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The grain size-dependent solute concentration in nanograined materials

Tongyi Zhang (Hong Kong University of Scienc)

Prager Medal Symposium in honor of George Weng: Micromechanics, Composites and Multifunctional Materials

Wed 10:45 - 12:15

MacMillan 117

Taking advantages from both Gibbs and McLean adsorption isotherms, we developed a Gibbs-approach based adsorption isotherm for grain boundary (GB) segregation in nanograined (ng) polycrystals. An excess GB thickness was introduced to describe the excess of GB atomic volume in comparison with the atomic volume in lattice. The GB bulk modulus was then determined with the excess GB thickness and a universal function. The newly developed adsorption isotherm is able to analyze simultaneously stresses, concentrations and their coupling behaviors in grains and GBs, showing that the apparent solute concentration could be greatly enhanced in ng materials, due to a large grain boundary volume fraction and a considerable increase in the lattice concentration that is, in turn, boosted by the concentration-induced stresses. Using the experimental data of lattice strain and sample strain of the nanograined Pd, with an averaged grain size of 10 nm and in thermodynamic equilibrium with a H2 partial pressure, we determined H concentrations and stresses, as a function of the H2 partial pressure, in both grains and grain boundaries. More importantly, we determined the intrinsic properties of grain boundaries, such as the grain boundary bulk modulus, the grain boundary excess thickness, the difference in chemical potential between grains and grain boundaries, etc. With the determined intrinsic properties, the Gibbs-approach based adsorption isotherm predicted the H segregation in grain boundaries of nanograined Pd with an averaged grain size of 5 nm. The predication was verified by other reported experiment data.