LIQUID METAL INDUCED EMBRITTLEMENT (LMIE), also known as liquid metal embrittlement, is the reduction of the fracture resistance of a solid material during exposure to a liquid metal. References in the literature to liquid metal embrittlement were first made over 100 years ago (Ref). Commonly, LMIE is described as the catastrophic brittle failure of a normally ductile metal when coated with a thin film of a liquid metal and subsequently stressed in tension. The fracture mode typically changes from a ductile to a brittle intergranular or brittle transgranular (cleavage) mode; however, there is no change in the yield and flow behavior of the solid metal. As shown in Fig. , embrittlement manifests itself as a reduction in fracture stress, strain, or both. Fracture can occur well below the yield stress of the solid. The stress required to propagate a sharp crack or a flaw is significantly lower than that necessary to initiate a crack in the liquid metal environment. In most cases, the initiation or the propagation of cracks appears to occur instantaneously, with the fracture propagating through the entire test specimen. The velocity of crack or fracture propagation can be as large as 10 to 100 cm/s (4 to 40 in./s) ( Ref, , , ). Examination of LMIE fracture surfaces shows complete coverage by the liquid metal. Liquid metal is in intimate contact with the solid and is usually difficult to remove. The fracture mode becomes apparent when special techniques are used to remove the liquid (Ref). It should be emphasized that gross amounts of liquid are not necessary; even micrograms of liquid lead can cause LMIE in 75 mm (3 in.) thick steel tubes (Fig. ). In fact, one monolayer is sufficient to cause LMIE (Ref). The fracture is usually brittle intergranular, with little indication of crack branching or striation to indicate slow crack propagation. The presence of liquid at the moving crack tip appears necessary for fast fracture. However, in high-strength brittle metals, a crack initiated in the presence of liquid may propagate in a brittle manner in the absence of liquid. In low-strength metals, a transition may occur from brittle-to- ductile failure under similar circumstances. Although far less frequent than brittle fracture, embrittlement can also occur by a ductile dimpled rupture mode in certain steels, copper alloys, and aluminum alloys. The embrittlement then manifests itself as the degradation of the fracture properties of the solid metal. Fracture is not limited to polycrystalline metals and the presence of grain boundaries. Embrittlement of amorphous alloys ( Ref, ) and single crystals (Ref, ) has been observed. Single crystals of zinc and cadmium exhibited brittle cleavage fracture as a result of LMIE (Fig. ). These are but a few of the manifestations of LMIE. Embrittlement is not a corrosion, dissolution, or diffusion-controlled intergranular penetration process but is considered to be a special case of brittle fracture that occurs in the absence of an inert environment and at low temperatures. Time- and temperature-dependent processes are not considered to be responsible for the occurrence of LMIE. Also, in most cases of LMIE, little or no penetration of liquid metal into the solid metal is observed. The embrittlement of the solid metal coated with liquid metal or immersed in the liquid does not depend on the time of exposure to the liquid metal before testing (Fig. ) or on whether the liquid is pure or presaturated with the solid. Usually, one of the prerequisites is that the solid has little or no solubility in the liquid and forms no intermetallic compound to constitute an embrittlement couple (Ref). However, exceptions to this empirical rule have been noted (Ref).

From: ASM Handbook Volume 13A, Corrosion: Fundamentals, Testing, and Protection (ASM International)
Published: 2003
Pages: 381-392