Heat capacities and thermal expansion coefficients of iron triad metals

Abstract

One of the complex problems relating to the thermodynamics of a substance is creating an adequate description of its thermal properties. For example, the Einstein and Debye models (as well as in various modifications of these models) the heat capacity is calculated only when mechanical vibrations of the lattice are taken into account. This leads to the impossibility of describing the increase in heat capacity with increasing temperature for most substances, including iron triad metals. In addition, there is not a single theoretical construction capable of calculating the temperature dependences of the heat capacity and thermal expansion coefficient during polymorphic transformations and structural, magnetic and other phase transitions in the system. They appear on the charts in the form of final jumps, peaks and holes. As a result, there is a need to develop a new approach to the calculation of thermal characteristics. It should take into account the occurrence of local equilibrium in small areas in initially non-equilibrium metal samples for research. The nonequilibrium of a sample can be caused by the presence of impurity atoms, defects, volatile components in it, residual stresses, the occurrence of irreversible processes, etc. For the analytical description of arrays of measured values, experimenters use different exponential expressions in different temperature intervals, sometimes with negative powers. Such theoretical and experimental approaches cannot be considered satisfactory. Therefore, for the creation of the new model, it is necessary to choose such values that would be sensitive to changes in the state of the system. Within the framework of the proposed model of a two-phase locally equilibrium region, such quantities are the absolute temperature of the system, the order parameter in the form of the difference in volume fractions of coexisting ideal phases, the phase composition of the ordering phase, and its derivative with respect to temperature. The developed model allows to calculate the temperature dependences of the heat capacities and coefficients of thermal linear expansion of the iron triad metals (Fe, Co, Ni) with a change in the aggregation state (crystal - liquid), the presence of structural, magnetic and other phase transitions. It has been shown that the used expressions adequately describe the experimental data in a wide temperature range, and also allow to extend the plotted curves to experimentally unexplored regions. A possible structural transition in cobalt at a temperature of about 1600 K, the existence of which requires additional experiments, was established. The obtained expressions are distinguished by their simplicity and universality of applicability; they can be used to create an automatic calculation of the thermophysical properties of not only iron triad metals, but also other solid substances

Keywords

• thermodynamic model
• order parameter
• structural rearrangement
• polymorphic transformations
• transition metal