A Third Generation Calphad Description of Fe: Revisions of Fcc, Hcp and Liquid

Abstract

The thermodynamic description of pure Fe was re-assessed using updated third generation Calphad models and taking into account the most recent theoretical data. In the present re-assessment, a critical evaluation was performed for the magnetic properties of the fcc and hcp phases yielding different descriptions from those accepted in the SGTE database. The selected magnetic properties enable us to model the thermodynamic properties of fcc satisfactorily without using the magnetic two-state model. A new method was applied to extrapolate the thermodynamic data for the solid phases, i.e. a single Gibbs energy expression is used for the entire temperature range for the bcc, fcc and hcp phases. The description of the liquid phase was re-evaluated following a recent suggestion on estimating the electronic heat capacity coefficient for the liquid phase. Overall, the present Calphad description can reproduce the selected experimental and theoretical data well.

Third Generation of Unary Calphad Descriptions and the Avoidance of Re-Stabilized Solid Phases and Unexpected Large Heat Capacity

“…[9] 2 Problems Created by Using a Single Temperature Range for Unary Gibbs Energy This work is prompted by the publication of the paper ''A third generation Calphad description of Fe: revisions of fcc, hcp and liquid'' by He et al in this issue. [4] However, the problem of solid phase re-stabilization at high temperature noted in that work is also observed in the latest version of 3rd generation data for pure Al [3] and pure W. [5] It is obviously imminent to the method applied for these unary Calphad descriptions because the parameters a and b in Eq 1 are generally positive, resulting in an unlimited growth of C p of solid phases at high T, far above C p (Liquid).…”

“…The remedy suggested for the artifact of solid phase restabilization [3][4][5] is the use of modified Calphad software, outlined in section 2.4 of the detailed work on the EEC by Sundman et al [10] The author entirely agrees not only that the axiom of the EEC is truly valid but also that the EET must occur well before that solid phase would become more stable than the liquid, [10] thus, it indicates a suitable maximum temperature to cease the unphysical extrapolation of solid data. The statement by Sundman et al [10] in section ''3.…”

“…[19,20] Numerous examples of overlooked solid phase re-stabilization at high temperature, taken from published Calphad assessment, were revealed in that work. [19] The latest revision of the 3rd generation data, [3][4][5] breaks this principle because with these new unaries there will be no binary or multicomponent system in which the thermodynamic equilibrium can be found simply based on the minimum of Gibbs energy functions without the incorrect re-stabilization of solid at high temperatures.…”

“…In the third-generation data for pure elements [1][2][3][4][5] the molar Gibbs energy function of an element i in any crystalline phase u, is generally described by an equation of the following form…”

“…E 0 is a total energy at 0 K and the parameters a, b and n are determined from the molar heat capacity, C p , where n usually takes the values 2, 3 or 4. h E is the Einstein temperature for the phase u of this element. The focus of this work is on the proposal made for the latest version of the 3rd generation data to use a single set of these parameters in Eq 1 to cover the temperature range from 0 to 6000 K [3][4][5] as opposed to a second high temperature range. [2] Note that H SER i is the molar enthalpy of the element i at 298.15 K and 1 bar in its standard element reference (SER) state.…”