Microstructure and high-temperature wear behavior of CoCrFeNiW_x high-entropy alloy coatings fabricated by laser cladding

Highlights

• Laser-cladded CoCrFeNiW_x (x = 0, 0.25, 0.5, 0.75, 1.0) high-entropy alloys coating were fabricated.

• There are unmelted W phase and intermetallics in W-doped CoCrFeNiW_x coatings.

• The microhardness and the wear resistance of the coatings at room temperature are improved significantly with the increasing W content.

• The temperature-dependent wear rate and the corresponding wear mechanism are revealed.

Abstract

High-entropy alloys (HEAs) coatings of CoCrFeNiW_x (x = 0, 0.25, 0.5, 0.75, 1.0) were fabricated on AISI1045 steel via laser cladding. The microstructure, microhardness and high-temperature performance of the W-doped HEA coatings were investigated. The results show that the W-doped CoCrFeNiW_x HEA coatings are composed of a face-centered cubic (FCC) solid solution matrix with unmelted W phases and FCC/intermetallics eutectic structures. The microhardness of the coatings is improved significantly with the increasing W content. The CoCrFeNiW coating exhibits good wear resistance at an elevated temperature of 600 ℃ due to the formation of the oxide layer covering the worn surface. However, the sublimation of WO₃ in the oxide film reduces the protective ability to wear at 800 ℃.

Introduction

High-entropy alloys (HEAs), as new alloy material, are customarily defined as an alloy system composed of five or more components with equal atomic ratio or nearly equal atomic ratio [1], [2]. The microstructure of HEAs is mainly composed of solid solutions with face-centered cubic (FCC) or body-centered cubic (BCC) lattice structures. In the solid solution phase, atoms with different radii randomly occupy lattice nodes, resulting in serious lattice distortion and high configurational entropy of the alloy system [3]. Previous studies have shown that HEAs with high microhardness [4], [5], [6], excellent corrosion resistance [7], [8], and outstanding high-temperature properties [9], [10], [11] can be prepared by reasonable allocation of atomic ratios. HEAs have attracted more and more attention due to their unique microstructure and excellent mechanical properties.

CoCrFeNi and its derivatives, as representative high-entropy alloy systems, are investigated extensively by many researchers. It is found that the equiatomic CoCrFeNi HEA is a simple FCC solid solution phase, which has outstanding ductility and fracture toughness at cryogenic temperatures [12]. The elongation to failure of the as-cast CoCrFeNi HEA is 50% [13]. However, the strength of a single-phase FCC lattice structure at high-temperatures is undesirable, which restricts the further application of the HEA. Alloying is an effective method to strengthen the single-phase HEAs, and the properties of HEAs can be regulated by doping specific elements with different contents. Ma et al. [14] prepared AlCoCrFeNi HEA with a single-phase BCC structure by adding Al. The yield strength, elongation, and hardness of the HEA are 1373 MPa, 24%, and 520 HV, respectively. He et al. [15] found that the CoCrFeNiMn HEA has excellent toughness and plasticity. The tensile strength and yield strength of the HEA are 500 MPa and 220 MPa, respectively, and the elongation is as high as 61.7%. In addition, some progress has been made in the study of CoCrFeNi HEA systems with Ti [16], Mo [17], W [18], [19], and other elements. The various characteristics of each element can benefit the performance of alloys [20].

In the existing reports, CoCrFeNi HEA and its derivatives are the most promising coating material for tribological applications among the HEAs developed [21]. Chen et al. [22] found that the σ phase (Cr-Fe) precipitates in the as-cast Al_0.6CoCrFeNi HEA after cold rolling and annealing, which significantly improves the wear resistance at 25–800 ℃. Verma et al. [23] studied the tribological properties of CoCrFeNiCu_x HEA at high-temperatures and discussed the self-lubricating effect of Cu. However, the Cu element may produce serious micro-segregation in HEAs [24]. Liu et al. [25] investigated the influence of the Ti element on the microstructure and high-temperature tribological properties of AlCoCrFeNi HEA. They found that the high-temperature wear resistance of the HEA increases with the increase of Ti content. Joseph et al. [26] found that the addition of Al is beneficial to the formation of the BCC phase and dense enamel layer, which significantly improves the yield strength and tribological properties of Al_xCoCrFeNi HEA at high-temperature. As an element with a high melting point and a large radius, tungsten is expected to have a beneficial effect on the mechanical and high-temperature wear properties of the CoCrFeNi-based HEAs, according to the cocktail effect. Thus, the effects of W on the microstructure of CoCrFeNi HEA, as well as the high-temperature performance of CoCrFeNiW, were investigated in this work.

In this work, CoCrFeNiW_x HEA coatings were successfully synthesized on medium carbon steel via laser cladding. The microstructural evolution, the microhardness, and the thermal stability of the CoCrFeNiW_x HEA coatings were evaluated. The high-temperature wear behavior of the CoCrFeNiW_1.0 coating was then discussed in detail. The results may provide a reference for applying CoCrFeNi-based HEAs in high-temperature fields.