On the prediction and the formation of the sigma phase in CrMnCoFeNix high entropy alloys

Abstract

The almost limitless variations in potential compositions of high entropy alloys necessitates the use of computational methods when attempting to optimise for any given application. However, the accuracy of the current thermodynamic approaches commonly being used for this purpose remains under debate, as relatively few validatory studies have been performed. Within the CrMnFeCoNi family of alloys, the formation of the phase and how it is influenced by compositional variations is of particular interest for elevated temperature structural applications. Here, the role of Ni on the formation of the phase has been studied through a systematic series of CrMnFeCoNix alloys, 0 x 1.5, following 1000 hour exposures at temperatures typically found to promote formation. Ni was found to have a significant effect on the phase stability of these alloys, suppressing the phase such that a single solid solution phase was the only stable phase in the CrMnFeCoNi1.5 alloy, whilst the CrMnFeCo alloy formed the phase during solidification. The corresponding thermodynamic predictions varied dramatically from the experimentally observed microstructures, indicating that the underlying databases require further optimisation. Interestingly, it was found that a relatively simple electronic structure based approach, New PhaComp, provided much more accurate predictions of the observed phase formation in the CrMnFeCoNix and CrMnxFeCoNi systems and could be manipulated to obtain formation temperatures. As such, this method could be extremely useful to those wanting to design CrMnFeCoNi high entropy alloys that are free from the phase.

Nitrogen Interstitial Alloying of CoCrFeMnNi High Entropy Alloy through Reactive Powder Milling

“…THERMOCALC software version 2018 (Thermo-Calc Software AB, Solna, Sweden) was used to calculate the phase composition prediction. Unfortunately, the currently developed HEA database (TCHEA1) could not be used as it does not involve interstitial elements, and its accuracy was questioned [ 16 ]. As such, THERMOCALC calculations were realized using an Ni-based alloy database (TCNI9) instead.…”

Effect of Co on the phase stability of CrMnFeCoxNi high entropy alloys following long-duration exposures at intermediate temperatures

“…One such approach is the CALPHAD method [25], which has proven highly effective in more conventional alloy systems. However, the underpinning thermodynamic databases are predominantly based on the phase equilibria of binary and, to a lesser extent, ternary systems and an increasing body of evidence shows that the fidelity of their predictions in higher order systems, particularly at near-equiatomic compositions, is unreliable [18,[26][27][28][29][30][31]. Furthermore, many studies of phase stability in HEAs have focused on temperatures in excess of 1000˚C [18,28,30], where single-phase solid solutions are more likely to form due to the enhanced entropic contribution to Gibbs energy.…”

“…This is particularly true when investigating phase stability at low homologous temperatures, where there may be severe kinetic limitations to phase formation, as evidenced in recent radiotracer studies [33][34][35]. So far, comparisons of CALPHAD predictions, using the TCHEA1 database, with experimental results have shown that the calculations significantly underestimate the temperature range of stability of the phase [27][28][29].…”

“…It has been well established that Cr is the main forming element in the CrMnFeCoNi system and as such promotes the formation of the phase [13,14,28,[36][37][38][39][40][41]. In addition, systematic studies of the effect of Mn and Ni have recently been carried out, which assessed the phase stability following long-term exposures at temperatures between 500 and 900˚C [27,29]. This is the temperature range in which second phase precipitates have been observed in the equiatomic alloy [9,10].…”

“…Therefore, to elucidate the role of Co on phase equilibria in the CrMnFeCoxNi system, this manuscript presents a systematic study of the phase equilibria of three alloys, where the atomic ratio was x = 0, 0.5, 1.5. All of the alloys were homogenised just below their solidus temperatures prior to long duration exposures of at least 1000 h at 500, 700 and 900˚C; the temperature range in which phase decomposition has been shown to occur in other alloys within the CrMnFeCoNi system [9,10,19,27].…”