The Effects of Temperature and Microstructure on Slip Localization in Microtextured Ti-6Al-2Sn-4Zr-2Mo under Dwell Fatigue

Abstract

Near-alpha titanium alloys, such as Ti-6242, experience a reduction in fatigue lifetime when the peak stress is held for each loading cycle. This type of sustained peak loading is known as dwell fatigue, and it mimics the long durations of high mean stress that gas turbine engine disks experience during aircraft takeoff. The significant reduction in lifetime, known as the dwell debit, is attributed to the phenomenon of load shedding. Load shedding occurs when an alpha grain well oriented for slip, termed a soft grain, lies adjacent to an alpha grain poorly oriented for slip, termed a hard grain. At the same macroscopic load, the soft grain stress relaxes and loses its ability to carry load, thereby requiring the hard grain to carry a higher load, creating stress localization. The region of high local stress acts as a favorable location for early crack nucleation relative to standard low cycle fatigue models. This phenomenon is known to be affected by local microstructure and temperature, but the underlying mechanisms are not well understood. This work utilized Digital Image Correlation with Scanning Electron Microscopy and Electron Backscatter Diffraction at multiple length scales to experimentally investigate and statistically quantify the effects of temperature and microstructure on strain localization and damage accumulation of Ti-6242 during dwell fatigue loading. Plastic strain accumulation was observed in heavily microtextured Ti-6242 under dwell fatigue loading at room temperature, 120°C, and 200°C. After 2 and 200 dwell cycles, plastic deformation maps of the 4 mm x 2.5 mm and 500 µm x 500 µm with over 150 million data points were collected to investigate the full gauge response and the response of individual grains. Slip traces traversing through 1 to as many as 140 grains were segmented from each deformation map. The length of each slip trace, orientation, and Schmid factors of the grains each trace traverses through, and the active slip family for each trace were studied to identify the critical grain characteristics that allow for the percolation of long-range plastic slip. The room temperature and 120°C cases had significantly more plastic slip activity than the 200°C case. At every temperature, plasticity occurred primarily as long-range basal slip traces through grains with a high basal Schmid factor that were located in subfeatures of microtextured regions, as defined by the current prevailing methods. While there was almost no prismatic slip activity at any temperature or length scale, a significant amount of short-range pyramidal slip occurred in regions near the long-range basal slip traces. These slip family trends were clear after 2 dwell cycles and remained constant with additional cycling to 200 dwell cycles. These findings indicate the microstructure of the material, specifically the distribution of basally soft grains, dictates the location of plastic strain accumulation from very early dwell cycling regardless of temperature. Microstructural regions that behave as a unit under plastic deformation are bound by regions with a low basal strength-to-stiffness, a metric which relates the propensity basal Schmid factor and the effective elastic modulus of a microstructural region. These results are relevant to the phenomenon of load shedding and the dwell fatigue debit, and they can be used to inform and improve current and future temperature-dependent dwell fatigue models and MTR segmentation strategies.