Research ArticleFatigue behavior of CoCrFeMnNi high-entropy alloy under fully reversed cyclic deformation
Introduction
High-entropy alloy (HEA) has drawn much attention worldwide since the reports in 2004 [1,2]. The characteristic of multi-principal elements of HEA is different from the one-principal element in traditional alloys, which opens vast space for alloy design [[3], [4], [5], [6]]. In the last several years, HEAs with superior mechanical properties have been reported, i.e. tensile properties [[7], [8], [9], [10]], fracture toughness [11], wear resistance [12], indicating that they can be potentially applied in industry. However, the fatigue behavior of HEAs has been rarely reported [[13], [14], [15]].
High-cycle fatigue (HCF) strength is crucial for the long-term service of engineering components since it represents the endurance limit under cyclic loading. It is thus desirable to improve the fatigue strength of materials. Previous studies indicate that fatigue strength has a positive relationship with the tensile strength though there exist exceptions [[16], [17], [18]]. In general, high strength can be obtained in ultrafine-grained (UFG) or nanocrystalline (NC) materials by imposing high plastic strains via various plastic deformation techniques [19]. However, due to the introduction of profuse defects during plastic deformation, these UFG or NC materials are under nonequilibrium state and energetically unstable [20]. For specified materials such as UFG Cu, grain coarsening can even occur during fatigue tests at ambient temperature, which can be detrimental for the fatigue strength enhancement [[21], [22], [23]]. In order to ameliorate the thermal and mechanical stability of these UFG and NC materials prepared by plastic deformation, subsequent annealing treatment was suggested to release the extra defects prestored in the specimens [24].
When the plastically deformed materials are annealed, either partially recrystallized or fully recrystallized structures can be obtained, and both can exhibit superior tensile strength and ductility [25,26]. Previous study indicates that the fatigue strength of the partially recrystallized Cu alloy depends strongly on the recrystallized grain size irrespective of the volume fraction of recrystallized region, and thus the recrystallized grains should be kept fine in order to improve the fatigue strength [27]. In contrast, fully recrystallized materials with homogeneous UFG structures can exhibit superior fatigue properties, since the high-density defects have been drastically removed after heat treatments [16]. These fully recrystallized UFG materials have the characteristics of low dislocation density, uniform grain size distribution and high fraction of high-angle grain boundaries; moreover, they can possess high yield strength as well as adequate uniform elongation, indicating that they draw excellent strain-hardening capability [25,28,29]. Recent studies demonstrate that fully recrystallized specimens with UFG structures can be obtained in HEAs [7,30,31] even after simple cold rolling and subsequent annealing process. It is thus desirable to investigate the fatigue properties of these UFG materials with fully recrystallized structures.
Section snippets
Experimental
In this work, a CoCrFeMnNi HEA solid solution with simple face-centered cubic (FCC) structure was selected to investigate the fatigue properties. The cast CoCrFeMnNi HEA ingot with a diameter of 110 mm was fabricated by a novel magnetic levitation melting technique. The ingot was solution treated at 1100 °C for 2 h, hot forged at 1000 °C to rods with a diameter of 30 mm and cold rolled to sheets with a final thickness of 2.5 mm. Detailed information on the melting and processing procedures can
Microstructures of the CG and UFG HEAs
Microstructures of the CG ( = 30 μm) and UFG ( = 0.65 μm) CoCrFeMnNi specimens are shown in Fig. 1. Fig. 1(a) exhibits the inverse pole figure (IPF) map of the CG HEA, which consists of equiaxed grains and profuse growth TBs. Fig. 1(b) demonstrates the misorientation angle distribution, which roughly follows the theoretical misorientation angle distribution of randomly oriented grains except the burst at the misorientation angle of 60°, which is related to the annealing TBs. The fraction of
Conclusions
Fully recrystallized CoCrFeMnNi specimens with UFG ( = 0.65 μm) and CG ( = 30 μm) structures were fabricated by simple cold rolling and annealing processes, and the tensile properties and high-cycle fatigue properties were investigated. The main results can be summarized as follows:
- (1)
The UFG HEA can exhibit superior microstructural stability and no grain coarsening was observed during fully reversed high-cycle fatigue tests.
- (2)
The fatigue strength can be ameliorated by refining the grain size. The
Acknowledgments
Y.Z. Tian acknowledges the IMR Foundation for “Young Merit Scholars”. This work was supported by the National Natural Science Foundation of China (Nos. 51501198 and 51331007).
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