Microstructural response of He+ irradiated FeCoNiCrTi0.2 high-entropy alloy
Introduction
High-entropy alloys (HEAs), as emerging alloys composed with four or more principal elements in equal-molar or near equal-molar fraction [1], open a new way to design alloys and draw great attention in material science community. Due to their excellent mechanical properties [2,3], great corrosion resistance [4], strong oxidation resistance [5,6], and promising radiation damage tolerance [[7], [8], [9], [10], [11], [12]], HEAs have been proposed as the candidate for structural materials in advanced nuclear systems. Currently, the radiation responses of HEAs under heavy ions or self-ions bombardment have been well studied [[7], [8], [9], [10], [11], [12]]. The great phase stability of AlxCoCrFeNi HEAs under a very high damage level of ∼100 dpa has been demonstrated by Xia et al. [7], and it was explained by the unique effect of HEAs, i.e., high mixing entropy. Kumar et al. found that [8] FeNiMnCr HEA has better radiation resistance than FeCrNi austenitic stainless steels under Ni ion irradiation up to 10 dpa in a temperature range from 400 °C to 700 °C, by showing smaller dislocation loops and undetectable voids. Moreover, Lu et al. found that [9] with increasing compositional complexity in single-phase concentrated solid-solution alloys, the growth of the radiation-induced defects was delayed and the radiation-induced segregation (RIS) was significantly suppressed. He et al. [13] also investigated the elemental segregation in several HEAs subjected to 1250 kV electron irradiations, and they found that the actively segregating elements are alloy-specific, which could be rationalized based on the atomic size difference and enthalpy of mixing between the alloying elements. Recently, Chen et al. [14] and Yan et al. [15] investigated the helium behavior in the FeCoNiCr HEA from different perspectives, and they both found that the FeCoNiCr HEA shows a better bubble formation resistance. For the underlying mechanism of HEAs' radiation tolerance, Zhang et al. [11] conducted ab initio electronic structure calculations and physical property measurements, and found that the chemical complexity can cause a substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivities, which subsequently suppress the evolution of defects.
Most of above-mentioned researches are focused on the effect of compositional complexity on the radiation resistances of HEAs. The correlation between their intrinsic properties and the evolution of radiation-induced defects is still unclear. For example, the low stacking fault energy (SFE) of some HEAs has been deemed as the culprit of their unique deformation mechanism [16], whether it will make a different microstructural response in radiation circumstance? Moreover, the microstructural response of HEAs under He+ irradiation has not been fully studied, which is very important for HEAs' application in nuclear system [17,18]. Thus, from both scientific interests and potential applications, we added 0.2 M ratio of Ti in the FeCoNiCr matrix to form a new solid solution HEA of FeCoNiCrTi0.2, which is expected to have a lower SFE than the parent alloy. Its microstructural evolution under He+ irradiation at elevated temperature was thoroughly investigated. Our results show that the behavior of radiation-induced dislocation loops was significantly influenced by SFE.
Section snippets
Experimental
The alloy with a composition of FeCoNiCrTi0.2 was produced by arc melting Fe, Co, Ni, Cr and Ti metals with high purity (>99.9%) in an argon atmosphere. Then the drop-casted sample was homogenized at 1373 K for 5 h and then water quenched to get a solid solution state. As a novel material, the crystalline structure of the pristine FeCoNiCrTi0.2 HEA was firstly examined. Fig. 1 shows the X-ray diffraction (XRD) pattern and its microstructure characterized by scanning electron microscope (SEM).
Results and discussions
It has been demonstrated that the addition of Ti in the FeCoNiCr HEA usually introduces the intermetallic phase with L12 structure at intermediate temperatures [21,22]. Therefore, before the microstructure characterization of radiation-induced defects, we conducted a series of selected-area electron diffraction (SAED) experiments along the irradiated depth to detect the phase stability of HEA during irradiation. An inset image in Fig. 2 (b) displays a SAED pattern (taken along the zone axis of
Acknowledgement
This work was supported by Hong Kong Research Grant Council (RGC) [Grant No. CityU 11212915].
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