Contact: Jörg Weissmüller
Alloy phase diagrams document equilibrium states of multicomponent systems, for instance in the temperature-composition domain. They form an indispensable engineering data base for materials selection and optimization in technological applications. For nanoscale materials, the energetics of the many interfaces has a significant impact on the driving forces for phase transformations and on the resulting equilibrium states. Phase diagrams of nanoscale alloys therefore depend on the grain- or particle size as an additional dimension in configuration space. Many of the relevant issues still await a systematic study. Our experiments on matrix-isolated nanoparticles and on nanocrystalline metal hydrides provide insights that can be generalized to nanoscale phase equilibria in a wider context.
Alloy Effects in Nanostructures
NanoStruct. Mater. 3 (1993), 261
J. Weissmüller and C. Lemier
On the Size-Dependence of the Critical Point of Nanoscale Interstitial Solid Solutions
Phil. Mag. Lett. 80 (2000), 411
J. Weissmüller, P. Bunzel and G. Wilde
Two-Phase Equilibrium in Small Alloy Particles
Scripta Mater. 51 (2004), 813
C. Lemier and J. Weissmüller
Grain Boundary Segregation, Stress and Stretch: Effects on Hydrogen Absorption in Nanocrystalline Palladium
Acta Mater. 55 (2007) 1241
| || |
The free energy, G, of nanocrystalline alloys with a pronounced tendency for grain boundary segregation exhibits a minimum when plotted as the function of the grain size, D. The minimum corres- ponds to a metastable state of the alloy in which grain growth is suppressed and the nanoscale grain structure remains stable to elevated temperature.
The Gibbs phase rule imposes restric- tions on the topology of alloy phase diagrams. For instance, three phases can only coexist in a single point in the temperature-composition domain. These rules are apparently violated in certain nanoscale systems, such as the Bi-Cd alloy nanoparticle in the figure.