Alpha-case Formation in Ti–6Al–4V in a Different Oxidizing Environment and Its Effect on Tensile and Fatigue Crack Growth Behavior

$$\alpha + \beta$$ titanium alloy is known for its high strength to low weight ratio, excellent mechanical properties and superior corrosion resistance, which make it a favorable material for aerospace and biomedical applications. However, on exposure to oxygen and nitrogen at elevated temperatures, a hard and brittle $$\alpha$$ -layer develops on the titanium alloy surface, which is referred to as ‘ $$\alpha$$ -casing.’ In the present study, Ti–6Al–4 V was heat treated above the $$\beta$$ -transition temperature and cooled at different rates in water, air and furnace to obtain lamellar morphologies of different $$\alpha$$ -lath thicknesses. Tensile and compact tension (CT) specimens with three different lamellar morphologies were obtained by heat treatment in such a way that one set (A) had ‘ $$\alpha$$ -case’ and other set (B) was devoid of such casing. Water quenched (WQ) samples formed a very thick $$\alpha$$ -case with numerous micro-cracks compared to the air and furnace cooled samples. On tensile loading, the WQ sample with $$\alpha$$ -case failed in fully brittle mode and the ductility of the air-cooled (AC) and furnace cooled (FC) samples with $$\alpha$$ -case was reduced significantly. It was also observed that the fatigue crack growth rate (FCGR) increased considerably as the apparent effective crack front area was reduced due to $$\alpha$$ -case formation.