Elsevier

Vacuum

Volume 192, October 2021, 110481
Vacuum

Short communication
Double-stage hardening behavior of a lightweight eutectic high entropy alloy in the course of low cycle fatigue

https://doi.org/10.1016/j.vacuum.2021.110481Get rights and content

Highlights

  • Unique crack growth behavior during room temperature fatigue of high entropy alloy.

  • Crack trapping between dendritic branches and retardation of the growth stage.

  • Double-stage cyclic hardening in tensile and compressive mode of low cycle regime.

  • Fragmentation and distribution of dendrite well resist against crack propagation.

  • Higher stress levels during cyclic loading compared with monotonic one.

Abstract

A unique crack growth behavior was detected during room temperature cyclic loading of a eutectic high entropy alloy, which caused the occurrence of two-stage cyclic hardening under the low cycle regime in tensile and compressive half cycles. This was attributed to the crack trapping between the dendritic branch, which caused retardation of the growth stage until the crack sheared and pass through the dendrite. The mechanical fragmentation of the dendrite and distribution through the microstructure could effectively resist crack propagation. This led to higher stress levels during cyclic loading compared with monotonic loading under both tensile and compressive modes of deformation.

Introduction

High-entropy alloys (HEAs), which usually consist of five or more elemental constituents with a nearly equal ratio, possess attractive properties such as high hardness, oxidation/corrosion resistance, and outstanding structural stability. In this respect, they have attracted extensive attention from theoretical and industrial points of view. Interestingly, the properties of these compositionally complex alloys can also be tuned in a wide range by changing the type and concentration of their constituents [1,2]. The presence of casting defects within the developed microstructures, such as micro-segregation or cavities, may serve as stress concentration regions and lead to the formation of discontinuities and micro-crack, thereby deteriorating the mechanical strength and ductility values [3,4].

To overcome such problems, the eutectic HEAs alloys have been developed in which owing to the occurrence of eutectic isothermal transformation and elimination of solidification temperature range, both segregation and shrinkage cavity can be alleviated and cast-ability is improved [3]. The concept of eutectic high entropy alloy (EHEA) was first introduced by Lu et al. through considering the composition of AlCoCrFeNi2.1, consisting of both FCC and BCC phases arranged as eutectic structure [5]. The majority of previous researches regarding the deformation behavior of high entropy alloys have focused on the microstructure evolution in the course of monotonic loading. In this respect, Hemphill [3] showed that in Al0.5CoCrCuFeNi alloy the aluminum-oxide-rich particles have been formed during the melting and solidification and subsequent homogenization processes, and provide nucleation sites for micro-cracks due to the stress concentration at the particles-matrix interfaces.

It is worth to mention that the damages may be more seriously accumulated through the cyclic fatigue loading, which is known as a crucial property for any structural component in servicing condition. However, only a few studies have been directed to investigate the fatigue behavior of HEAs. Apparently, in the case of EHEAs, reduction of the casting defects can significantly affect the mechanical properties, especially in the course of cyclic loading where the crack initiation and crack propagation are more prominent [6]. In addition, the eutectic cast structure characteristics (such as dendrite morphology and dendrite arm spacing) would play an important role in determining the fatigue behavior of EHEAs. In this context, the present work has been steered toward studying the effect of casting structure characteristics resulting from eutectic solidification and on the cyclic behavior of the susceptibility for crack initiation and propagation with special emphasizing on the effect of dendritic morphology.

Section snippets

Materials and methods

The master alloy of AlCoCrFeNi2.1 was prepared from commercially pure elements (Al, Co, Ni: 99.8 wt %; Cr, Fe: 99.5–99.5 wt %). The experimented material was received in vacuum arc remelted condition, the chemical composition of which is listed in Table 1. The fatigue specimens were prepared according to ASTM standard-E606 [7] in flat sheet geometry and mirror-polished to remove any stress concentration. The material was subjected to a fully reversal (R = −1) strain-controlled push-pull fatigue

Result and discussion

Fig. 1a shows the XRD pattern of the initial microstructure verifying the existence of two constituent FCC and BCC phases with L12 and B2 order phase structure, respectively. The corresponding phase map (Fig. 1b) also reveals the matrix is composed of both FCC and BCC phases. The volume fractions of FCC and BCC phases were measured to be 77% and 23 %, respectively. The BCC phase is mostly located in inter-dendritic regions. In conventional casting structures, the formation of dendrites is

Conclusion

The room temperature cyclic loading behavior of a eutectic high entropy alloy was investigated under the low and high cycle regimes. Owing to the specific pattern of eutectic solidified microstructure and dendritic morphology, a unique crack growth behavior was detected, which caused cyclic hardening in tensile and compressive half cycles. Under the low strain amplitude, the expected fatigue limit was achieved and the nucleation-controlled process was led to the development of fish-bone type

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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