THE EFFECTS OF PROLONGED THERMAL EXPOSURE ON THE FRACTURE AND FATIGUEBEHAVIOR OF ALUMINUM-LITHIUM ALLOY-8090

Aluminum-lithium alloys are currently being considered for application s at moderately elevated temperatures; accordingly, a study has been m ade on the effects of prolonged (100 and 1000 hours overaging) thermal exposure at 149-degrees-C and 260-degrees-C on the mechanical propert ies of a peak-aged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits an excellent combination of strength (approximately 500 MPa) and toughness (35 MPa square-root m) with mod erate tensile elongation (4 pct). Overaging at 149-degrees-C results i n a approximately 50 pct reduction in ductility and toughness, primari ly associated with the growth of equilibrium phases along grain/subgra in boundaries, resulting in formation of solute-depleted precipitate-f ree zones and coarsening of matrix delta' and S precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchan ged. Thermal exposures at 260-degrees-C, conversely, lead to dramatic reductions in strength (by approximately 50 to 80 pct), toughness (by approximately 30 pct) and fatigue-crack propagation resistance; crack- growth rates at all DELTAK levels above approximately 5 MPa square-roo t m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution of delta', severe coarsening of S and T2 precipitates in the matrix, and formation of equilibrium Cu- and Mg-rich intermetallic phases in the matrix and along grain boundar ies. The resulting lack of planar-slip deformation and low yield stren gth of 8090 following overaging exposures at 260-degrees-C increase th e cumulative crack-tip damage per cycle and reduce the tendency for cr ack-path deflection, thereby accelerating fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has bette r strength retention and generally superior fatigue-crack growth prope rties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7150 alloys.