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 polycrystal line nickel is investigated at 700 degrees C. Short intergranular films of micrometric thickness with a very acute tip are observed by SEM. Intergranu lar penetration of Bi-Ni alloy induces strong intergranular brittleness tha t extends far ahead of the micrometric film tip. Very long intergranular fi lms of nanometric thickness are identified by AES after in situ intergranul ar 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 (I-Bi/I-Ni) are correlated to the thicknes s of the intergranular film. Detailed procedures related to the determinati on of the experimental I-Bi/I-Ni ratio as well as all assumptions related t o the quantification model are clearly stated. Atomic densities, attenuatio n lengths and retrodiffusion factors are calculated whereas effective cross -sections, probabilities of de-excitation by the Auger process and the tran smission 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 degrees C. The uncertainty mainly comes from the difficulty in determi nation of the Auger electron collection angle, due to the use of polycrysta ls. 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 (C) 2001 John Wiley & Sons, Ltd.