Effect of μ-precipitates on the microstructure and mechanical properties of non-equiatomic CoCrFeNiMo medium-entropy alloys

Highlights

• The effect of μ-precipitates in the non-equiatomic CoCrFeNiMo alloys is discussed.

• Further addition of Mo and decrease in annealing temperature enhances the formation of μ phase.

• Formation of μ phase retards the recrystallization and grain growth by Zener pinning effect.

• The present non-equiatomic CoCrFeNiMo alloys exhibits superior tensile properties by multiple strengthening mechanisms.

Abstract

Non-equiatomic Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₅ (Mo5) and Co₁₈Cr_12.5Fe₅₅Ni₇Mo_7.5 (Mo7.5) medium-entropy alloys were synthesized by vacuum induction melting, cold rolling, and subsequent annealing treatment at various temperatures (900–1200 °C) and they were investigated to exploit the precipitation strengthening in addition to solid solution strengthening of alloys. The effect of annealing temperature and Mo content on the microstructure and mechanical properties are systematically analyzed. From the microstructural analysis, a Mo-rich μ phase is observed in the face-centered cubic (fcc) matrix. Increasing the Mo content and low annealing temperature enhance the formation of μ phase, which is consistent with the thermodynamic calculation results. The formation of μ phase effectively enhances the strength of the Mo7.5 alloy by precipitation strengthening, and suppression of grain growth and recrystallization by Zener pinning effect. These lead to superior combination of tensile strength as high as 1100 MPa and large ductility. Our results provide insights not only into μ-phase strengthening of fcc-structured alloys, but also into the future development of high-performance MEAs.

Introduction

High-entropy alloys (HEAs) and medium-entropy alloys (MEAs), consisting of multi-elements in an equiatomic or near-equiatomic composition, have attracted increasing attention due to their unusual properties compared with conventional alloys [[1], [2], [3], [4], [5]]. As a result of the high configurational entropy, these alloys tend to form a single-phase solid solution rather than intermetallic compounds. Since the first paper of HEAs by Yeh et al. [6], numerous HEAs and MEAs have been proposed with respect to their crystal structures, such as fcc-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) [[7], [8], [9], [10]]. Among them, fcc-structured HEAs and MEAs, such as CoCrFeMnNi and CoCrNi alloys, have come to the fore as candidate materials for cryogenic applications because of their outstanding combination of tensile properties and fracture toughness even down to the liquid nitrogen temperature.

However, they show relatively low yield and tensile strength at room temperature. Motivated by this weak point of fcc single phase alloys, extensive efforts have been made to effectively improve the room-temperature tensile properties of fcc-structured alloys, such as solid-solution hardening [11], grain refinement [12], deformation strengthening [13], and metastability-engineering strategy [14,15]. These efforts suggest that careful manipulation of alloying elements and composition can provide optimized microstructure for the desired properties. Along with these attempts, it has recently been found that non-equiatomic composition of some multicomponent alloy systems also exhibits a single fcc structure in spite of its relatively low configurational entropy [12,16]. Furthermore, well-tuned compositional variations from equiatomic to non-equiatomic alloy system can lead to improved mechanical properties, as suggested by Tasan et al. [16]. Zhou et al. [17] have studied the relationship between configurational entropy and composition of n-element multicomponent alloy system with one matrix element (n = 2–10). They observed that not only equiatomic ternary or quaternary alloys but also many non-equiatomic multicomponent alloys can be regarded as MEAs due to their configurational entropy between 1R and 1.5R (R is the gas constant).

As an alternative method for strengthening alloys, an appropriate compositional modification can lead to precipitation strengthening through the formation of secondary precipitates. The homogeneous distribution of hard intermetallic particles in the matrix act as effective obstacles to dislocation movement, thereby enhancing the strength of the material [18]. Different kind of topologically close-packed (TCP) phases, such as σ, μ, Laves, etc., are observed in HEAs [19]. It is well known that these intermetallic particles usually have an extremely high hardness and strengthen materials, but often cause degradation of mechanical performance due to the brittle cracking of TCP phases [20]. It is reported that the well-designed intermetallics-containing steel achieved both high strength and tensile ductility [18]. Therefore, it is necessary to manipulate the shape, dimension, and distribution of intermetallic particles for optimization of strength and ductility of materials.

Some researchers have studied the microstructure and mechanical properties of Mo-containing alloys such as VNbMoTaW [21], AlCoCrCuFeNiMo_x [22], CoCrFeNiMo_x [23]. Majority of the earlier investigations have focused on the phase evolution and mechanical properties of Mo-containing alloys. For example, the addition of Mo into the CoCrFeNi alloys led to the formation of σ and μ phases due to limited solubility of Mo in the fcc matrix and modifies the mechanical properties of CoCrFeNiMo_x [23]. Ming et al. found that tailoring the thermal-mechanical processing enables the evolvement of fcc-structured Mo-added alloys to nanoscale μ phase precipitated fcc solid solution [24]. But, the effect of precipitates on recrystallization and grain growth behavior combined with the evolution of tensile properties with respect to the annealing temperature has not been reported so far in Mo-added alloys.

In this work, the mechanical properties and microstructural evolution of cold-rolled and subsequently annealed Co_22.5+x−yCr_12.5Fe₅₅Ni_10−xMo_y alloys were studied. The purpose of this study was to investigate the effect of precipitates on microstructure evolution combined with mechanical properties of Mo-added alloys. Working toward this aim, the Co-Cr-Fe-Ni-Mo alloy system was chosen, which can exhibit the formation of μ phase in the fcc matrix upon heat treatment [23,24]. In the present alloy system, the inexpensive Fe element was increased up to 55 at%, while high-cost alloying elements Co and Ni were decreased for cost-effectiveness. The Cr is of high value for its high corrosion resistance. However, many previous studies have reported the Cr-rich σ phase in HEAs, and higher Cr concentration may lead to the increase in the volume fraction of the σ phase in the CoCrFeMnNi system [25]. Therefore, 12.5 at% of Cr was selected. The Mo content was adjusted not only to promote solid solution and precipitation strengthening effects but also to investigate the effect of precipitates on microstructure evolution. Hence, we selected the two non-equiatomic CoCrFeNiMo alloys with compositions of Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₅ and Co₁₈Cr_12.5Fe₅₅Ni₇Mo_7.5 (denoted by Mo5 and Mo7.5, respectively). The configurational entropy of the Mo5 and Mo7.5 alloys were ∼1.27R. Therefore, the present alloys can be regarded as MEAs. The fraction of equilibrium phases existing at the temperatures ranging from 400 °C to 1600 °C were determined by thermodynamic calculations. With the increased Mo additions, the thermodynamic calculation results show that the forming temperature range of μ phase increases, i.e., Mo element enhances the formation of μ phase. Based on the equilibrium phase distribution results, the microstructure of the annealed specimens was varied by controlling the annealing treatment and the role of precipitates was investigated in detail.