Low-cycle fatigue behavior and surface treatment of a twinning-induced plasticity high-entropy alloy

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Abstract

The low-cycle fatigue life and cyclic deformation behavior of a metastable high-entropy alloy were investigated in this study. Additionally, the effects of the ultrasonic nanocrystal surface modification (UNSM) process on tensile properties and fatigue life were evaluated. Heat treatment following the cold rolling of Fe40Mn40Co10Cr10 alloy plates resulted in the formation of a coarse grain size of 46.7 ± 19.6 μm. Additional mechanical twins were activated during tensile testing compared to the equiatomic CoCrFeMnNi alloy, leading to increased elongation. However, mechanical twins in cyclic loads appear only at high strain amplitudes. Therefore, similar to the CoCrFeMnNi alloy, the slip of dislocations was the dominant cyclic deformation mechanism and the fatigue life of the alloys were comparable. The UNSM process formed a deformed gradient layer with a thickness of approximately 200 μm on the alloy surface. In addition to mechanical twins and low-angle grain boundaries, ultrafine grains with diameters as small as 200 ± 120 nm are detectable in this layer. Although this surface treatment successfully increased the yield strength by more than 70%, fatigue crack initiation was accelerated and fatigue crack growth resistance was degraded.

Introduction

High- and medium-entropy approaches to alloy design have opened new pathways to obtain desired properties more conveniently when using various amounts of alloying elements [1,2]. The lower Gibbs free energy associated with higher configurational entropy prevents the formation of complex intermetallics and results in stable phases [3,4]. Despite the lower entropy of non-equiatomic variants of well-known CoCrFeMnNi high-entropy alloys (HEAs) compared to equiatomic variant (i.e., Cantor alloy), the former ones exhibit a strong resemblance to the latter [5,6]. Therefore, different deformation mechanisms such as transformation-induced plasticity and twinning-induced plasticity (TWIP) can be triggered using non-equiatomic HEAs. Despite the superior cryogenic fracture resistance of Cantor alloy, which is attributed to the activation of mechanical twinning [7], the high price of its alloying elements is a drawback for their use as industrial material. Fe40Mn40Co10Cr10 is a metastable HEA designed to be cheaper than Cantor alloy, but with more activation of mechanical twinning at room temperature [6,8].

Surface mechanical attrition treatment [9,10], severe shot peening [11], surface mechanical rolling treatment [12], and ultrasonic nanocrystal surface modification (UNSM) have been developed to introduce severe plastic deformation on the surfaces of materials. These surface treatment methods can significantly increase the volume fraction of mechanical twins (MTs), refine the grain size, and induce martensitic phase transformations near the surface of face-centered cubic (FCC) metals [10]. The residual stress and fine-grained structures generated on surfaces by these methods can improve strength and fatigue limits [[13], [14], [15]]. The UNSM, where a static load is imposed simultaneously with low-amplitude ultrasonic frequency vibrations, has been successful at producing ultrafine grains on the surfaces of HEAs [16,17].

To use HEAs in industrial applications, their cyclic deformation behavior and fatigue life must be investigated. Because MTs have been identified as strong barriers to fatigue crack growth [18,19], it is necessary to study the fatigue behavior of the Fe40Mn40Co10Cr10 HEA, which is also known as the TWIP HEA. Although there have been a few studies on the cyclic deformation behavior of Cantor alloy [[20], [21], [22], [23], [24]], there have been no reports on the fatigue performance of other cheaper variants. Additionally, the effects of UNSM on the low-cycle fatigue (LCF) behavior of different materials have not yet been investigated. Therefore, the strain-controlled fatigue life and effects of UNSM on the cyclic deformation behavior of the target HEA were studied and the underlying mechanisms were evaluated.

Section snippets

Experimental procedures

An ingot of Fe–40Mn–10Cr–10Co (at.%) with a weight of approximately 10 kg was produced using vacuum induction melting. A slab with dimensions of 100 × 70 × 30 mm was hot-rolled at 1100 °C through nine passes with a 10% thickness reduction in each pass to a final thickness of 12 mm. After homogenization heat treatment at 1200 °C for 2 h, cold rolling was performed through 10 passes with a 10% thickness reduction in each pass. Next, heat treatment was performed at 1100 °C for 30 min. Flat

Initial microstructure and tensile properties

The initial microstructure of the UT exhibited a uniform distribution of grain size with an average value of approximately 46.7 ± 19.6 μm (Fig. 2a) and a gradient microstructure was observed in the ST (Fig. 2b). UNSM resulted in a deformed microstructure near the surface in the ST. The cross-sectional EBSD image quality (IQ) map near the surface of the ST is presented in Fig. 2c, where various colors are used to represent boundaries with different misorientation angles. This map reveals that

Cyclic deformation behavior

The variation in the maximum stress with the number of cycles at different Δεt2 values is presented in Fig. 10. The cyclic hardening observed during the initial cycles in the UT is a typical behavior in FCC alloys and is attributed to the multiplication of primary dislocations [39]. Despite the slight softening observed in the UT following initial hardening at Δεt2= 0.85%, stability governed the cyclic behavior at Δεt2< 0.85% (Fig. 10a). The similar cyclic softening observed in the final stage

Conclusions

The cyclic deformation behavior of the metastable HEA Fe40Mn40Co10Cr10 (at.%), was studied through strain-controlled LCF tests. Additionally, the effects of UNSM on its tensile properties and fatigue life were investigated. The important conclusions drawn from this study are as follows.

  • 1

    Although a smaller amount of alloying elements led to a lower YS for the target HEA compared to the CoCrFeMnNi alloy, more activation of mechanical twinning as a result of its lower stacking fault energy resulted

CRediT authorship contribution statement

Seyed Amir Arsalan Shams: Writing – original draft, Methodology, Investigation, Formal analysis. Gyeonghyeon Jang: Investigation. Jae Wung Bae: Methodology, Writing – review & editing. Auezhan Amanov: Methodology, Writing – review & editing. Hyoung Seop Kim: Writing – review & editing, Supervision, Funding acquisition. Taekyung Lee: Writing – review & editing. Chong Soo Lee: Resources, Writing – review & editing, Supervision, Funding acquisition, Project administration.

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.

Acknowledgments

The authors appreciate Dr. Jae Nam Kim of GIFT, POSTECH for his contribution in TEM analysis.

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