LOW-CYCLE FATIGUE BEHAVIOR OF 2 AL-LI ALLOYS

The low cycle fatigue (LCF) behavior of two aluminum-lithium alloys was investigated. Efforts were aimed at understanding the effects of microstructure 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 microstructure of each alloy. Direct-chill cast and rolled X2095 exhibited immediate cyclic softening followed by a plateau region. Strain was distributed in a homogeneous manner throughout the microstructure. Mechanically alloyed and forged AA5091 (formerly referred to as 905XL) also deformed in a homogeneous manner, but the cyclic response was characterized by initial softening followed by gradual hardening. Over-strain loops 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 fatigue crack during a tensile overload. The over-strain response suggested that the intrinsic material response does not contribute to crack retardation in X2095 or AA5091.