Yield strength and yield to tensile ratio in a multiphase heterogeneous Ti-Al-Fe-Si alloy with via rolling and one-step solution treatment

Aerospace titanium alloy fasteners, typically represented by Ti-6Al-4V and Ti-3Al-5Mo-4.5V, rely heavily on expensive alloying elements such as V and Mo, which substantial increase production costs. To reduce cost, this study explores a Ti-4.5Al-2.5Fe-0.25Si (wt%, TAFS) alloy by substituting V and Mo with Fe, an abundant and highly effective β-stabilizing element with a high molybdenum equivalence and strong solid-solution strengthening capability. A post-rolling single-step solution treatment was employed to tailor the yield-to-tensile strength ratio (YSR) for short-process manufacturing. The alloy exhibits a broad YSR tunability from 0.63 to 0.85 over the investigated solution temperature range. Specifically, the minimum YSR of 0.63 is achieved at 900 °C, while the maximum YSR of 0.85 is obtained at 940 °C. A critical transition temperature of 910 °C is identified: below 910 °C, the alloy exhibits “double yielding”, whereas this behavior is effectively suppressed above it. With increasing solution temperature, the volume fraction of secondary α/α′ martensite increases gradually, leading to a progressive microstructural transition. At 900 and 910 °C, the low fraction of α/α′ preserves the potential for transformation-induced plasticity (TRIP) and martensite reorientation, contributing to the low YSR. In contrast, at 920 °C, the higher α/α’ fraction enhances strength through martensitic strengthening and multiphase interfacial interactions. The heterogeneous multiphase microstructure is governed by back-stress hardening arising from the synergistic effects of dislocations, dispersed precipitates, and phase interfaces. These results demonstrate that the alloy can be processed in a low-YSR state for cold heading and subsequently adjusted to a high-YSR state after processing, providing a cost-effective route for manufacturing aerospace titanium fasteners.