Flight Validation of Non-Conforming Titanium Zirconium Molybdenum (TZM) for Focal Plane Assemblies

Titanium-zirconium-molybdenum (TZM) is often used in telescope and spectrometer detector assemblies as a structure-detector interface due to its thermal expansion compatibility with HgCdTe-based focal plane arrays. This alloy has been used in several flight missions at NASA Jet Propulsion Laboratory (JPL) including James Webb Space Telescope, Euclid, and more recently, Spectro-Photometer for the History of the Universe, Epoch of Reionization (SPHEREx) and Contribution to ARIEL Spectroscopy of Exoplanets (CASE). On these two recent projects, TZM was purchased to ASTM B386-19 Alloy 364 SR (stress-relieved) and JPL Materials and Processes discovered several non-conformances in chemical composition, mechanical performance, and microstructure, as part of the initial material characterization. LECO Combustion Testing, Inductively Coupled Plasma (ICP) analysis, uniaxial tensile testing per ASTM E8-04, metallographic evaluation for microstructural discrepancies, and Vickers’s microhardness testing were employed to assess usability of the non-conforming material.During fabrication of SPHEREx’s TZM Linear Variable Filter (LVF) baffle, a large crack was identified in the wall of the mount. A detailed investigation uncovered significant differences between SPHEREx and CASE TZM. The stock plates from both projects were found to have significantly different microstructures. SPHEREx grains had a large Length-to-Diameter (L/D) ratio, with uniform grain sizes across the plate thickness. CASE TZM was found to have smaller L/D ratios, with approximately 55% larger grains at the edge of the plate than at the center of the plate. CASE TZM microstructure unveiled a partially recrystallized material, where stress-relieved state was specified in procurement documents. SPHEREx material was found to be within ASTM B386-19 requirements for chemical composition of oxygen, nitrogen, and carbon, all of which are alloying elements of that can cause the formation of embrittling inclusions. In contrast, CASE TZM exceeded maximum limits for oxygen and nitrogen. Comparing mechanical properties, CASE TZM was discovered to perform at only 75% of the mechanical strength minimum requirements, whereas SPHEREx TZM was within specification requirements.The SPHEREx TZM L/D ratio was found to contribute to the cracking of the LVF mount, due to the high anisotropy and concentration of grain boundaries in the short transverse direction resulting brittle response. Subsequent fabrication processes were adjusted, including material handling and fixating, as a result of these conclusions.Fabricated CASE parts were cross-sectioned to identify their location within the stock plate in order to determine the effects of the grain structure anisotropy. Microhardness measurements were taken at different locations in the plate thickness and used to approximate ultimate strength values. These values were used to assess design conservatism in the structural performance and to validate the material for flight.As a result of the characterization and testing methods, both SPHEREx and CASE were able to integrate TZM parts for flight despite non-conformances in received stock material. Additionally, the overall process was carefully documented in order to develop guidelines for future qualification. This work demonstrates that careful attention should be devoted to material evaluation in future applications of TZM, including heat treatment verification, microstructural characteristics, and related potential anisotropies in mechanical properties.