EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Titanium alloys oxidation leads to the simultaneous formation of an oxide layer and an oxygen-enriched layer below the oxide scale due to high solubility of oxygen in titanium. A few tenths of atomic percent of dissolved oxygen is sufficient to weaken the mechanical properties of titanium alloys. It is thus necessary to take into account the impact of this oxygen-enriched layer to predict mechanical behaviour of oxidized thin components. The mechanical properties of samples with various oxygen-enriched layers are studied with room-temperature and high-temperature tensile tests on ultra-thin Ti6242S samples more or less pre-oxidized paired with digital image correlation techniques. Ultra-thin samples were prepared with different thicknesses ranging from 100 μm to 1 mm and then exposed at 650°C for durations between 40h to 120h. The purpose was to explore different ratios between the thickness of the oxygen-enriched layer and sample thickness (5% to 50%). The thickness of the oxygen-enriched layer was characterised with microhardness tests and electron probe micro analyser (EPMA) profiles.
Fracture surfaces have been investigated with scanning electron microscopy to identify the brittle and ductile areas and to quantify their proportion in the different pre-oxidation conditions.The measured thicknesses of brittle areas were in good agreement with the thickness of the oxygen-enriched layer characterised by EPMA and microhardness techniques.
Tensile tests revealed that the oxygen-enriched layer in Ti6242S induced a decrease in yield strength and ductility. Mechanical properties have been studied as a function of sample thickness and proportion of oxygen-enriched section. At room temperature, the decrease of these mechanical properties seems proportional to the proportion of oxygen-enriched section. A good correlation has been found between the losses of ductility, the oxygen gradient and analyses of fracture surface.