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Isothermal oxidation experiments in air were performed on Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) with a bimodal microstructure in the temperature range 811 K to 922 K (538°C to 649°C) for up to 500 hours, and a-case depths were quantified using metallography. Alpha-case depth followed a parabolic variation with time. Alphacase depths in excess of 10 lm formed above 811 K (538°C) and 100-hour exposures. An activation energy of 244 kJ/mol was estimated for diffusion of oxygen in the a phase of Ti-6242.The high chemical affinity of titanium to oxygen (indicated by Ti-O bond energy of 2.12 eV, comparable to the Ti-Ti bond energy of 2.56 eV [1] ) and the high interstitial solid solubility of oxygen in a-titanium (about 14.5 wt pct or 34 at. pct in pure titanium [2] ) cause significant oxygen ingression during air exposure at high temperatures, resulting in the simultaneous formation of an oxide (TiO 2 ) scale on the surface and an oxygen-rich a layer underneath the scale. Formation of an oxygen-rich a layer is a result of the oxygen gradient, oxygen migration through the n-type anion-defective TiO 2 scale, and the relative ease of interstitial diffusion. This layer is commonly referred to as a case, since it is a continuous, hard, and brittle zone of oxygen-stabilized a phase. Alpha-case forms during casting, [3] processing, [4] and elevated temperature exposure in service. Alphacase formed during casting or processing is completely removed via machining or chemical milling. [5] Alphacase formed during service often limits the maximum service temperature of titanium alloys, since a significant amount of less ductile a case results in the formation of surface cracks under tensile loading. Incorporation of oxygen leads to anisotropic lattice distortions, thus hindering dislocation mobility and changing the deformation behavior from a wavy to a planar slip mode. [6] The low local ductility and the large slip offsets at the surface can cause low overall ductility or early crack nucleation under cyclic loading conditions. The service conditions that affect the oxidation kinetics are environment, temperature, stress, and exposure time. For long-term elevated-temperature applications such as blades, disks, and impellers in gas turbine aero-engines, a-case depth is critically important in addition to creep resistance and strength. [7] For shortterm elevated-temperature applications such as thermal protection systems and hot structures that include thinwall components, rapid degradation across the entire cross section could occur due to a-case formation. [8] The high-temperature application of conventional titanium alloys is therefore limited to a temperature regime below which diffusion rates through the oxide scale are slow enough to prevent excess oxygen content being dissolved in the bulk material, resulting in no significant a-case depth.While the a-case formation and its deleterious effects on component life are well known, [9] quantitative studies on a case are scant. Shamblen and Redden [10] determined the air contamination rate...

Grain growth is generally driven to minimize the overall grain boundary energy. However, for low-angle grain boundaries the requirement that lattice planes be continuous across the boundary gives rise to a coupling between the normal motion of the grain boundary and the tangential motion of the lattice. We show through phase-field crystal simulations this coupling in polycrystalline systems can give rise to a rigid body translation of the lattice as a grain shrinks. The process is mediated by significant climb of the dislocations in the boundary and dislocation reactions at the trijunctions. Thus the grain growth process is coupled to vacancy diffusion processes as well as the dynamics of grain trijunctions. Moreover, grain shrinkage can cease because of dislocation behavior near the trijunction, illustrating that this coupling can have an influence on the grain growth process in polycrystals.

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