Elsevier

Acta Materialia

Volume 98, 1 October 2015, Pages 288-296
Acta Materialia

Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys

https://doi.org/10.1016/j.actamat.2015.07.030Get rights and content

Abstract

The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic FexMn62−xNi30Co6Cr2 based high entropy alloys, where x ranges from 22 to 42 at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems.

Introduction

High entropy alloys (HEA) are multicomponent (5 or more) massive solid solutions with an equiatomic or a near equiatomic composition [1], [2], [3], [4], [5], [6]. The original ideal of investigating multicomponent alloys in equal or near-equal proportions represents a new alloy exploration strategy [7], [8], [9], [10], [6]. Instead of starting from a corner of a phase diagram with one prevalent base element, it has been suggested that new materials could be identified by directly producing equiatomic compositions with multiple components. The term “high entropy alloys” was introduced by Yeh et al. [2], based on the hypothesis that the high configurational entropy would stabilize the solid solution phase over competing intermetallic and elemental phases [11]. A well-studied HEA is the Cantor alloy i.e. Co20Cr20Fe20Mn20Ni20 (at.%) which develops a single phase fcc solid solution e.g. [9], [12], [13], [14], [15], [16]. Recently, it has been shown that a non-equiatomic composition of this alloy system also exhibits a single phase fcc solid solution irrespective of its slightly lower mixing entropy [17], [18].

The objective of this study is two-fold. One focus is the prediction and analysis of the phase stability of this alloy system i.e. FexMn62−xNi30Co6Cr2 (at.%, x = 22, 27, 32, 37, and 42), while varying the Fe and Mn contents, and maintaining the compositions of Cr, Co and Ni constant. The configurational entropy of these alloys ranges from 1.295 to 1.334 kB/atom (kB is the Boltzmann constant) which yields 80–83% of that in equiatomic composition (1.6094 kB/atom) as shown in Fig. 1.

Another focus is to explore the feasibility of using the CALPHAD (CALculation of PHAse Diagrams) method for future knowledge based approaches to the design of HEAs. Compared with other approaches for designing HEAs (e.g. empirical rules [19], [20], [21], [22], [23], [24], [25], [26], or ab initio based methods [27], [28]), the CALPHAD method provides an optimal balance between efficiency and accuracy. On the other hand, most multicomponent systems are not fully covered by the available CALPHAD databases. Instead, current CALPHAD simulations of multicomponent systems are based on the extrapolation from binary, ternary, and, (perhaps) quaternary systems [29]. Hence, the accuracy of the corresponding predictions yielded by using a CALPHAD approach needs to be critically evaluated.

This paper is organized as the follows: The synthesis, heat treatment and methods of phase identification are described in Section 2. The employed CALPHAD method will be briefly introduced in Section 3. The experimental results will be presented in Section 4.1. In Section 4.2 we show the calculated phase diagram, as well as corresponding CALPHAD simulations to predict the phases and compositions resulting from solidification and annealing followed by water quenching. We then compare and discuss the experimental and theoretical results in Section 5. In Section 6, we further examine the predictive capacities of CALPHAD by comparing the corresponding simulation results with observations presented in the experimental study by Otto et al. [12]. Finally, we summarize this study in 7.

Section snippets

Alloy preparation: synthesis and heat treatment

The conventional approach for investigating the microstructure and mechanical properties of novel metallic structural materials typically consists of iterative experimental loops including synthesis of a single charge, hot and/or cold deformation, heat treatment, machining of tensile specimens and mechanical testing. Specifically, when the alloy system of interest has not been completely understood, e.g. in terms of thermodynamic stability and corresponding process parameters, this conventional

CALPHAD simulations

The CALPHAD (CALculation of PHAse Diagrams) simulations were performed on the platform of Thermo-Calc [31] using the database TCFE7 (Fe-alloys database version 7).

Phase diagrams calculated by CALPHAD are by definition equilibrium phase diagrams, while most corresponding experiments represent materials that are often far away from their equilibrium state.

In order to resolve this problem two other types of CALPHAD-based simulations were performed besides the prediction of the actual equilibrium

Results of DSC and EBSD characterization

For the as-cast alloys, the DSC traces presented in Fig. 3 show no endothermic or exothermic peaks for all alloys, indicating that no solid state phase transformation has taken place between room temperature and melting. It can be noted that with an increase in the Fe concentration, the melting point also increases.

Below 600 K, there are fluctuations on the DSC curves, which are more pronounced for the larger heating rate of 10 K/min. This is due to the fact that for a given heating rate, a

Comparison between experiments and CALPHAD simulations

For HEAs with their intrinsically slow diffusion kinetics, 3 possible scenarios are conceivable to occur during cooling after solidification: (1) the high temperature phase is retained; (2) due to segregation during non-equilibrium solidification, the local chemical composition can be close to the chemical compositions of the low temperature phases. In case that long distance diffusion is not required – which is actually a characteristic feature of HEAs owing to their intense mixing state – at

Revisiting HEAs in equiatomic composition

In Sections 4.1 Results of DSC and EBSD characterization, 4.2 Results of the CALPHAD calculation, 5 Comparison between experiments and CALPHAD simulations, we revealed that for the here investigated HEAs, CALPHAD provides very consistent results when compared to experiments. By considering both the relevant enthalpy and entropy contributions, the CALPHAD models should ideally be able to predict the phase formation in other HEA systems. Hence, in order to test the versatility and suitability of

Summary and conclusions

In this study, we experimentally and theoretically investigated the phase stability of non-equiatomic composition HEAs, i.e. FexMn62−xNi30Co6Cr2. Here, we summarize our study:

  • 1.

    DSC measurements on the as-cast FexMn62−xNi30Co6Cr2 materials from room temperature (300 K) to 1573 K exhibited no solid state phase transformation. EBSD measurements on the materials annealed at 1473 K for 2 h followed by water quenching revealed a single fcc solid solution phase.

  • 2.

    The equilibrium phase diagram obtained for Fex

Acknowledgments

D.M expresses his gratitude to Prof. Gerhard Inden for his lectures on ThermoCalc given at the Max-Planck Institut für Eisenforschung GmbH, Düsseldorf, Germany. Funding by the European Union is gratefully acknowledged, provided under the 7th Framework Programme (FP7/2007–2013) through the European Research Council (ERC) Advanced Grant SMARTMET agreement 290998.

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