Enhancing low-cycle fatigue performance of high-efficient laser additively manufactured TC11 titanium alloy: Mechanisms of cyclic softening/hardening and microstructural refinement

The enhancement of low-cycle fatigue (LCF) performance of large structural components manufactured from TC11 titanium alloy fabricated via laser powder directed energy deposition (LP-DED) in the aerospace field is a challenging issue in achieving high reliability. This study systematically investigates the LCF behavior of TC11 titanium alloy fabricated via LP-DED and subjected to three double annealing regimes. Fully reversed strain-controlled fatigue tests were conducted under strain amplitudes ranging from 0.4 % to 1.2 %, accompanied by comprehensive microstructural characterization using EBSD, TEM, and fractography. Results reveal that cyclic softening/hardening is governed by the competition between back stress from geometrically necessary dislocations (GNDs) accumulation and friction stress from dislocation annihilation. The 950 °C + 550°C-annealing treated specimen demonstrated enhanced LCF life due to finely refined α phase regions with an average thickness of 4.4 μm, which facilitated crack deflection through localized stress redistribution and strain incompatibility. Microstructural analysis demonstrated that basketweave structures with discontinuous α_GB phases enhanced resistance to crack propagation by accumulating dislocations at phase boundaries, delaying microvoid coalescence. The critical role of annealing temperature in tailoring microstructural heterogeneity and dislocation dynamics is underscored by these findings, which provide valuable insights for enhancing the performance of titanium alloys fabricated via LP-DED process in aerospace applications.