Microstructural Evolution and Phase Formation in 2nd-Generation Refractory-Based High Entropy Alloys.

Vladimir Popov,Vladimir Popov Jr,Eyal Eshed,Alexander Katz-Demyanetz,Igor Gorbachev,Alexey Kovalevsky,Natalya Larianovsky

doi:10.3390/ma11020175

Abstract

Refractory-based high entropy alloys (HEAs) of the 2nd-generation type are new intensively-studied materials with a high potential for structural high-temperature applications. This paper presents investigation results on microstructural evolution and phase formation in as-cast and subsequently heat-treated HEAs at various temperature-time regimes. Microstructural examination was performed by means of scanning electron microscopy (SEM) combined with the energy dispersive spectroscopy (EDS) mode of electron probe microanalysis (EPMA) and qualitative X-ray diffraction (XRD). The primary evolutionary trend observed was the tendency of Zr to gradually segregate as the temperature rises, while all the other elements eventually dissolve in the BCC solid solution phase once the onset of Laves phase complex decomposition is reached. The performed thermodynamic modelling was based on the Calculation of Phase Diagrams method (CALPHAD). The BCC A2 solid solution phase is predicted by the model to contain increasing amounts of Cr as the temperature rises, which is in perfect agreement with the actual results obtained by SEM. However, the model was not able to predict the existence of the Zr-rich phase or the tendency of Zr to segregate and form its own solid solution—most likely as a result of the Zr segregation trend not being an equilibrium phenomenon.

Highlights

IntroductionHigh entropy alloys (HEAs) are, in relative terms, newcomers to the world of metallurgy
High entropy alloys (HEAs) are, in relative terms, newcomers to the world of metallurgy.These alloys are composed of roughly equimolar proportions of metallic elements [1,2,3]
X-ray diffraction (XRD) was utilized as a complementary characterization technique to verify the phase identification performed according to scanning electron microscopy (SEM)

Summary

Introduction

High entropy alloys (HEAs) are, in relative terms, newcomers to the world of metallurgy. These alloys are composed of roughly equimolar proportions of metallic elements [1,2,3]. HEAs should possess a single disorderly solid solution phase, which lends them their high entropy characteristic [1]. It is common practice to classify the refractory element-based HEAs according to generations (1st, 2nd, and 3rd) based solely on their chronological formulation. The 1st-generation HEAs were the original compositions belonging to this family—Mo25 Nb25 Ta25 W25 and Mo20 Nb20 Ta20 V20 W20 , both are BCC and possess a single solid solution [4,5]. The one significant drawback to such an application is the high density and low corrosion resistance exhibited by the refractory elements, separately or alloyed [5]

Methods

Results

Discussion

Conclusion