Effect of Microstructural Evolution on Ultra-High-Cycle-Fatigue Behavior of Two-Phase Titanium Alloy Suitable for Ultrasonic Scalpel Applications

Two-phase titanium alloy, pivotal in ultrasonic scalpels, exhibits working dynamics similar to fatigue behavior under axial vibration loading (R = −1) exceeding 20 kHz, with its ultra-high-cycle fatigue (UHCF) performance being crucial for clinical applications. This study investigates the UHCF properties of the Ti6Al4V alloy by evaluating microstructure variations and provides insights into the mechanism of nanograin formation and expansion in the internal crack initiation sites. Key findings indicate that a partially recrystallized microstructure (annealed at 650°C) exhibits the highest fatigue life, with enhanced resistance to crack initiation attributed to elongated α grains, moderate texture intensity, and optimal basal slip activation. Internal small-scale inclusions, which precede deformed α grains, can also serve as initiation sites for cracks in the UHCF regime. The formation of nanograins at crack initiation sites is primarily driven by the slip of basal <a> dislocations, with their subsequent growth influenced by the type of surrounding grain boundaries. This study provides a profound understanding of the relationship between dislocation motion and internal crack initiation in Ti6Al4V alloy, offering valuable insights for optimizing the microstructural design of ultrasonic scalpels to enhance clinical durability.