AES quantification of intergranular film thickness in the Ni–Bi system with respect to the liquid metal embrittlement phenomenon

Intergranular penetration of liquid bismuth‐rich alloy in solid polycrystalline nickel is investigated at 700° C. Short intergranular films of micrometric thickness with a very acute tip are observed by SEM. Intergranular penetration of Bi–Ni alloy induces strong intergranular brittleness that extends far ahead of the micrometric film tip. Very long intergranular films of nanometric thickness are identified by AES after in situ intergranular fracture. A quantitative model aimed at the determination of the actual thickness of these films is developed and described in detail. The measured bismuth/nickel intensity ratios (IBi/INi) are correlated to the thickness of the intergranular film. Detailed procedures related to the determination of the experimental IBi/INi ratio as well as all assumptions related to the quantification model are clearly stated. Atomic densities, attenuation lengths and retrodiffusion factors are calculated whereas effective cross‐sections, probabilities of de‐excitation by the Auger process and the transmission efficiency of the analyser are suppressed from the final formulae due to reference measurements on the bulk Bi–Ni embrittling alloy, which is used as a bicomponent standard of known composition. Numerical application gives a value of 2–4 nm for the thickness of the film formed during 8 h at 700° C. The uncertainty mainly comes from the difficulty in determination of the Auger electron collection angle, due to the use of polycrystals. Interest for the use of bicrystals is underlined. It is postulated that the mechanisms of liquid metal embrittlement (LME) should be analysed with respect to the tip of this nanometric film. Copyright © 2001 John Wiley & Sons, Ltd.

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