Investigation of Ti-6Al-4V alloy response to atmospheric re-entry exposure

Ti-6Al-4V is a widely used aerospace alloy for its high strength-to-weight ratio and high operating temperature properties. Despite widespread use, titanium and its alloys have been shown to ignite in oxygen and nitrogen rich test streams. The reactivity of titanium is attributed to the high solubility for oxygen that increases with temperature, accelerating the oxidation rate and resulting in a combustion reaction. When introduced to the monatomic oxygen rich and high enthalpy re-entry environment, Ti-6Al-4V X-link components from the space shuttle Columbia exhibited accelerated oxidation and combustion behavior. Ti-6Al-4V metal plates tested in the simulated re-entry environment of an arc-jet facility have also exhibited ignition and combustion responses. A forensic material characterization study of the X-links and arc-jet samples is necessary to gain a better understanding of the material response in the dynamic and extreme environment of re-entry. Materials characterization methods, in the form of microstructural analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and microhardness testing, are used to identify the current state of the degraded Ti-6Al-4V material and glean knowledge about the alloy response during re-entry. Data from X-link components is compared to the Ti-6Al-4V samples previously tested in the simulated re-entry arc-jet facility conditions. Several similarities in microstructural features and oxide compound detection suggest that qualitative and quantitative conclusions can be drawn from comparison of actual and simulated re-entry conditions. Furthermore, subtleties between oxidation, ignition, and combustion reaction thresholds were identified. A thorough materials characterization of these materials will yield insight into the discrimination between different material responses of titanium alloys. Widespread use of titanium alloys in space vehicles warrants a thorough characterization of their potential failure modes to help ensure the safe and reliable operation of future spaceflight vehicles