Abstract
In this paper we present and discuss phase equilibria in the quaternary Al-Cr-Fe-Ni and the quinary Al-Co-Cr-Fe-Ni alloy systems based on experimental data from DTA/DSC, SEM/EDS and SEM/EBSD on as-cast and isothermally annealed samples. These data as well as literature data were used for developing a new Al-Co-Cr-Fe-Ni thermodynamic description by the CALPHAD approach. Considerable efforts in this direction have been made already and commercial databases for high entropy alloys are available, e.g. TCHEA4 and PanHEA. We focus on comparing our new thermodynamic database with computations using TCHEA4 for two section planes i.e. quaternary AlxCoCrFeNi and quinary AlxCrFe2Ni2, where x is the stochiometric coefficient. According to our new thermodynamic description a single-phase field BCC-B2 is stable over a wider temperature range in both section planes, giving rise to dual-phase microstructures along solid state phase transformation pathways. In the section plane AlxCoCrFeNi the two-phase field BCC-B2+σ predicted by the new database is stable between 600°C and 800°C, while in TCHEA4 this phase field extends to nearly 1000°C. Furthermore, our new database showed that the solidification interval with primary BCC-B2 phase in quinary as well as quaternary section planes is narrow, being in a good agreement with presented micro-segregation measurements. Additionally, computed phase fields and phase-field boundaries in the quinary section plane correspond well to the experimental results reported in the literature.
Highlights
High entropy alloys (HEAs) and, in particular, alloys from the Al–(Co)–Cr–Fe–Ni alloy system have been subject to numerous investigations due to promising mechanical, magnetic and electrochemical properties (Butler and Weaver, 2016; Gao et al, 2016; Chen et al, 2017; Shi et al, 2017)
In this work we presented phase equilibria in the Al–Co–Cr– Fe–Ni HEA system from annealing experiments and CALPHAD modeling of the quinary alloy system
In the first part we described Differential thermal analyses (DTA)/DSC measurements and microstructures obtained by isothermal annealing for a series of AlxCoCrFeNi samples with 0.3 < x < 1.5
Summary
High entropy alloys (HEAs) and, in particular, alloys from the Al–(Co)–Cr–Fe–Ni alloy system have been subject to numerous investigations due to promising mechanical, magnetic and electrochemical properties (Butler and Weaver, 2016; Gao et al, 2016; Chen et al, 2017; Shi et al, 2017). Some of them e.g., additive manufacturing, may involve high cooling rates leading to non-equilibrium microstructures which can be further optimized through heat treatment schemes For both alloy design and processing technologies it is important to rely on critically assessed thermodynamic background information. In this work we present research results on thermodynamic equilibria in the system Al–Co–Cr–Fe–Ni including key experimental data for selected alloys and annealing conditions as well as a thermodynamic description based on CALPHAD modeling. Differential thermal analyses (DTA) were performed with heating and cooling rates of 20 K/min on as-cast samples in alumina crucibles under high purity Ar atmosphere These measurements were conducted in a high temperature differential scanning calorimeter type Pegasus DSC 404 F3 (NETZSCHGerätebau GmbH) operated in DTA mode with two different sensors, i.e., DSC-cp and DSC-DTA both with an S-type thermocouple. The EBSD measurements were performed mainly in mapping mode under an accelerating voltage of 25 kV and the working distance of around 16 mm
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