Enhanced fatigue crack propagation resistance of Al-Cu-Li alloys via regulating grain structure and precipitation behavior

The fatigue crack propagation (FCP) resistance of Al-Cu-Li alloys was improved via regulating grain structure and precipitation behavior, and their effect on the FCP rate was further elucidated. It was revealed that modifying the solution heating rate significantly affected the grain structure and FCP behavior. Specifically, the decreased solution heating rate resulted in an alternative distribution of recrystallized grains and sub-grain bands, with a larger recrystallized-grain size and increased sub-grain band density. This microstructural evolution altered the reversible plastic zone (Rp) scale at the crack tip, where the R_p/D ratio (D: recrystallized-grain size) in samples with 2 °C/min solution heating rate (2HR) ranged from 0.31 to 1.22, leading to transgranular crack propagation and decreased FCP rates. Furthermore, the sub-grain zones in 2HR sample exhibited low tilt and twist angle differences and featured shearable T₁ precipitates, promoting crack deviation and bifurcation. In contrast, the specimens subjected to direct solution treatment (DST) showed a by-passing mechanism during T₁ precipitates-dislocation interaction. The shearable T₁ precipitates contributed to strain energy release, while the by-passed T₁ precipitates facilitated intergranular crack propagation. Thus, the FCP resistance of 2HR sample was significantly enhanced compared to DST sample. These findings provided a novel approach to improving the FCP resistance of Al-Cu-Li alloy through controlled microstructural design.