Microstructure evolution in a nanocrystalline CoCrFeNi multi-principal element alloy during annealing

Pham Tran Hung,Megumi Kawasaki,Jae-Kyung Han,János L Lábár,Jenő Gubicza

doi:10.1016/j.matchar.2020.110807

Pham Tran Hung, Megumi Kawasaki + Show 3 more

Open Access

https://doi.org/10.1016/j.matchar.2020.110807

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Abstract

Experiments were conducted for the study of the evolution of the microstructure in a nanocrystalline CoCrFeNi multi-principal element alloy during annealing. The nanocrystalline state was achieved by high-pressure torsion (HPT) which is a well-defined severe plastic deformation technique. The heat treatment of the nanocrystalline CoCrFeNi alloy was performed in a differential scanning calorimeter (DSC). It was found that the thermogram contains two exothermic peaks with maxima at about 680 and 870 K. For further analysis, a different set of the samples were annealed to temperatures below and above the two DSC peaks. It was revealed the first exothermic peak was related to the decrease of the density of lattice defects (dislocations and twin faults) while the grain size remained consistent. The comparison of the measured and the calculated released heat values suggested that during this recovery a high concentration of excess vacancies was annihilated (about 10−3). The second exothermic peak corresponded to the recrystallization of the microstructure when the grain size increased from about 60 nm to about 660 nm. It was revealed that the hardness of the nanocrystalline CoCrFeNi alloy remained unchanged during recovery (first DSC peak) while recrystallization (second DSC peak) caused a decrease of the hardness from about 5100 MPa to about 3200 MPa.

Highlights

Multi-principal element alloys (MPEAs) or complex concentrated alloys (CCA), including high-entropy alloys (HEAs), are designed with multiple principal elements of equal or near equal molar ratios [1,2]
The present study revealed an excellent phase stability of the CoCr FeNi MPEA processed by high-pressure torsion (HPT) since neither decomposition nor forma tion of chemical heterogeneities was observed
The present study suggests that the stacking fault energy (SFE) of CoCr FeNi MPEA is relatively low (20 mJ/m2), the homologous temperature of recrystallization is only slightly lower than that for traditional alloys with higher SFE, and in addition recovery precedes recrystallization in the differential scanning calorimeter (DSC) thermogram

Summary

Introduction

Multi-principal element alloys (MPEAs) or complex concentrated alloys (CCA), including high-entropy alloys (HEAs), are designed with multiple principal elements of equal or near equal molar ratios [1,2]. This is in contrast with conventional alloys involving one principal element with a minor amount of additional constituents. In order to achieve a high-entropy configuration, the materials would ideally maintain a single-phase solid solution state, which is not always present in MPEAs. For instance, CoCrFeMnNi be comes unstable after prolonged exposure to intermediate temperature [11], and severely deformed HfNbTiZr decomposes into multiple phases after annealing [12]. Both theoretical and experimental studies [13,14,15,16]

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