INTRINSIC FATIGUE-CRACK GROWTH-RATES FOR AL-LI-CU-MG ALLOYS IN VACUUM

The influences of microstructure and deformation mode on inert environ ment intrinsic fatigue crack propagation were investigated for Al-Li-C u-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount o f coherent shearable delta' (Al3Li) precipitates and extent of localiz ed planar slip deformation were reduced by composition (increased Cu/L i in X2095) and heat treatment (double aging of AA8090). Intrinsic gro wth rates, obtained at high constant K(max) to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy depend ent; da/dN varied up to tenfold based on applied DELTAK or DELTAK/E. W hen compared based on a crack tip cyclic strain or opening displacemen t parameter (DELTAK/(sigma(ys)E)1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growt h resistance. Tortuous fatigue crack profiles and large fracture surfa ce facets were observed for each Al-Li alloy independent of the precip itates present, particularly delta', and the localized slip deformatio n structure. Reduced fatigue crack propagation rates for X2095 in vacu um are not explained by either residual crack closure or slip reversib ility arguments; the origin of apparent slip band facets in a homogene ous slip alloy is unclear. Better understanding of crack tip damage ac cumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.