Effect of crystal structure and grain size on corrosion properties of AlCoCrFeNi high entropy alloy

https://doi.org/10.1016/j.jallcom.2020.158056Get rights and content

Highlights

  • Correlation between phase fraction and corrosion resistance is elucidated.

  • Phase fraction was controlled by sequential alloying strategy.

  • Optimum BCC:FCC ratio of ~3:2 was observed for FeCrNiAlCo sequence.

Abstract

High entropy alloys (HEAs) have drawn considerable attention owing to their unique properties such as high fracture toughness, good strength–ductility combination and enhanced corrosion resistance. In this study, the corrosion resistance dependence on the crystal structure and grain size of AlCoCrFeNi HEA is investigated. AlCoCrFeNi HEA with different mixture of body centered cubic (BCC) and face centered cubic (FCC) phases is produced using sequential alloying and tested for corrosion resistance in 3.5 wt% NaCl solution. CoNi+Fe+Cr+Al (84% BCC), FeCr+Ni+Al+Co (62% BCC) and AlNi+Co+Cr+Fe (38% BCC) alloy sequences have corrosion potential of −454, −299 and −524 mV (vs. SCE), and corrosion current of 14, 0.4 and 12 µA, respectively. FCC is a tight binding lattice with higher packing fraction than BCC which made it better for corrosion resistance, but FCC is rich in elements like Co, Ni, and Fe which are easily corroded. These two competing effects lead to a nearly optimum corrosion resistance for FeCr+Ni+Al+Co alloy sequence with 62% BCC and 32% FCC. It is also observed that an increase in grain size improves corrosion resistance. The influence of different chemical elements, crystal structure and microstructure (coarse vs. nanocrystalline) on corrosion resistance is discussed.

Introduction

High entropy alloys (HEAs) are multicomponent alloys with at least five elements, which are mixed in equiatomic or near equiatomic proportions [1]. HEAs tend to form simple solid solutions of all their constituent elements because of their high configurational entropy of mixing [2], [3]. HEAs have shown good strength–ductility combination [4], [5], [6], [7], high fracture toughness [8], [9], enhanced corrosion and oxidation resistance [10], [11], tribological properties [12], [13], and magnetic properties [14]. AlCoCrFeNi is one of the most commonly studied HEAs, with enhanced structural [15], [16] and corrosion properties [17], [18], [19], [20]. The corrosion resistance of coarse-grained cast AlCoCrFeNi alloy has been studied in NaCl and H2SO4 solutions [21], and microcrystalline AlCoCrFeNi coatings have been shown to improve the corrosion resistance of 304 stainless steel [18].

The corrosion resistance of an alloy depends on several material properties including chemical composition [22], microstructure like crystal structure and grain size [23], and macrostructure like porosity and surface finish [24], [25]. The corrosion resistance of an alloy is often improved by changing the chemical composition of the alloy [26]; however, this also leads to changes in the microstructure, and then deconvoluting the effects becomes difficult [27]. In microcrystalline AlxCoCrFeNi alloy, the corrosion resistance was improved by reducing the Al content, however, this also led to changes in the crystal structure of the alloy which potentially affects the behavior as well [21]. Furthermore, changing the chemical composition can have drastic effects on other material behaviors [28]. This motivates us to study and improve the corrosion resistance of multicomponent alloys by keeping the chemical composition fixed and changing the microstructural parameters namely, crystal structure and grain size.

We had recently proposed a new strategy of multicomponent alloy synthesis, termed sequential alloying in which we can change the crystal structure without impacting the overall composition of the alloy. Sequential alloying explores the path dependence of alloy formation during mechanical alloying (MA) [29]. In sequential alloying, first a binary alloy of any two constituent elements of a multi-component alloy is formed, and then the remaining elements are alloyed in subsequent steps until the final multicomponent alloy is formed. AlCoCrFeNi alloy synthesized using MA forms both body centered cubic (BCC) and face centered cubic (FCC) phases [30], [31], [32]. We have demonstrated that varying the sequence in which elements are mixed allows for different fractions of BCC and FCC phases to be obtained in AlCoCrFeNi [29]. Thus, sequential alloying provides a salient advantage in that alloys of the same chemical composition, but with different phase fractions can be formed. In addition to the crystal structure, microstructural property like grain size also impacts the behavior of an alloy [23]. The grain size of an alloy can be controlled through different synthesis routes [33]. For e.g., MA generally yields a nanocrystalline alloy whereas vacuum arc melting results in a coarse-grained alloy [34].

