Liquid-metal embrittlement of 7075 aluminum and 4340 steel compact tension specimens by gallium

Liquid-metal embrittlement of aluminum alloy 7075-T6 and AISI 4340 steel by gallium has been observed previously. Unfortunately, quantitative data regarding the level of embrittlement are limited due to the difficulty of testing fracture mechanics specimens. This study uses L-T compact tension specimens to determine the threshold stress intensity for crack initiation during exposure to gallium. Sample surfaces were wet with gallium prior to exposure at 35°C by removing the crack tip and side groove oxide film with a scalpel in the presence of gallium. Both 4340 steel and 7075 Al were embrittled by Ga as measured by threshold stress intensity. Kthi, the stress intensity at crack initiation as detected by direct current potential drop (DCPD), for 4340 and 7075 was 33% and 5% of KIci, (the fracture toughness as detected by DCPD), respectively. In contrast to air-exposed samples, extremely rapid crack propagation to failure of Ga-exposed samples was observed. Fractography revealed intergranular fracture of Al 7075 and 4340 steel following Ga exposure.