Effects of Al addition on the microstructure and mechanical property of Al_xCoCrFeNi high-entropy alloys

Abstract

A five-component Al_xCoCrFeNi high-entropy alloy (HEA) system with finely-divided Al contents (x in molar ratio, x = 0–2.0) was prepared by vacuum arc melting and casting method. The effects of Al addition on the crystal structure, microstructure and mechanical property were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vickers hardness tester. The as-cast Al_xCoCrFeNi alloys can possess face-centered cubic (FCC), body-centered cubic (BCC) or mixed crystal structure, depending on the aluminum content. The increase of aluminum content results in the formation of BCC structure which is a dominant factor of hardening. All the BCC phases in the as-cast alloys have a nano-scale two-phase structure formed by spinodal decomposition mechanism. The Al_0.9CoCrFeNi alloy exhibits a finest spinodal structure consisting of alternating interconnected two-phase microstructure which explains its maximum hardness of Hv 527 among the alloys. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants, overall hardness demonstrate that the formation of a single FCC solid solution should have Al addition <11 at.% and the formation of a single BCC solid solution requires Al addition at least 18.4 at.% in the Al_xCoCrFeNi system.

Introduction

The conventional strategy for developing alloys is to select one or two elements as main component for primary properties and other minor elements as alloying addition for modifying microstructure and properties [1], [2]. This traditional alloy concept has led to most of traditional alloys which are composed of single principal metal element, such as Al-, Cu-, Mg-, Ni-, Fe-(steel), Ti-, and Sn-based alloys. However, this conventional approach also restricts the number of alloys that can be studied and utilized. In view of this limitation, a novel alloy approach coined as high-entropy alloys (HEAs) was proposed by Yeh [3], [4], [5] to broaden the alloy field. A HEA was originally defined as an alloy composed of at least five principal metallic elements, each ranging from 5 to 35 at.% [5]. In the past decade, many high-entropy alloy systems composed of five to nine metal elements e.g., Al_xCoCrCuFeNi [6], [7], [8], [9], [10], Al_xCoCrFeNiTi [11], Ti_xCoCrCuFeNi [12], CoCrMnNiFe [13], CuNiAlCoCrFeTiMo [14], [15], AlCoCrCuFeNiSi [16], Al_xTiVCrMnFeCoNiCu [17], MoNbTaW, MoNbTaVW [18], [19], and Al_xCrCuFeNi [20] have been studied and characterized for their microstructures and properties.

Traditional metallurgical theory suggests that multiple alloying elements will lead to the formation of many intermetallic compounds which not only cause brittleness but also become difficult to analyze [21]. However, high-entropy alloys tend to enhance the formation of simple solid solution structures (FCC, BCC or mixed) [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [22] rather than many complex phases due to high mixing entropy effect [3], [4], [5]. It is this effect to make HEAs abundant of possible opportunities to have promising properties such as high elevated-temperature strength [7], [18], [19], [23], and excellent wear [7], [8], [23], [24], corrosion [25], [26] and oxidation resistance [21].

Recently, a five-component HEA system of Al_xCoCrFeNi alloy [27], [28], [29], [30] modified by excluding Cu element from Al_xCoCrCuFeNi alloy system was synthesized and characterized for its microstructures, mechanical properties, work-hardening and age-hardening behavior. However, the relationship between microstructures and mechanical properties was less mentioned and explained. To understand the effects of Al addition in the Al_xCoCrFeNi alloy system (x value in molar ratio, from 0 to 2.0), the crystal structure, casting microstructure and mechanical property of fourteen alloys were investigated using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Vickers hardness tester. The resultant microstructures and mechanical properties due to the variation of Al content are discussed, which provides essentials of this HEA system for further research and applications.