Mechanistic insight into cooperative slip system activation under cyclic loading in a near-alpha titanium alloy

Fatigue crack initiation in polycrystalline alloys involves cyclic irreversibility at the scale of individual slip bands, highlighting the importance of understanding the exact mechanism of slip system activation. This study statistically assessed both the slip direction and slip plane along with the cyclic plastic strain accumulation of individual slip bands, using electron backscatter diffraction combined with high-resolution digital image correlation in a TIMETAL®834 alloy subjected to cyclic four-point bending at up to 90% of the proof stress. Under the testing conditions, basal <a> slip provided higher effective shear strain compared to prismatic <a> slip. Two concurrently activated <a> type Burgers vectors, rather than a single one, regularly contributed to basal slip traces with significant variations in their relative contributions across different slip traces and notably, even along the same slip trace within a single grain. This behaviour is driven by the need to maximise geometric alignment with neighbouring slip bands, revealing a mechanism whereby the local slip accommodation directly stimulates the cooperative activation of slip systems in adjacent grains, particularly within clusters of similarly oriented grains with well-aligned basal planes, regardless of grain boundary misorientation. For the first time, the geometric alignment was precisely quantified by evaluating the geometric compatibility parameter for basal-basal slip pairs using the measured slip direction, rather than inferring it from Schmid factor considerations. Importantly, although the localised strain accumulation nearly saturates during the earliest fatigue stages, the exact slip directions continue to evolve with increasing cyclic number to enhance the slip alignment between neighbouring grains.