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

Abstract

\(\alpha + \beta\) 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.

Graphical abstract

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Correspondence to Sushil K. Mishra.

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Seth, P., Jha, J.S., Alankar, A. et al. Alpha-case Formation in Ti–6Al–4V in a Different Oxidizing Environment and Its Effect on Tensile and Fatigue Crack Growth Behavior. Oxid Met (2021). https://doi.org/10.1007/s11085-021-10079-y

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Keywords

  • \(\alpha\)-case
  • Heat treatment
  • Tensile test
  • FCGR test
  • Lamellar microstructure
  • Fractography