A Study on Alpha-Case Depth in Ti-6Al-2Sn-4Zr-2Mo

Abstract

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...

Effect of microstructure on oxygen rich layer evolution and its impact on fatigue life during high-temperature application of α/β titanium

“…give an estimated ORL thickness [29].While this data was not used for subsequent analysis of the rate of oxygen diffusion in each microstructure, it is often seen in the literature [10,29]and is useful for comparison between techinques. In order to quantify the oxygen ingress depth objectively, an image processing technique shown in Figure 5(a) was applied consisting of the following steps:…”

“…temperatures is considered rich in oxygen, and this layer isreferred to hereafter as an oxygen rich layer (ORL). A number of efforts have been devoted to the first type of models that predict the evolution of the ORL by modeling oxygen diffusion in α/β titanium alloys [15][16][17][18]. In these models, the thickness of the ORL is assumed to be small compared to the thickness of the bulk, allowing the application of Fick's second law of diffusion to estimate the oxygen concentration profile and concomitant ORL thickness at various temperatures and exposure durations [19].…”

“…In these models, the thickness of the ORL is assumed to be small compared to the thickness of the bulk, allowing the application of Fick's second law of diffusion to estimate the oxygen concentration profile and concomitant ORL thickness at various temperatures and exposure durations [19]. Many of these efforts utilize experimental data and/or simulations to calibrate Fick's second law diffusion coefficient [15][16][17][18]20]. An excellent summary of oxygen diffusivity in various titanium alloys has been provided by Liu and Welsch [21].…”

“…The large variability of microstructures in α/β titanium alloys [23]makes it difficult to extract unique trends in diffusion modeling parameters directly from published literature., In particular, analysis of the Arrhenius plots given by McReynolds et al [18]gives an estimated diffusion coefficient of a Ti-6242Ssample with a bimodal microstructureof ~4 x 10 -3 μm 2 /s at 649°C, based on optical measurements of ORL thickness. This is approximately 20 times higher than the value reported by Shamblen and Redden [17] at 639°C(D (639°C) ≈2 x 10 -4 μm 2 /s), which is close to the diffusion coefficient reported for pure alpha titanium(2.105 x 10 -4 μm 2 /s)by Brockman et al [20],who used a representative volume element (RVE) approach and 2dimensional FEM simulations based on electron backscatter diffraction data to simulate oxygen diffusion in microstructure constituents of Ti-6242S with a globular microstructure.…”

“…The model calibration was conducted using electron probe microanalysismeasurements of oxygen distribution after exposure at 650°C.Although the materials used in Refs. [18] and [20] were the same alloy (Ti-6242S), the calibrated model parameters were different, possibly due to the difference between bimodal and globular microstructures and/or the procedures used to experimentally measure the ORL thickness experimentally.…”