Room temperature fracture processes of a near-a titanium alloy following elevated temperature exposure

Near-a 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. Introduction Near-a titanium alloys, like Ti–6Al–2Sn–4Zr–2Mo (Ti-6242) and Ti-5.68Al-4.05Sn-3.65Zr-0.68Nb-0.52Mo0.33Si (IMI-834), are known for maintaining strength at elevated temperatures. Because of its high solid solubility in a-titanium, inward diffusion of interstitial oxygen occurs simultaneously with oxide growth during elevated temperature excursions. The oxide scale is typically made up of TiO2 although Ti3AlN has been found in IMI-834 upon exposure to temperatures exceeding 750 C [1]. Although protective at room temperature, the TiO2 scale formed by high temperature exposure grows to be thicker and may spall during repeated thermal excursions, which limits the maximum service life. In solid solution, oxygen can strengthen the a-phase under quasi-static deformation rates [2]; however, it can be detrimental to fatigue and fracture properties through its effect on slip behavior. It has been demonstrated that oxygen strongly affects slip character causing a shift from wavy to planar glide with increasing oxygen concentration [2, 3]. Physically, planar slip results in a decrease in tensile ductility and toughness during monotonic loading [4, 5]. With regard to fatigue, Mahoney and Paton [6] have shown that oxygen does not strongly affect fatigue crack growth behavior over a wide range of DK when its concentration is relatively small (0.06–0.18 wt %). In contrast, Bache et al. [7] have shown that high concentrations of O (0.51 wt %) in mill-annealed Ti–6Al–4V resulted in a 5to 10-fold increase in the long crack growth rate in air at DK \ 12 MPa m for R = 0.01 and a smaller, but measureable, increase at larger DK. At a load ratio of 0.5, material with moderate (0.165 wt %O) and high (0.4 wt %O) oxygen content both exhibited increased crack growth rates compared to low oxygen material (0.09 wt %O) over the entire range of DK investigated A. L. Pilchak (&) W. J. Porter R. John Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, OH 45433, USA e-mail: [email protected] W. J. Porter University of Dayton Research Institute, Dayton, OH 45469, USA 123 J Mater Sci (2012) 47:7235–7253 DOI 10.1007/s10853-012-6673-y