Twinning engineering of high-entropy alloys: An exercise in process optimization and modeling

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Abstract

In a bid to improve the mechanical properties of high-entropy alloys, particularly, their strain hardening capability, we adapted the time-proven concept of ‘twinning engineering’, developed in the context of TWIP steels, to twinning-assisted high-entropy materials. The strategy chosen involved a two-step thermomechanical processing that consisted of low-temperature pre-straining and subsequent annealing. This approach was trialled on CoCrFeMnNi as an exemplary high-entropy alloy. The annealing conditions selected ensured that the deformation twins generated under low-temperature deformation were retained, whilst the dislocation density was recovered. The viability of this strategy was convincingly confirmed for room temperature deformation of the alloy. A constitutive model accounting for the effect of the pre-straining induced deformation twins was proposed. It was shown to provide a reliable description of the low-temperature and room-temperature deformation behavior of CoCrFeMnNi when deformation twins are involved.

Introduction

The role of deformation-induced twinning in the mechanical response of face-centered cubic (FCC) metals and alloys is now well established. It is known that deformation twins (DTs) act as strong obstacles to dislocation motion, thus reducing the mean free path of dislocations. This mechanism is often referred to as the dynamic Hall-Petch effect [1,2]. Most recently, DTs have been utilized to strengthen FCC high-entropy alloys (HEAs) at both room temperature [[3], [4], [5]] and cryogenic temperatures [[6], [7], [8], [9]]. In previous work [[10], [11], [12]], an interesting metallurgical route, which was initially proposed for twinning-induced plasticity (TWIP) steels [13,14], was applied to enhance the yield strength of an FCC CoCrFeMnNi high-entropy alloy. The specific approach adopted DTs for this HEA involves two steps [12]: (1) pre-straining at 77 K to generate DT and (2) annealing at 500 °C for 1 h to reduce the dislocation density by static recovery. The philosophy behind this treatment was that in this way the material would be provided with strain hardening capability under subsequent deformation. Ideally, the DTs generated at 77 K should be retained. Indeed, it was found [12] that during the static recovery at 500 °C, the DTs remained thermally stable. As a result, the CoCrFeMnNi alloy that underwent this processing schedule exhibited a high yield strength and substantial strain hardening at room temperature, surpassing the conventional grain boundary strengthening [12].

While this processing strategy is promising, there is still a challenging task of optimizating it in order to get the greatest possible benefits in terms of the mechanical performance of HEAs. Looking again at TWIP steels, we note that DTs were found to be thermally stable up to ~625 °C [13]. Notably, Bouaziz et al. [13,14] reported that an excellent combination of yield strength and uniform elongation of TWIP steels was achieved through various processing routes combining cold-rolling and recovery heat treatment. Translating that to the CoCrFeMnNi alloy, one may expect that annealing at a temperature in excess of 500 °C with the aim of recovering stored dislocations, whilst retaining DTs may be promising. It is anticipated that the pre-existing nano-sized deformation twin boundaries generated as a result of the proposed twinning engineering can substantially improve the strength and the strain hardening capability of the CoCrFeMnNi alloy.

In the present study, we attempted to use this kind of ‘twinning engineering’ for CoCrFeMnNi as a popular exemplar of HEAs. Thermal stability of DTs and the mechanical properties of the alloy with different processing histories were systematically investigated to optimize the heat treatment regime with respect to its tensile properties. A constitutive model describing the microstructural evolution and the associated strain hardening behavior of the CoCrFeMnNi alloy following the thermomechanical treatment identified as an optimum one was also established. The results of the experimental investigation and modeling of the deformation behavior of alloy CoCrFeMnNi are presented below. We will show that the model provides a faithful description of the mechanical behavior of the material after the proposed processing steps. It is thus believed that its use will facilitate design of HEAs with the desired mechanical properties.

Section snippets

Experimental procedures

An ingot of a Co20Cr20Fe20Mn20Ni20 (atomic percent, at%) alloy was cast via vacuum induction melting in an argon atmosphere. The ingot was homogenized at 1,100 °C for 6 h in an argon atmosphere, followed by water quenching. To reduce the surface roughness and remove oxides, the homogenized ingot was mechanically milled before cold-rolling. The alloy was cold-rolled from an initial thickness of 7 mm down to 1.5 mm. The cold-rolled plate was annealed at 800 °C for 1 h in an argon atmosphere, and

Results and discussion

Fig. 1(a and b) show the microstructure of the annealed alloy. Its average grain size is 11.54 ± 4.19 μm. The kernel average misorientation (KAM) map presented on Fig. 1(b) demonstrates that the alloy is fully recrystallized. The recrystallized fraction was determined using a KAM value of less than 0.5 [15]. Using this standard [15], the procedure returned a value of 95% for the recrystallized fraction. Accordingly, the material was nominally considered as fully recrystallized. The average KAM

Conclusions

In summary, an optimization of two-stage thermomechanical processing of the equiatomic Co20Cr20Fe20Mn20Ni20 HEA was carried out to test a twinning engineering strategy for improving the mechanical properties of HEAs. The strategy included generating deformation twins at low temperature (77 K) deformation followed by annealing. The optimization step targeted the selection of the annealing treatment regime achieving excellent mechanical properties. The microstructural evolution of the alloy and

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Jongun Moon: Conceptualization, Methodology, Data curation, Formal analysis, Investigation, Visualization, Writing – original draft. Olivier Bouaziz: Conceptualization, Methodology, Formal analysis, Validation, Supervision, Writing – review & editing. Hyoung Seop Kim: Conceptualization, Validation, Resources, Supervision, Project administration, Funding acquisition, Writing – review & editing. Yuri Estrin: Conceptualization, Methodology, Formal analysis, Validation, Writing – review & editing.

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

This work was supported by (1) Future Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT [2016M3D1A1023384], (2) Basic Science Research Program through the NRF of Korea funded by the Ministry of Education [2021R1A6A3A03044109], and (3) the NRF grant funded by the Korea government (MSIP) [NRF-2021R1A2C3006662].

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