Predictive Gibbs-energy approach to crystalline/amorphous relative stability of nanoparticles: size-effect calculations and experimental test with Cu-Zn alloys

J. PELEGRINA; F. C. GENNARI; A. FERNÁNDEZ GUILLERMET; A. CONDÓ

Sao Pablo

Workshop; Discussion Meeting on thermodynamics alloys TOFA 2016; 2016

TOFA

Ball milling experiments performed in the last decade in various systems opened the question about the stability of crystalline nanoparticles with respect to the same group of atoms but in the amorphous state. In principle, this problem should be treated in thermodynamic terms, by comparing the Gibbs energy of the competing phases. However, the often limited equilibrium information available on crystalline phases, or the general lack of experimental data on the amorphous phase, makes it difficult to establish the respective Gibbs energy functions. The general purpose of the present work is to develop a predictive approach to this problem, and assess its accuracy by confronting thermodynamic calculations with experimental observations on Cu-Zn nanoparticles. The bases of the approach are as follows. First, the present Gibbs energy formalismmakes use of the ?lattice-stability? concept currently applied in so-called CALPHAD (?Calculation of Phase Diagrams?) modeling work. Second, the enthalpy of formation of the alloy phases is treated in theframework of the Miedema model, with special attention to the parameters for the amorphous phase. Third, the size-dependent surface contribution to Gibbs energy is accounted for. With the current thermodynamic description, the sizes of the crystalline nanoparticles which are stable with respect to the amorphous aredetermined by calculation. These predictions are confronted with the actual sizes of the nanoparticles generated by subjecting ?-Cu-Zn powders to low energy milling treatments. On this basis, a discussion is reported of the accuracy of the present approach. In particular, the parameters in the Gibbs energy description which crucially affect the agreement between calculations and experiments are highlighted.