Influence of temperature and strain-rate on liquid metal embrittlement of an aluminum alloy containing bismuth inclusions

Abstract

Elevated temperature, slow strain-rate tensile tests were carried out on a large-grained A1-0.23 wt% Ti alloy containing 1.5 wt% Bi in the form of inclusions present at grain boundaries. It was found that a marked reduction in ductility occurred over a very narrow temperature range (~ 10°C) centred about the Bi melting point for notched specimens tested at a strain-rate of 10-3 s-1. Tension tests carried out on smooth samples at three strain-rates 10-2, 10-3 and 10-4 s-1) indicate that an increase occurs in the severity of embrittlement at higher rates of strain. This observed degradation in mechanical properties is clearly a manifestation of classical liquid metal embrittlement (LME). Fractographic evidence indicates a transition occurs from transgranular fracture by void coalescence to intergranular fracture by grain boundary wedge crack nucleation and growth once the Bi inclusions become molten. At higher temperatures there is a brittle-to-ductile transition after which the strains associated with grain boundary sliding are readily accommodated by grain boundary migration. It was found that a wedge crack nucleation and growth model accounts for the abrupt transition to intergranular fracture due to a reduction in fracture surface energy associated with the melting of Bi.