Diffusion and Segregation in Grain and Interphase Boundaries
Grain Boundary Diffusion and Segregation in Compacted and Sintered Nanocrystalline Alloys
We have intensively investigated self-(Fe,Ni) and Ag solute diffusion behaviour in well compacted and sintered
nanocrystalline (grain size d ≈ 100 nm) g-Fe-40 wt % Ni alloys. The very sensitive radiotracer technique was
applied using 63Ni, 59Fe isotopes and 110m Ag radiotracer for
measuring the segregation to interfaces of nano-sized grains. In the nanocrystalline material the individual
nano-scaled grains turned out to be clustered in micrometer-sized agglomerates and there existed two types of internal
interfaces with different length scales and diffusion characteristics. The boundaries between the nano-grains
and between the agglomerates of these nanocrystallites act as different short circuit paths. The systematics of
grain boundary self- and solute diffusion in such a bimodal structure was outlined and a model was developed to
treat the measured tracer diffusion profiles in a physically correct way.
It was discovered that in comparison
to the simultaneously performed analogous measurements in coarse-grained Ni-Fe alloys GB diffusion in
coarse-grained and nano-sized Fe-Ni materialis very similar. However diffusion of Fe, Ni and Ag in the
agglomerate boundaries is two to three orders of magnitude faster. The segregation of Ag in the GBs is very
strong and could be determined quantitatively only in the nanosized material due to the here available very large
number of interfaces, despite of the extremely low solid solubility of Ag in Fe-Ni material. Due to this reason
similar segregation measurements were unsuccessful in coarse-grained alloy material.
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