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

Abstract

The effect of Co on the phase stability of the CrMnFeCoxNi family of alloys, where the atomic ratio x = 0, 0.5, 1.5, has been experimentally established following 1000 hour heat treatments at 700 and 900˚C and up to 5000 hours at 500˚C. All the alloys were single phase fcc in the homogenised condition, except for CrMnFeNi which also contained bcc precipitates that remained present following exposures at 900˚C and 700˚C. The exposures at 900˚C and 700˚C also resulted in the formation of phase precipitates in the CrMnFeNi and CrMnFeCo0.5Ni alloys but not in the CrMnFeCo1.5Ni alloy. These data, in conjunction with results previously published in the literature, conclusively establish that Co stabilises the fcc matrix at elevated temperatures. However, at 500˚C, further bulk decomposition in the CrMnFeNi alloy was observed and produced a fine-scale intergrowth of a NiMn L10 phase and CrFe phase. Grain boundary precipitates were also observed following exposure at 500˚C in the CrMnFeCo0.5Ni and CrMnFeCo1.5Ni alloys. Four different phases were observed on the grain boundaries of the CrMnFeCo0.5Ni alloy (Cr carbide, , FeCo B2 and NiMn L10), whilst only two phases were found on the grain boundaries of the CrMnFeCo1.5Ni alloy (Cr carbide and NiMn L10). The experimental observations facilitated an assessment of the fidelity of current thermodynamic predictions of phase equilibria. All the phases predicted were observed experimentally and the stability fitted the experimental observations well. However, at lower temperatures, thermodynamic predictions were less consistent with experimental observations, underpredicting the extent of the B2 phase field and failing to predict the formation of the L10 phase.

Inner relaxations in equiatomic single-phase high-entropy cantor alloy

“…Depending on composition and processing routes, HEAs exhibit variations in their phase constitution and microscopic morphology [20]. Several HEA systems based on Cr, Mn, Fe, Co, Ni and Al were proposed to be single-phase face-centered cubic (fcc) structures in quite broad compositional and temperature intervals [21][22][23]. The equiatomic CrMnFeCoNi HEAthe classical Cantor alloyforms a stable fcc solid solution above ~800 °C that can be retained down to room temperature (RT), if the alloy is cooled at a sufficiently high rate [24,25], whereas AlxCrFeCoNi HEAs for x < 0.3 are fcc at RT after cooling rapidly from above 1000 °C [26][27][28][29][30].…”

The influence of Fe variations on the phase stability of CrMnFexCoNi alloys following long-duration exposures at intermediate temperatures

“…The light-contrast precipitates contained around 45 at.% Cr and were equally depleted of Mn, Fe and Co relative to the matrix, but contained only ~ 5 at.% Ni. This composition is indicative of the σ phase, which has been widely observed in alloys of the CrMnFeCoNi system [6][7][8][9]11,26,46]. The dark-contrast precipitates, on the other hand, contained ~ 70 at.% Cr and less than 10 at.% of the other elements (C was omitted from quantification as SEM-EDX is unreliable for measuring the concentration of light elements).…”

“…However, when these predictions are extended to higher order systems, the model parameters often require adjustment to accurately describe the Gibbs energies and hence equilibrium phase stability [24]. In particular, it has been observed that predictions often fail to accurately describe the extent to which intermetallic phases persist into multicomponent space [12,[26][27][28]. Therefore, the use of appropriate experimental data in the development of CALPHAD databases is essential.…”

“…However, with an understanding of the effect of each element on the microstructural equilibrium, the alloy composition can be tuned to supress the formation of undesirable phases. It is widely accepted that Cr is the main σ forming element in CrMnFeCoNi-based alloys [30][31][32][33][34][35][36], and recently, a series of detailed systematic studies have investigated the effect of Mn, Co and Ni following long-duration heat treatments at temperatures between 500 and 900˚C [12,26,27].…”

Structure and Magnetic Property of High‐Entropy Intermetallics Based on SmCo5 with the Equiatomic Ratios at Sm and Co Sites

“…[2] In general, N should be not less than 4 to meet the requirement of ΔS c > R. According to the crystallographic knowledge, the initially proposed high-entropy alloy (HEA) can be considered as a solid solution, which is formed by substituting the transition metals or metals for pure metal with equiatomic or near-equiatomic ratios ranging from 5 to 35 at%. The composition design of HEA is initially focused on the solid solution of pure metal with equiatomic ratio at metal crystal site, such as face-centered-cubic (FCC)-based CoCrFeMnNi and CrMnFeNiCo x ; [3,4] body-centered-cubic (BCC)-based HfNbTaTiZr and NbMoTaVW; [5,6] and hexagonal-closepacked (HCP)-based GdDyTbLuY and HoDyYGdTb. [7][8][9] HEAs show excellent mechanical and physical properties such as high strength and ductility, [10,11] good corrosion resistance, [12,13] excellent cryogenic and high temperature performance.…”