CRSS evolution with changing grain size for a dual-phase Ti-6Al-2Sn-4Zr-2Mo-Si alloy having a martensite structure

Duplex martensitic microstructures have recently attracted significant attention due to their unique ability to combine high strength, excellent ductility, and superior work-hardening properties, making them ideal for structural applications. This study explores the micromechanisms underlying the tensile properties of a Ti-6Al-2Sn-4Zr-2Mo-Si alloy with hot-rolled T-split textures and various microstructures, tested along two tensile directions: TensileD//RD (referred to as 0°) and TensileD⊥RD (referred to as 90°). A comparative analysis highlights the superior work-hardening capacity and isotropic behavior of duplex (α+α′) and (α+α") microstructures containing martensite, compared to equiaxed (α+β) microstructures. The enhanced work-hardening observed in the duplex microstructures is attributed to mechanisms such as variant reorientation in the martensitic phases, the pronounced mechanical contrast between the harder α phase and the softer α'/α" phases, and the interactions between α slipping and α'/α" twinning. Macroscopic Hall-Petch relations further clarify how microstructures influence strength and ductility. Specifically, duplex (α+α′) microstructures exhibit improved ductility due to lower Hall-Petch constants and diminished grain boundary effects. Interestingly, reverse Hall-Petch behavior is observed in the duplex (α+α") microstructure at 90°, which is associated with the presence of α" martensite. Slip trace analysis is conducted to determine the experimental Critical Resolved Shear Stress (CRSS) ratios and qualitatively assess the impact of grain size and tensile direction on the activation of slip systems. Multiscale simulations are then utilized to calculate CRSS values and investigate the roles of deformation modes and crystallographic texture in shaping the macroscopic behavior of duplex (α+α") microstructures. At 0°, the T-split texture and the facilitation of prismatic<a> slip between adjacent prismatic grains result in a low Hall-Petch constant and minimal grain boundary effects, acting as soft grains. In contrast, basal<a> and pyramidal<c+a> systems exhibit much higher Hall-Petch constants, behaving as hard grains and significantly contributing to work hardening. At 90°, basal<a> slips uniquely display reverse Hall-Petch behavior, which is linked to the macroscopic reverse Hall-Petch phenomenon. This behavior is thought to stem from the presence of α" martensite and the hot-rolled texture, combined with the tensile direction of 90°, which triggers a shift in the dominant mechanism around basal grains from intragrain dislocation movement to grain boundary sliding.