Diffusion and Segregation in Grain and Interphase Boundaries
Investigation of the grain boundary pre-melting phenomenon due to solute segregation, studied for the
case of Bi in Cu
The embrittlement of copper by Bi atoms is a typical example of a severe detrimental phenomenon in
materials science. It was recently proposed in the literature that the strong Bi segregation in Cu can cause
a liquid-like film at the grain boundaries (GBs), when the alloy is in the single phase (solid solution) region in the
equilibrium phase diagram. We have recently performed GB diffusion experiments of Bi in pure Cu and in
Bi-doped Cu(Bi) alloys to prove the given predictions. Because, if a liquid-like GB-phase is indeed
formed, the GB diffusivity should be dramatically enhanced. First of all the segregation of Bi in Cu was
determined quantitatively by our well-established radiotracer technique with the isotope 207Bi.
Furthermore 64Cu and 207Bi GB tracer diffusion was measured in a series of
dilute Cu-Bi alloys in the single phase of solid solution and in the two-phase (solid plus liquid) regions of the
equilibrium phase diagram. It was shown that Bi segregates extremely strongly in Cu GBs. An abrupt
increase of the GB diffusivities of both Cu and Bi by about two decades was observed at certain
Bi contents which are without any doubt in the single-phase region and are definitely less than those
associated with the bulk solidus concentration. The present results convincingly showed the occurrence of a
pre-melting phase transition in GBs of the Cu-Bi system.
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