Fatigue Behaviour and Crack Initiation in CoCrFeNiMn High-Entropy Alloy Processed by Powder Metallurgy

Zdeněk Chlup,Monika Vilémová,Jiří Matějíček,Stanislava Fintová,Ivo Kuběna,Hynek Hadraba

doi:10.3390/met9101110

Abstract

Single-phase equiatomic five-element high entropy alloy CoCrFeMnNi was prepared by powder metallurgy. Two materials with ultra-fine-grained microstructure were prepared by spark plasma sintering (SPS) of ball-milled powder at two sintering times (5 and 10 min), assigned as HEA 5 and HEA 10, respectively. Basic microstructural and mechanical properties were evaluated. The median grain size of the microstructures was determined to be 0.4 and 0.6 μm for HEA 5 and HEA 10, respectively. The differences in the microstructure led to a significant change in strength and deformation characteristics evaluated at room temperature. The effect of cyclic loading was monitored by three-point bending fatigue test. The results show that even relatively small change in the microstructure causes a significant effect on fatigue life. The fatigue endurance limit was measured to be 1100 MPa and 1000 MPa for HEA 5 and HEA 10, respectively. The detailed fractographic analysis revealed that abnormally large grains, localised in the microstructure on the tensile loaded surface, were a typical fatigue initiation site. The formation of (nano) twins together with dislocation slips caused the crack nucleation because of the cyclic loading.

Highlights

High-entropy alloys are usually defined as single-phase alloys composed by at least five elements in the equiatomic composition stabilised by the configurational entropy [1,2]
The results show that even relatively small change in the microstructure causes a significant effect on fatigue life
The commonly used median grain size D50 was determined from the electron backscatter diffraction (EBSD) data to demonstrate the differences between both microstructures later illustrated on mechanical response

Summary

Introduction

High-entropy alloys are usually defined as single-phase alloys composed by at least five elements in the equiatomic composition stabilised by the configurational entropy [1,2]. A large variety of HEAs exists, but single-phase equiatomic five-element HEA Co-Cr-Fe-Mn-Ni, the so-called Cantor alloy, is one of the most studied [5]. The single-phase solid solution high-entropy alloys have a potential for solid solution hardening and if the single-phase fcc lattice structure is reached, a large number of slip systems ensures good ductility of the material [6,7,8,9]. The preferential processing method of this type of HEA is some variant of casting usually followed by heat treatment [5,8,9,10,11]

Methods

Results

Discussion

Conclusion