Investigation of material response to atmospheric re-entry exposure of sub-structural Ti-6Al-4V components recovered from Space Shuttle Columbia

Abstract

Aluminum and titanium char-layer deposits discovered on overhead windows recovered from the Space Shuttle Columbia accident revealed unexpected material response. It was observed that titanium melted and deposited before aluminum: behavior inconsistent with their relative melting temperatures. In close proximity to the windows, payload bay door (PLBD) latch rollers, containing Ti-6Al-4V, exhibited significant thermal damage that could not be explained by re-entry heating alone. The roller was considered a likely source of the deposition as damage was localized to the titanium region and the component was evaluated to identify features of ignition and/or combustion. A second examination was conducted on arc-jet samples obtained from a previous work that studied Ti-6Al-4V combustibility in a simulated re-entry environment. The samples were compared to the analysis of the PLBD roller and both exhibited features suggestive of titanium combustion. The PLBD roller possessed microstructural similarities to the arc-jet specimens that reacted more aggressively to the simulated environment.