Grain-Scale Stress States in Microtextured Ti64: Implications for Dwell Fatigue

The room-temperature fatigue life of titanium alloys with a significant alpha phase fraction is substantially reduced when the peak stress is sustained for a few minutes on each stress cycle. Furthermore, these alloys often form microtextured regions during forging which are known to increase the dwell debit; however, the mechanisms for this reduction are still not well understood. In this research, an experimentally collected 3D microstructure of equiaxed Ti-6Al-4V containing microtextured regions is used as input for linear elastic finite element analysis of grain-level stress states relevant to dwell fatigue. The directional strength to stiffness ratio and a metric related to local plane strain are evaluated around the microtextured regions. Strength to stiffness ratios indicate that misoriented grains within hard microtextured regions will begin to yield at macroscopic stress levels below the yield strength, and, under certain loading conditions, they will yield at lower macroscopic stresses than any other location in the microstructure. Under uniaxial tension, boundaries between hard and soft microtextured regions experience some degree of plane strain loading, which may be linked to enhanced load shedding in these areas during dwell fatigue.