Ab initio thermodynamics of the (Cu,Ni)(In,Sn) intermetallics: energetics, thermophysical properties and comparisons with experiments and CALPHAD type extrapolations

S. RAMOS DE DEBIAGGI; C. DELUQUE TORO; G. F. CABEZA; A. FERNÁNDEZ GUILLERMET

Montevideo

Workshop; 5th Workshop on novel methods for electronic structure calculations, and First Southamerican Congress on Materials; 2013

Universidad de la República

At present, the ab initio methods are recognized as a valuable predictive route to establish the thermodynamics of various types of substances. For instance, the T= 0 K enthalpy of formation (EOF) as well as other thermophysical cohesion-related properties has been considered as a key information in establishing trends in the relative stability of various type of phases across the Periodic Table. A particularly demanding application of such methods is the development of thermodynamic databases for the phenomenological (so-called CAPHAD-type) calculation of the phase diagram for complex intermetallic systems. In this case it is necessary to calculate the properties of various stable, non-stable and hypothetical compounds. The present contribution aims at summarizing the experience gained in a systematic ab initio density functional theory study of the intermetallic phases (IPs) of the (Cu,Ni)(In,Sn) systems. These materials are currently a subject of considerable theoretical and experimental interest in connection with the application of In-Sn alloys as lead-free micro-soldering alloys, when Ni or Cu act as contact materials. The work involves extensive calculations at 0 K to evaluate the structural and equation of state parameters, the electronic DOS and the EOF of the various stable and metastable Cu-In, Cu-Sn, Ni-In and Ni-Sn compounds. A discussion of the composition dependence of their properties and a critical comparison with experiments is also included. In addition, several non-stable ("hypothetical") compounds involved in the most recent CALPHAD modeling of Cu-In-Sn and the Ni-In and Ni-Sn phase diagrams are studied. Systematic differences are found between the 0 K ab initio calculated EOFs of the evaluated IPs and the experimental data at T=298 K, which is the usual reference temperature adopted in thermochemical compilations at finite temperature. A proper comparison of the ab initio and thermodynamic values requires the knowledge of the integrated heat-capacity difference between the compound and the elements from 0 to 298 K, an information which actually is not available from the experiments. It is a purpose of the present contribution to explore the use of ab initio methods and the quasiharmonic approximation to calculate the vibrational heat-capacity for one of the compounds included in the systematic study. With the new information obtained an attempt is reported to estimate the effect of temperature upon the calculated EOFs.