Thermodynamic Analysis of the Mg–RE–Zn (RE = Y, La) Ternary hcp Phase Using the Cluster Variation Method

Abstract

In this study, thermodynamic properties of the MgREZn (RE = Y, La) ternary hcp phase at finite temperature have been investigated by means of first-principles calculations combined with the cluster variation method (CVM). Free energy calculation, including the configurational entropy, shows that the MgYZn ternary hcp phase has a tendency to phase separation. Conversely, the MgLaZn ternary system does not exhibit such behavior even around room temperature. Furthermore, the calculated spinodal region extends to a broader composition range and the maximal spinodal temperature reaches above 1000 K for the MgYZn system. Conversely, the spinodal region for the MgLaZn system is a limited narrow region near the Mg-rich side, and the maximal spinodal temperature is 300 K. Formation enthalpies calculated on the basis of recent information from structure analyses do not show a definite difference in these two ternary systems. Therefore, we propose that the dominant factors in the formation of a novel long period stacking ordered structure include spinodal decomposition as well as structure transformation from 2H to other structures having periodic stacking faults.

Thermodynamic stability of Mg–Y–Zn ternary alloys through first-principles

“…At T = 500K, F form of structures on hcp is negative at all through composition and possesses two extreme values. Through calculation of formation free energy of SQSs, Mg-Y-Zn alloy exhibits phase separation into Mg-and Y-Zn-rich phase and this result is consistent with previous research by Iikubo et al [2], which suggests the validity of this simulation based on SQSs. Using these optimized SQSs, we proceed our discussion to calculate bulk modulus, ordering temperature, and the effect of stacking difference on phase stability.…”

“…In order to clarify relationship between thier formation process and resultant properties in terms of application for structural materials, considerable number of experimental as well as theoretical studies have been carried out. Previous theoretical studies mainly address formation process and thermodynamic stability of Mg-based LPSO alloy, including (i) the tendency of phase separation confirmed by Cluster variation method [2], (ii) in-plane ordering of clusters consisted of Y, Zn substitutional atoms [3], and (iii) systematic understanding of energetic stability with respect to variety of substitutional atoms into Mg-based alloys [4]. Although these previous theoretical works partly clarify themodynamic stability of LPSO phases, they did not sufficiently discuss about (i) relative stability in terms of disordered phases or (ii) effect of lattice vibration on stability of LPSO.…”

Nanoclusters first: a hierarchical phase transformation in a novel Mg alloy

“…However, the periodic stacking faults in the LPSO structures, as shown in Fig. 1 starts another discussion from a structural phase transformations viewpoint, i.e., stacking-fault driven transformation 29 . However, neither of the mechanisms appears to be compatible with the two-dimensional ordering kinetics of the present alloys described above.…”

Thermodynamic Origin of Solute-Enriched Stacking-Fault in Dilute Mg-Zn-Y Alloys

“…5i) compared with those of the Mg-Y and Mg-Zn systems [41]. As a result, the spinodal decomposition line [42,43] can be traced as indicated by arrow in Fig. 8a, and the determined SESF composition (xfcc) is found to be indeed located within the spinodal range.…”