A systematic investigation of the combined effects of the crystal structure (BCC vs. FCC) and grain size (coarse vs. nanocrystalline) on the corrosion properties of HEA has not been reported in the literature. In this work, to study the effect of crystal structure, we synthesized three sequences of AlCoCrFeNi alloy starting from different initial binary systems, namely AlNi (B2 binary), FeCr (BCC binary), and CoNi (FCC binary) [35]. For comparison, we have also synthesized an AlCoCrFeNi alloy using conventional MA, wherein all elements are mixed together in one step and milled for 10 h. Milled powders were then sintered using spark plasma sintering (SPS). SPS was used to attain high-density pellets and retain the nanocrystallinity of milled powders [36]. To study the effect of grain size, we synthesized AlCoCrFeNi alloy through vacuum arc melting to compare its corrosion properties with nanocrystalline AlCoCrFeNi.

We will use the following notation systems henceforth: for all sequential alloys, the order of elements in an alloy notation indicates the sequence of elemental addition in the alloy formation. For example, ‘FeCrNiAlCo’ refers to the alloy formed by the sequence FeCr+Ni+Al+Co. The alloy formed using conventional MA is termed the ‘base alloy’, whereas the alloy prepared through vacuum arc melting is termed the ‘cast alloy’.

Section snippets

Experimental details

MA was carried out in a high-energy planetary ball mill (Fritsch Pulverisette P-5) at 300 rpm with a ball to powder ratio of 10:1 using WC vials and balls, and toluene was used as a process control agent. For sequential alloys, we first milled the as-received elemental powders (>99.5%) until the grain size of each element was< 50 nm [29]. Then we mixed two of these nanocrystalline elemental powders to form a binary alloy, and the remaining nanocrystalline elements were added stepwise. During

Microstructure

All the as-milled powders exhibited a BCC+FCC mixture along with WC contamination and FeCrNiAlCo also showed Cr7C3 impurity phase (see Supplementary Information). The relative intensity ratio (RIR) was used for the calculation of phase fractions. RIR was calculated from the integrated intensity of the deconvoluted highest-intensity BCC and FCC peaks. We had shown in our previous work that the RIR method of phase fraction calculation gives results in close agreement with Rietveld refinement for

Synthesis

The phase fraction of the as-milled sequentially alloyed powders is strongly affected by the starting binary alloy. AlNiCoCrFe alloy had the highest BCC content as it started with AlNi binary. AlNi is a B2 binary system and has the highest negative enthalpy of mixing (ΔHmix = −22 kJ/mol) among all binaries [44]. Starting with this strong initial binary system led to the final alloy with the highest BCC content. CoNiFeCrAl alloy starting from CoNi binary has the lowest BCC content as CoNi is a

Conclusions

Sequentially alloyed AlNiCoCrFe, CoNiFeCrAl, and FeCrNiAlCo, base alloy and cast alloy were synthesized. Corrosion resistance dependence on microstructural properties like grain size, dislocation density, and crystal structure was studied. Corrosion resistance improved with increasing grain size and decreasing dislocation density. The base alloy had a larger grain size and showed lower Icorr values compared to CoNiFeCrAl alloy. The cast alloy had excellent corrosion resistance due to its large

CRediT authorship contribution statement

Abhinav Parakh: Conceptualization, Methodology, Writing - original draft preparation. Mayur Vaidya: Conceptualization, Formal analysis. Nitish Kumar: Investigation. Raghuram Chetty: Supervision, Writing - review & editing. B.S. Murty: Resources, Supervision, 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

The authors would like to thank Dr. Lakshman Neelakantan, Department of Metallurgical and Materials Engineering, Indian Institute of Technology (IIT) Madras, for the useful discussions. The authors would also like to express their gratitude to Mr. Anil Prasad, Mr. K. Guruvidyathri K, Mr. Mohan Muralikrishna, Mr. Anirudha Karati, and Mr. Rahul John for their assistance in carrying out some of the experiments and for the discussions. The authors would like to thank IIT Madras for financial

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