Abstract
Digital image correlation (DIC) and crystal plasticity simulation were utilised to study cold dwell behaviour in a coarse grain Ti-6Al alloy at 3 different temperatures up to 230 °C. Strains extracted from large volume grains were measured during creep by DIC and were used to calibrate the crystal plasticity model. The values of critical resolved shear stresses (CRSS) of the two main slip systems (basal and prismatic) were determined as a function of temperature. Stress along paths across the boundaries of two grain pairs, (1) a `rogue' grain pair and (2) a `non-rogue' grain pair, were determined at different temperatures. Load shedding was observed in the `rogue' grain pair, where a stress increment during the creep period was found in the `hard' grain. At elevated temperatures, 120 °C was found to be the worst case scenario as the stress difference at the grain boundaries of these two grain pairs were found to be the largest among the three temperatures. This can be attributed to the fact that the strain rate sensitivity of both prismatic and basal slip systems is at its greatest in this worst case scenario temperature.
Highlights
Titanium alloys with predominantly α-phase (HCP crystalline structure) are widely used in aero engines and gas turbines at moderate temperatures, due to their excellent specific strength and corrosion resistance [1]
Two reasons for these difference could be: (1) the Ti6Al alloy tested in this work had lower Al content (≈ 5.8wt%) compared to that in the literature, (2) strain rates in our work extend to lower values
critical resolved shear stresses (CRSS) of the two major slip systems of α-Ti were quantified as a function of temperature by calibrating the crystal plasticity finite element (CPFE) model from experimentally measured strain evolution in individual grains during creep
Summary
Titanium alloys with predominantly α-phase (HCP crystalline structure) are widely used in aero engines and gas turbines at moderate temperatures, due to their excellent specific strength and corrosion resistance [1]. In such applications, titanium alloy components often subjected to extremely complicated stress state. Titanium fan blades and compressor disks experience long hours of high stress hold (dwell), which often results in a drastic lifetime reduction for these titanium components [2, 3] This phenomenon that seems to happen at relatively lower temperatures (typically below 200 ◦C) has been termed ‘cold dwell fatigue’. Local time dependent plasticity could occur in grains
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