Microstructure and mechanical properties of multiprincipal component CoCrFeNiMo_x alloys

Abstract

Four multiprincipal component CoCrFeNiMo_x (x = 0, 0.3, 0.5, and 0.85, in molar ratio) alloys were synthesized by vacuum arc-melting in a copper mould. The effects of variations in the amount of Mo on the microstructure and mechanical properties were investigated. The CoCrFeNi alloy exhibits a single face-centered cubic solid solution, whereas a (Cr,Mo)-rich σ phase is observed in the face-centered cubic matrix after the addition of Mo into the alloy. A (Mo,Cr)-rich μ phase appears on the fringes of the σ phase in the CoCrFeNiMo_0.85 alloy. The hardness of face-centered cubic matrix and the σ-phase content increase with increasing Mo concentration, resulting in an increase in the alloy hardness, from HV135 to HV420. The compressive strength of the alloy improves as the Mo content increasing, but a simultaneous degradation of the ductility is observed. Accordingly, as the Mo content increases from 0 to 0.85, the yield stress and compressive strength rise from 136 MPa and 871 MPa to 929 MPa and 1441 MPa, respectively, and the fracture strain lowers from 75% to 21%. The solid-solution strengthening of the face-centered cubic matrix and the formation of the σ/or σ + μ phases are the two main reasons for the strengthening of the alloy.

Introduction

Alloys without a major element—that is, alloys constructed using at least five principal elements—were first explored by Yeh et al. since 1996 and published in 2004 [1], [2], [3]. Such an alloy, termed a high-entropy alloy, is defined as containing n principal elements (typically n≧ 5), each with concentrations between 5 at.% and 35 at.%. Because of the high mixing entropy ΔS_mix of alloys with multi-principal elements, the microstructure of high-entropy alloys usually displays some simple solid solutions during solidification, instead of complex phases or inter-metallic compounds [1], [2], [3], [4], [5], [6], [7], [8], [9]. The Al–Co–Cr–Cu–Fe–Ni alloy system was the first to be studied in terms of its microstructure and mechanical properties, and has since been widely investigated [6], [10], [11], [12], [13], [14]. Despite the segregation of Cu in interdendritic regions, the crystal structure of the alloys displayed simple face-centered cubic (FCC) and/or body-centered cubic (BCC) solid solutions, depending on the concentrations of each principal element. Al strongly promotes the formation of BCC phases; a study of Al_xCoCrCuFeNi alloys showed that the crystal structure was transformed from FCC to BCC as the Al content was increased [10].

Recently, to expand and enrich the field of multiprincipal component alloys, some researchers have studied the microstructure and compressive properties of Mo-containing alloys such as VNbMoTaW, AlCoCrCuFeNiMo_x, and AlCoCrFeNiMo_x[15], [16], [17]. These alloys generally showed BCC or BCC + α (unidentified Mo-rich phase) microstructure at room temperature, and exhibited high compressive strengths but inferior fracture strains, with values near 3%. These poor fracture strain properties resulted from the presence of the hard BCC solid solution, reducing the ductility of the alloy. By contrast, if a high-entropy alloy is formed with a strong but ductile FCC matrix and a hard secondary phase, a good ductility can be approached although some strength may be sacrificed.

The mechanical properties of FCC high-entropy alloys are seldom reported. Motivated by this fact, we did not include the strongly BCC-forming Al in the alloy studied here, but instead prepared CoCrFeNiMo_x alloys. The effects of variations in the Mo content on the microstructure and properties of the alloys were investigated in detail.