Microstructure-controlled low-cycle fatigue of 2026 Al alloy in T4 and T6 tempers: roles of SⅠ precipitates and grain-boundary GBPs / PFZs

Abstract

The 2026 aluminum alloy is a damage-tolerant structural material for aerospace applications, where low-cycle fatigue (LCF) resistance is critical to structural safety. In this work, strain-controlled LCF tests with strain amplitudes of 0.6%–0.9% were performed on a 2026 Al alloy in two typical tempers (T4 and T6), and the fatigue behavior was correlated with microstructural evolution. The fatigue life was evaluated using the Coffin–Manson relationship, while an energy-based hysteresis model was employed to extract the intrinsic fatigue damage capacity W₀ and the damage-transition exponent β. The T4 temper exhibits more homogeneous intragranular deformation, assisted by dispersed quenched-in dislocation loops and thermally stable Al₃Zr dispersoids, together with the absence of grain-boundary precipitates, leading to a microvoid-dominated damage mode. Consequently, the T4 temper shows high fatigue ductility (εf' = 17.3, c = −1.44) and a higher damage capacity (W₀ = 2511 × 10⁶ J/m³). In contrast, the T6 temper contains a high density of shearable S_Ⅰ precipitates, which sustain a higher fatigue strength (σf' = 1127 MPa) but promote localized cyclic deformation and microcrack-dominated damage with subsurface crack initiation. Within the energy-based framework, the larger β in the T6 temper indicates a more nonlinear conversion from cyclic hysteresis work to effective damage under localized deformation, consistent with its crack-initiation mode. These results clarify the distinct roles of S_Ⅰ precipitates and intragranular dislocation-loop / Al₃Zr features in controlling LCF response, providing guidance for tailoring temper states of Al–Cu–Mg alloys for different fatigue-service scenarios.