Microstructure, fatigue crack growth rate and low-cycle fatigue behavior of an Al-Li alloy sheet with different aging

The microstructures, fatigue crack growth (FCG) rates and low-cycle fatigue (LCF) behaviors of 1445 Al -Li alloy sheet with T6 aging at 170 C-degrees, single-step T8 (S -T8) aging at 150 degrees C and double-step T8 (D -T8) aging at 170 C-degrees following aging at 150 C-degrees for 4 h were investigated. The main precipitates in T6 and S -T8 samples are delta ' phases, but strain strengthening causes 40 MPa enhancement in the S -T8 sample yield strength (YS). Due to coherent shearable delta ' precipitates, the FCG rate curves of S -T8 samples are highly coincident. With the 2nd-step aging time extension, delta ' precipitates in D -T8 sample gradually coarsen, semi-coherent S ' phases form and their amount increases. The interaction between delta ' precipitates with dislocations transites from shearing to by-passing, leading to the YS enhancement and the FCG resistance decrease. Meanwhile, cyclic-hardening decreases and transfers to cyclic-softening during LCF at starin amplitude of 0.6% similar to 0.9%. At 0.9% strain amplitude, due to greater cyclicsoftening and cleavage fracture of S ' precipitates, the D -T8 sample with 2nd-step aging time of 120 h has a shorter LCF life. The D -T8 sample with 2nd-step aging time of 24 h possesses a comprehensive property of higher strength, lower FCG rate and better LCF life.