Alloy thermodynamics in nanostructures
- PDF / 415,067 Bytes
- 4 Pages / 576 x 792 pts Page_size
- 33 Downloads / 224 Views
The importance of the interactions between alloy atoms and topological defects for the thermodynamic properties of nanostructured alloys is pointed out. The McLean model for grain boundary segregation is extended to yield an expression for the total Gibbs free energy of an alloy polycrystal. This provides a simple conceptual basis for a qualitative discussion of the thermodynamic properties of nanocrystalline alloys. It is demonstrated that certain alloy poly- or nanocrystals may reach a metastable state, where the alloy is stable with respect to variation of its total grain boundary area.
A growing number of investigations on thermodynamic and structural properties of nanostructured alloys is lately being reported. A characteristic attribute of these systems is their high density of interfaces. For example, in a 3 nm grain size polycrystal, roughly half of the atoms are located on sites immediately adjacent to a grain boundary.1 Hence, the concentrations of topological defects and alloy atoms are comparable in nanostructured alloys. When discussing the properties of these solids, it is therefore of central importance to consider the interaction between alloy atoms and topological defects. While these interactions per se have been investigated for a long time, the resulting knowledge is often neglected in studies of nanostructures. The present paper aims at pointing out the importance of this issue for the thermodynamic properties of nanostructured alloys. Alloy atoms interact with grain boundaries via essentially three mechanisms2^1: first, part or all of the elastic strain energy due to atomic size mismatch is released when solute segregates to the grain boundaries; second, because the like and unlike atom coordination numbers in the grain boundary segregation sites differ from the lattice site coordination numbers, the electronic energies of interaction in the grain boundary sites are different from those in the lattice sites; third, the total "defect energy" of the grain boundaries is reduced when solute with a lower grain boundary energy segregates. As a result, the enthalpy of solution in the grain boundary segregation sites, A.HsplnGB, differs from the one in the crystal lattice, AH^liaa. The difference between both quantities, the enthalpy of segregation, is known to reach values of up to 100 kJ/mol. 2 Consequently, in thermodynamic equilibrium, the solute concentrations in lattice and grain boundary may differ by several orders of magnitude. This implies that the composition of nanostructured alloys may vary on a nanometer scale; hence, the alloys must not generally be considered to be homogeneous solid solutions or compounds. J. Mater. Res., Vol. 9, No. 1, Jan 1994
We now give an exemplary illustration of how the thermodynamic properties of a solid solution are affected when the concentration of topological defects is comparable to the concentration of solute atoms, and when there is a strong interaction between those components. For this purpose, the McLean model for grain boundary segregation2-5 is ex
Data Loading...
