The enthalpy of formation of NiAl

Abstract

NiAl and NiAl based ternary metallic systems have attracted much attention in recent years for potential use as high temperature structural materials [1–6]. Alloy development of such systems requires a thorough knowledge of the phase equilibria and thermodynamics of the systems. Using high temperature calorimeters [7–9], the enthalpies of formation for many alloy systems have been measured and the results incorporated into thermodynamic databases for modeling of their phase diagrams using software such as Thermocalc©R [10]. However, there exists some confusion regarding the value for the enthalpy of formation of NiAl. In one reference [11], comparison of published experimental data was made which are referred to different standard states, with the consequent conclusion that there is a wide discrepancy in the experimental NiAl data. In this paper, the enthalpies of formation of NiAl as a function of composition determined by two different calorimetric techniques are compared. The results are also compared to enthalpy of formation data for the stoichiometric alloy from several other sources. Two kinds of calorimeters are widely used in measuring the enthalpies of formation of intermetallic compounds with high melting points: the differential solution calorimeter and the direct synthesis calorimeter. The former is an indirect method because the compound is prepared before the enthalpy of formation is determined, while the latter is a direct one since the enthalpy of formation is determined during alloy formation. Using the differential solution calorimeter, Henig and Lukas [12] determined the enthalpy of formation of NiAl at 1100 K. During their experiment, the element Ni and the compound NiAl were dropped into the liquid aluminum separately and their heat of dissolution were measured in turn. The reactions involved in the measurement are: