Manipulation of Microstructure and Mechanical Properties in N-Doped CoCrFeMnNi High-Entropy Alloys

Heh Sang Ahn,Kook Noh Yoon,Ji Young Kim,Wook Ha Ryu,Min Seok Kim,Eun Soo Park,Jing Zhang

doi:10.3390/met11091487

Abstract

Herein, we carefully investigate the effect of nitrogen doping in the equiatomic CoCrFeMnNi high-entropy alloy (HEA) on the microstructure evolution and mechanical properties. After homogenization (1100 °C for 20 h), cold-rolling (reduction ratio of 60%) and subsequent annealing (800 °C for 1 h), a unique complex heterogeneous microstructure consisting of fine recrystallized grains, large non-recrystallized grains, and nanoscale Cr2N precipitates, were obtained in nitrogen-doped (0.3 wt.%) CoCrFeMnNi HEA. The yield strength and ultimate tensile strength can be significantly improved in nitrogen-doped (0.3 wt.%) CoCrFeMnNi HEA with a complex heterogeneous microstructure, which shows more than two times higher than those compared to CoCrFeMnNi HEA under the identical process condition. It is achieved by the simultaneous operation of various strengthening mechanisms from the complex heterogeneous microstructure. Although it still has not solved the problem of ductility reduction, as the strength increases because the microstructure optimization is not yet complete, it is expected that precise control of the unique complex heterogeneous structure in nitrogen-doped CoCrFeMnNi HEA can open a new era in overcoming the strength–ductility trade-off, one of the oldest dilemmas of structural materials.

Highlights

The single-phase FCC CoCrFeMnNi high-entropy alloy (HEA) has potential advantages in hardness, toughness, thermal stability, and excellent plasticity at room temperature and low temperature [1,2,3,4,5]
The N0.3 HEA (Figure 1b) exhibits a partially recrystallized microstructure with large-sized grains elongated along the rolling direction and small-sized equiaxed grains distributed along the shear deformed zone
We have not yet solved the problem of ductility reduction as the strength increases because the microstructure optimization is not yet complete in this study, there is a promising possibility to overcome the strength–ductility trade-off in the N-doped HEAs due to their unique complex heterogeneous microstructure through the precise control of the N contents or the post-processing conditions, which will merit further investigation

Summary

Introduction

The single-phase FCC CoCrFeMnNi high-entropy alloy (HEA) has potential advantages in hardness, toughness, thermal stability, and excellent plasticity at room temperature and low temperature [1,2,3,4,5]. The adjustment of the post-processing conditions, which generate heterogeneous microstructure, can be used to obtain the synergetic effect of various strengthening mechanisms [14,15,16,17,18]. Xiong et al [22] reported more systematic influences of nitrogen alloying on microstructural evolution and tensile properties of CoCrFeMnNi HEA treated by cold-rolling and subsequent annealing at different annealing temperatures (773–1173 K). To optimize the strengthening effect, a more in-depth study on how complex heterogeneous microstructures are formed through competition between recrystallization and precipitation according to the N contents and/or post-processing conditions and the strengthening effect caused by each structural difference is necessary. The strengthening behavior of the N-doped HEA with complex heterogeneous microstructure compared to CoCrFeMnNi HEA under the identical post-processing condition was carefully discussed based on intensive structural analysis

Experimental

Results and Discussion

Conclusions