In-Situ TEM Observation of Aluminum Embrittlement by Liquid Gallium

Abstract

Liquid metal embrittlement (LME) occurs when some solids contact certain liquid metals. Normally ductile materials will exhibit brittle fracture behavior because of interactions between the solid metal and adsorbed liquid atoms. The fracture characteristics of different solid-liquid metal couples vary greatly. A requirement is that the liquid wets the surface of a solid. Ductile materials fail over a period of time, by slow and often discontinuous crack growth. For embrittlement to occur, the liquid metal atoms must be adsorbed at the crack tip. If the crack extends beyond the adsorbed atoms, it may become blunted and arrest. Thus the time necessary for failure depends on the transport of liquid metal atoms to the crack tip. The aluminum-gallium system exhibits this type of time-dependent failure. Liquid gallium penetrates the grain boundaries so that failure occurs primarily by intergranular fracture at applied stress levels as low as zero, although transgranular cleavage is sometimes observed for applied stress intensities of 4--5 MPa-m{sup 1/2}. This penetration proceeds much faster than can be accounted for by grain boundary diffusion, and occurs in the absence of any applied stress. The driving force is generally considered to be the reduction in interfacial energy which results when amore » high energy grain boundary is replaced with two lower energy aluminum-gallium interfaces. This study reports the results of grain boundary penetration observed in-situ in the TEM.« less