Multi-scale fatigue crack growth behaviors within band mixed-grain structure in AlLi alloy

The effect of band mixed-grain structure controlled by annealing treatment before solid solution on fatigue crack growth (FCG) properties in T8 aged 2A97 AlLi alloy sheets was investigated systematically. The lamellar boundaries (LBs) and residual secondary phase particles inhibited the growth of recrystallized grains along normal direction. It ultimately resulted in a pronounced band mixed-grain structure. Annealing treatment significantly increased the fraction of sub-grains and the lower annealing temperature suppressed sub-grains coarsening. Fatigue cracks growing parallel to the sub-grain bands suffered great resistance leading to a significant reduction in FCG rate. At lower stress intensity factor range (ΔK), changes in stress field at the crack tip significantly diverted fatigue crack propagation direction, resulting in a millimeter-scale bending area in fatigue crack path. Moreover, the bending degree of fatigue crack is closely related with sub-grain size. Microscopic fatigue cracks grew mainly along the grain orientation boundaries (GOBs) caused by insufficient recrystallization. Its deflections and bifurcations were affected by the secondary phase particles and Schmid factor (SF). Large-sized brittle secondary phase particles tended to damage the fatigue properties of the matrix, while fine dispersoids reduced FCG rate. Bifurcations occurred when fatigue crack propagated in the R-cube orientated sub-grain regions with higher SF