The effect of oxygen on the structure and mechanical behavior of Aged Ti-8 Wt pct Al

Abstract

It is shown that for a Ti-8 wt pct Al alloy aged at a temperature high in the two-phase region (695°C) to precipitate the ordered α₂ phase, an increase in oxygen content from 600 ppm to 1200 ppm decreases the fracture strain from 20 to 1 pct elongation at room temperature and slightly increases the yield strength. The fracture mode is changed from dimpled rupture to predominantly cleavage. Further increase in oxygen content to 3000 ppm does not produce significant additional changes in ductility or yield strength. It is demonstrated that oxygen additions alter the position of the α/α + α₂ coherent solvus, resulting in formation of coherent α₂ in specimens containing ⪞ 1000 ppm oxygen aged at 968 K (695°C). For a given aging time the volume fraction of α₂ increases with increasing oxygen up to 1300 wt ppm and then levels off. The changes in mechanical behavior are attributed to the presence of α₂. The experimental evidence suggests that oxygen partitions preferentially into α₂.