welding. ESD trials using a Hastelloy X filler were successful for all material combinations. ESD places down very thin (5‐ to 10‐μm) layers per pass, and interactions between the substrates and the fill were limited (at most) to that layer. For the refractory metals, the fill only appeared to wet the surface, with minimal dilution effects. Microstructures of the deposits showed high weld metal integrity with maximum porosity on the order of a few percent. Some limited success was also obtained with MPW. In these trials, only the T‐111 tubes were used. Joints were possible for the T‐111 tube to the Hastelloy X bar stock, but the stiffness of the tube (resisting collapse) necessitated the use of very high power levels. These power levels resulted in damage to the equipment (concentrator) during welding. It is of note that the joint made showed the typical wavy bond microstructure associated with magnetic pulse/explosion bond joints. Joints were not possible between the T‐111 tube and the MarM 247 bar stock. In this case, the MarM 247 shattered before sufficient impact forces could be developed for bonding.In this study, a range of joining technologies has been investigated for creating attachments between refractory metal and Ni‐based superalloys. Refractory materials of interest include Mo‐47%Re, T‐111, and Ta‐10%W. The Ni‐based superalloys include Hastelloy X and MarM 247. During joining with conventional processes, these materials have potential for a range of solidification and intermetallic formation‐related defects. For this study, three non‐conventional joining technologies were evaluated. These included inertia welding, electro‐spark deposition (ESD) welding, and magnetic pulse welding (MPW). The developed inertia welding practice closely paralleled that typically used for the refractory metals alloys. Metallographic investigations showed that forging during inertia welding occurred predominantly on the refractory metal side. It was also noted that at least some degree of forging on the Ni‐based superalloy side of the joint was necessary to achieve consistent bonding. Both refractory metals were re...

Compatibility of Niobium Alloys and Superalloys in a Flowing He - Xe Power Conversion System

Proposed concepts for an ambitious mission to explore Jupiter’s three icy moons place significant demands on the various spacecraft systems. There are many challenges re lated to the high output power conversion systems being considered, and one example is the need to ensure system compatibility at all levels. The utilization of appropriate materials for component structures is important to ensuring long mission life. Re fractory metal alloys have attractive high -temperature properties in inert environments, but these alloys are sometimes susceptible to contamination. Potential material compatibility issues exist between refractory metal candidates and more conventional a lloys. Nb -1Zr has long been considered one of the most well characterized refractory alloys that is well suited for elevated -temperature use and liquid -metal compatibility. However, previous studies have suggested that niobium alloys can not co -exist in a closed system with traditional stainless steels or superalloys due to transport of contaminants. The relevance of this information to a proposed power conversion system is discussed. Also, experiments and fundamental calculations are being performed to determine contamination transport from candidate superalloys to Nb -1Zr in a closed system with an inert carrier gas. Potential protective schemes are explored to ensure system level compatibility between the refractory alloy Nb -1Zr and a nickel -based sup eralloy.

Metallic Concepts for Repair of Reinforced Carbon-Carbon Space Shuttle Leading Edges

[Abstract] The development of thermal protection systems (TPS) for re-entry vehicles has been a topic of interest since the inception of manned space flight. Re-entry materials systems are critical for a spacecraft to withstand the intense temperature and environmental effects that are associated with the re-entry profile. Recently, intense efforts have been ongoing to develop repair technologies for TPS systems of the Space Shuttle Orbiter to reduce the risk of another catastrophic loss like that of Columbia. In particular, attention has focused on the ability to apply an on-orbit repair of both the wing leading edges and the tiles on the Orbiter’s underbelly. Both large and small areas of damage to leading edges allow hot gases to enter and destroy the reinforced carbon-carbon (RCC), thereby leading to catastrophic failure. Repair of large areas of damage, greater than 25 centimeters in diameter, are particularly of interest since that is the widely accepted reason for the Columbia accident. This paper presents results on a metallic overwrap concept that can conform to the contour of the wing leading edges and prevent internal structural damage from the hot gases during re-entry, which can reach in excess of 1649 o C in a highly oxidizing plasma environment. Sheet samples (380µm thick) of several silicide-coated refractory alloys were evaluated and a series of coated Re-based concepts were down-selected for more rigorous testing in an arcjet testing facility. The Re sheet was coated with an R512E silicide coating to mitigate oxidation. Two additional concepts using Ir were examined. The first examined Ir as a surface layer without the silicide coating while the second added a layer of Ir below an outer layer of Re which also received the R512E silicide coating. The R512E coating, consisting of Fe, Cr, and Re silicides, is known to be brittle and was found to contain numerous fine cracks both perpendicular and parallel to the coating surface. Consequently, a “Type A” coating consisting of a mix of sodium silicate and SiC was applied to the surface to seal fine cracks 2 in the R512E coating as well as for emissivity considerations. The concepts with the R512E coatings (Re+R512E+Type A and Re+Ir+Re+R512E+Type A) survived the arcjet exposure that simulated atmospheric re-entry conditions forming a porous silca scale. However, the Ir coating alone did not sufficiently protect the Re. Part of the failure of the Ir coating, deposited by CVD, may have been due to poor bonding with the Re substrate observed in test samples. Although handling and bending of the coated Re sheet remains a concern, the coated Re materials look promising for repairing Orbiter wing leading edges exposed to temperatures exceeding 1649 o C.