Room temperature fracture processes of a near-α titanium alloy following elevated temperature exposure
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Near-α titanium alloys are used at higher temperatures than any other class of titanium alloys. As a consequence of thermal exposure, these components may develop locally elevated oxygen concentrations at the exposed surface which can negatively impact ductility and resistance to fatigue crack initiation. In this work, monotonic and fatigue fracture mechanisms of Ti–6Al–2Sn–4Zr–2Mo–0.1Si samples exposed to laboratory air at 650 °C for 420 h were identified by means of a combination of quantitative tilt fractography, metallographic sectioning, and electron backscatter diffraction. These mechanisms were compared and contrasted with those operative during similar tests performed on material is the as-received condition with uniform oxygen content. While faceted fracture was not observed during quasi-static loading of virgin material, locally elevated concentrations of oxygen near the surfaces of exposed samples were shown to change the fracture mode from ductile, microvoid coalescence to brittle facet formation and grain boundary separation at stresses below the macroscopic yield point. Similar features and an increased propensity for facet formation were observed during cyclic loading of exposed samples. The effects of this time-dependent degradation on monotonic and cyclic properties were discussed in the context of the effect of oxygen on crack initiation and propagation mechanisms.
This work was performed as part of the in-house research activities of the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXLM, Wright Patterson Air Force Base, OH. The financial support of the Air Force Office of Scientific Research through Task No. 09RX24COR, Dr. David Stargel, Program Manager, is gratefully acknowledged. Two of the authors were partially supported under onsite Air Force contracts FA8650-07-D-5800 (ALP), Dr. Ali Sayir, Program Manager, and FA8650-09- D-5223 (WJP) during the time this work was completed.
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