LOW-CYCLE FATIGUE BEHAVIOR OF 2 AL-LI ALLOYS

The low cycle fatigue (LCF) behavior of two aluminum-lithium alloys wa s investigated. Efforts were aimed at understanding the effects of mic rostructure on the cyclic stress-strain behavior and methods by which different microstructures accommodate plastic strain. These goals were achieved by analyzing the cyclic response and the deformed microstruc ture of each alloy. Direct-chill cast and rolled X2095 exhibited immed iate cyclic softening followed by a plateau region. Strain was distrib uted in a homogeneous manner throughout the microstructure. Mechanical ly alloyed and forged AA5091 (formerly referred to as 905XL) also defo rmed in a homogeneous manner, but the cyclic response was characterize d by initial softening followed by gradual hardening. Over-strain loop s were applied during some of the LCF tests, the purpose of which was to simulate the strain history of the material ahead of a growing fati gue crack during a tensile overload. The over-strain response suggeste d that the intrinsic material response does not contribute to crack re tardation in X2095 or AA5091.