Impact of Microstrucutre on Dwell Fatigue in Dual-Phase Titanium Alloys

Dual phase titanium alloys, such as Ti-6242, experience a significant reduction in fatigue lifetime when the peak load is held at each cycle. This type of sustained peak loading, also known as dwell fatigue, mimics the long periods of high mean stress experienced by titanium fan and compressor components during takeoff and cruise. The reduction in fatigue lifetime is known as the dwell debit, and is attributed to the phenomenon of load shedding. Both local microstructure and temperature are known to impact load shedding and thereby the macroscopic response of Ti-6242 when subject to dwell fatigue, but the underlying mechanisms are still under active investigation. This study utilized electron backscatter diffraction (EBSD) and digital image correlation (DIC) to characterize the role of local microstructure and temperature on load shedding during dwell fatigue. EBSD was used to determine local orientation and texture information, and DIC provided information about the heterogeneity of the strain distribution and plastic strain accumulation. Ex-situ tests were performed to investigate the link between the deformation of local microstructures and macroscopic damage. The resultant strain fields and orientation maps were statistically analyzed to provide quantitative insights into the impact of local microstructure on load shedding during dwell fatigue.

elevated stress. This allows for early crack nucleation and faceting, which significantly limits the lifetime of the component. [5] The redistribution of load is known to be affected by both local microstructure and temperature, but the exact mechanisms are still under active investigation . [6,7] Research shows that the most detrimental temperature for dwell fatigue in Ti-6242 alloys is approximately 65°C, and that the dwell sensitivity essentially disappears at temperatures above 200°C, though this may be alloy and microstructure dependent. [6,8] It has been suggested that the temperature depended is due to the decrease in slip resistance between the hard and the soft grains as the temperature increases. [9] With increasing temperature, the CRSS required for pyramidal slip decreases approach to the CRSS required for basal and prismatic slip. [10] The strength mismatch between the hard and soft grains diminishes, the stress is redistributed, and the hard grain boundary no longer acts as a pile up location. A different model attributes the primary cause of the temperature dependence is due to a thermally activated dislocation escape. During material loading, dislocations move through the material until they meet an obstacle that pins them in place. At elevated temperatures, the dislocations escape the obstacle via a thermally activated escape mechanism and continue moving through the material. At 60ºC, the dwell time and obstacle escape time are comparable, which allows additional dislocations to nucleate during the dwell and for a significant pile up to occur. At 200ºC, no load shedding occurs because the time required for dislocation escape is so short that the material reaches an equilibrium before the dwell. [6] For both thermally dependent models, it is further hypothesized that the thermal reduction of the dwell debit will results in more homogeneous deformation at the meso-scale, but this has yet to be thoroughly validated. [10] The effects of local microstructure on dwell fatigue is a complex parameter that still requires further research. The specific microstructural features of interest relative to load shedding are microtextured regions (MTRs). MTRs are defined as clusters of similarly oriented alpha grains and can be described by their intensity (ie size, area fraction, density, and angular spread). MTRs is known to assist in the formation of fatigue cracks on both the global and local level. Locally, soft MTRs have an increased potential for closely packed hard-soft alpha particles which allow for numerous crack initiation sites that can coalesce to create a large crack growth event. [11] On the global scale, clusters of soft grains can allow for easy slip transmission between grains of nearly identical orientation, which can create slip lengths and dislocation pile ups many times larger than an individual grain. [11,12,13,14] These mechanistic hypotheses have yet to be directly observed or rigorously tested. The present study investigates the mechanisms behind the temperature and microstructural dependence of load shedding to inform modeling and to improve dwell fatigue resistance and lifing of Ti-6242.
Electron Backscatter Diffraction (EBSD) and Digital Image Correlation (DIC) are combined to experimentally quantify the global and local damage relative to microstructure during dwell fatigue loading. DIC is a high-resolution deformation tracking methodology that tracks a random speckle pattern from image to image. Local microstructure information collected from EBSD is spatially aligned with plastic strain maps from DIC. The resulting aligned microstructural and damage maps are analyzed to determine the microstructural impact on load shedding and the dwell debit. Quantitative mean field and local strain information over several grains and textured regions provide statistically relevant data to identify critical factors impacting the dwell debit. The slip activity and the distribution of plastic strain within MTRs and grains elucidate how dislocations move and pile up during dwell loading. This information serves to validate and further inform the mechanistic understanding of load shedding.

II. Material and Experiments
Dog bones were machined via wire EDM from forged and hot-rolled Ti-6242. The materials was previously beta annealed and hot rolled to a 75% reduc on. The material was specifically processed to produce large MTRs on the order of 1-3mm. The samples were metallographically prepared via polishing down to a mirror finish with a 1:4 hydrogen peroxide: colloidal silica mixture. The resul ng surfaces were marked with pla num markers via vapor deposi on to define a region of interest of 4mm x 2.5 mm in the center of the gauge. The microstructure of the en re RIO was inspected using the EBSD, as shown in Figure 1. EBSD was performed with a step size of 2 µm over the 4mm by 2.5mm field of view to collect informa on about the alpha and beta phase of the material. The material has an equixed micrstructure with an average alpha nodule size of 10 µm.

III. Results and Discussion
By aligning the orienta on informa on collected with EBSD with the strain maps from SEM DIC, the long-range slip traces can be matched to the orienta on of the individual grains involved in the coopera ve slip transfer. Long-range slip in this study is defined as slip traces that extend beyond two grains. By extrac ng all the orienta on informa on from a segmented slip trace, the dominant orienta on of the grains involved in that slip trace can be found by determining the sta s cal mode of all the orienta ons. Each trace's dominant orienta on can be used to calculate the Schmid factor for that longrange slip trace. The dominant orienta on of all detected slip traces had a high basal Schmid factor, as shown Figure 4. Coopera ve slip can occur through mul ple grains where they individually exhibit a high basal Schmid factors and are co-located.

Figure 5: Standard Deviation of Basal Schmid Factor Per Slip Trace After 2 Dwell Cycles
Iden fica on of the ac ve slip systems of many of the long-range slip traces reveals that plas c deforma on is predominantly accommodated by basal slip with very li�le long-range prisma c slip Figure 6. Even in MTRs that exhibit a high pristma c schmid factor, li�le to no prisma c slip is observed. This trend is exhibited a�er only two cycles. Long-range pyramidal slip is also iden fied. This slip is hypothesized to exist to allow addi onal deforma on in regions that have already deformed by basal slip as well as to maintain compa bility at the interface between deformed and undeformed regions. More rigorous inspec on and further tes ng is required to determine the cause of this behavior.
With increased cycling, the total number of slip features increases and the rela ve ra os of iden fied basal, prisma c and pyramidal remain consistent. Basal slip dominates the iden fied deforma on at both early and late cycles.

IV. Conclusions
This ex-situ study of plastic strain accumulation in Ti-6242 under dwell fatigue loading inform our understanding of MTRs and their effects on dwell fatigue lifetimes. At room temperature, plastic strain accumulation is observed at MTR interfaces at the microscale. Also, contiguous slip occurs across continuous clusters of grains. These long-range slip features are dominantly basal slip and occur through grains with a high basal Schmid factor and a small spread in orientation. Further segmentation and statistical analysis of this data set and additional dwell fatigue experiments, specifically at elevated temperature, are needed to further elucidate the role of microstructure and temperate on load shedding.