Microstructure and the properties of FeCoCuNiSnx high entropy alloys

doi:10.1016/j.msea.2012.03.080

Materials Science and Engineering: A

Volume 548, 30 June 2012, Pages 64-68

https://doi.org/10.1016/j.msea.2012.03.080 Get rights and content

Abstract

FeCoCuNiSn_x high-entropy alloys (x denotes the adding the elements amount in atomic percentage) are prepared by an arc furnace. Their microstructure and mechanical properties are investigated. The results show that the alloys have a single FCC solution when Sn content is small, the microstructure of the alloys with increasing Sn content is FCC solution and Cu₈₁Sn₂₂ intermetallic compounds. The alloys possess the high strength and the plasticity. When Sn content is between 0.05 and 0.07, the maximum elongation strain and the maximum tensile strength can reach 19.8% and 633 MPa, respectively. The adding of Sn leads to the increase of tensile strength.

Highlights

► Based on a new alloying design idea, new FeCoCuNiSn_x alloys are prepared. ► The crystal structure of alloys is a single FCC solution when Sn content is small. ► The elongation strain and tensile strength of the alloy reach 19.8% and 633 MPa.

Introduction

The general strategy for developing alloys is to select one or two elements as principal components for primary properties and other minor elements for the acquisition of definite microstructure and properties. It is known that when the principal element has a high solubility; the strength of the alloys can increase due to the solution hardening. If the solution structure is kept, the alloy would have a better combination of strength and plastics. As the amount of alloying element increases, intermetallic compounds, however, are typically formed in the alloys due to the limited solid solubility, which results in enhanced strength at the expense of accompanying embrittlement. To realize the goal of a solution structure, a new alloy design concept, the high entropy (HE) alloy was explored by Yeh et al. based on the thermodynamic consideration, which consists of a simple FCC or BCC structure [1], [2], [3], [4], [5]. Following Boltzmann's hypothesis on the relationship of entropy complexity [2], the configurational entropy change, ΔS_conf, during the formation of a solid solution from n elements with equimolar fractions, may be calculated from the following equation: $Δ S_{conf} = - R ln (\frac{1}{n}) = R ln (n)$ where R is the gas constant. When n = 5, ΔS_conf = 1.61R, which approaches the magnitude of the melting entropy of the most intermetallic compounds (about R-2R). As results, intermetallic compounds are absent and a single solid solution is built with structures of a single FCC, a single BCC or FCC + BCC [2], [3], [4], [5]. Due to the above structural and compositional characteristics, the alloys are ductile and working-hardenable, and have high strength [3], [4], [5], [6]. Thus, the alloys have potential applications as high-temperature structure materials and work tools. Also, due to these possible applications, the properties of the alloys, such as strength, hardness, wear resistance, were investigated [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. To improve the properties of the alloys, the effects of alloying of B, Cr, Ti, Fe, Al and V elements on the structure and properties have been considered [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

As everyone knows, Sn has a low melting point of 505.06 K, but a high boiling point of 2876 K. Differing from other elements, there is a wide difference of about 2400 K between the two temperatures. Sn is very soft at room temperature, and has low solubility with other elements, therefore, when a small quantity of Sn adds in to the alloys, it will be segregated to grain boundary, accordingly improve the plastic property of the high entropy alloy [18].

In this contribution, FeCoCuNiSn_x alloys with different Sn contents are fabricated. The corresponding microstructures and mechanical properties are measured, the effects of Sn content on the alloy microstructure and tensile properties are investigated.

Section snippets

Experimental methods

FeCoCuNiSn_x alloys are prepared by a vacuum furnace under argon atmosphere. The purity of all the raw elemental metal is above 99.9%. The rod ingot of the alloy with a diameter of 6 mm is obtained in a cold copper hearth. The corresponding cooling rate is about 10³ K/s. Table 1 shows the composition of the alloys. The phase structure analysis of the samples was made by Rigaku D/max 2500 X-ray diffractometer at 50 kV and 250 mA. Scanning angles ranged from 20 to 120° with a scanning rate of 5°/min.

Results and discussion

Fig. 1 shows X-ray analysis results of the alloys with different Sn contents. When x = 0, the alloy is a single FCC crystal structure. Cu₈₁Sn₂₂ phase appears with increasing of Sn contents. Because Cu₈₁Sn₂₂ is hard and brittle phase, which will decrease the tensile strength and the toughness of the alloys. Fig. 2 shows the lattice constant of the alloys with different Sn contents. It can find that the lattice constant increases with the increasing Sn contents from 0.3586 nm to 0.3606 nm because Sn

Summary

FeCoCuNiSn_x high entropy alloys were prepared with a FCC crystal structure when Sn content is less than 0.05, Cu₈₁Sn₂₂ phase appears when Sn content is more than 0.05. The dendritic area is rich Fe and Co and the interdendritic area is rich Cu and Sn, the distribution of Ni is more uniform in dendritic region and interdendritic region. When Sn content is between 0.05 and 0.07, the alloys possess the high strength and the plasticity, the maximum elongation strain and the maximum tensile strength

Acknowledgements

The authors gratefully acknowledge the financial supports from NNSFC (Grant No. 50571040) and National Foundation of Doctoral Station (Grant No. 20100061110019).

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Microstructure and the properties of FeCoCuNiSnx high entropy alloys

Abstract

Highlights

Introduction

Section snippets

Experimental methods

Results and discussion

Summary

Acknowledgements

Corros. Sci.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Lett.

Mater. Sci. Eng. A

J. Alloys Compd.

Mater. Lett.

J. Alloys Compd.

J. Alloys Compd.

Microstructure and the properties of FeCoCuNiSn_x high entropy alloys