Microstructures, mechanical properties and corrosion resistances of FCC CoCrFeNiTi high-entropy alloys prepared by pre-alloyed powder mixing and vacuum laser-directed energy deposition

Abstract

In this study, vacuum laser-engineered directed energy deposition (V-LDED) was employed to fabricate CoCrFeNiTix (x = 0.1, 0.2, 0.3) high-entropy alloys (HEAs) by strategically mixing equiatomic pre-alloyed CoCrFeNi and CoCrFeNiTi powders. With increasing Ti content, the lattice distortion of the HEAs intensified, grains were refined, and precipitate content increased; however, the face-centered cubic (FCC) structure remained the predominant structure. The strength and plasticity of the HEAs initially increased and then decreased with the addition of Ti. The CoCrFeNiTi0.3 (Ti0.3) alloy exhibited the best mechanical properties, with a tensile yield strength (TYS) of 604 MPa, an ultimate tensile strength (UTS) of 882 MPa, and a plastic elongation of 13.5%. Compared to the Ti-free alloy, the TYS and UTS were increased by 124% and 83%, respectively. The CoCrFeNiTi0.2 (Ti0.2) alloy showed the best corrosion resistance with the corrosion potential (Ecorr), corrosion current density (Icorr), passivated film resistor (Rc), and charge transfer resistance (Rct) values of –0.208 V, 4.889 × 10–7 A/cm2, 7.03 × 103 Ω/cm2, and 8.50 × 105 Ω/cm2, respectively. The addition of Ti increased the Cr and Ti contents in the passive film, which are easily passivated elements. The multiple effects of Ti on the corrosion resistance were mainly attributed to the formation and composition of the passive film and density of the precipitates.

1. Introduction

High-entropy alloys (HEAs), which are distinguished by their multi-principal element compositions, exhibit a combination of unique high entropy, sluggish diffusion, lattice distortion, and cocktail effects, enabling exceptional mechanical strength and corrosion resistance [1]. Their complex chemistry challenges conventional manufacturing, making additive manufacturing (AM), particularly laser-based methods, critical for processing [2]. Laser additive manufacturing (LAM) techniques, including laser-directed energy deposition (L-DED) and laser powder bed fusion (L-PBF), utilize high-energy beams for precise layer-wise fabrication [3–4]. Although L-PBF is used under ultra-low-oxygen environments (<10 ppm) to achieve a high surface quality and minimal oxidation, this limits the size of the workpiece. Conversely, L-DED excels in terms of the deposition rate, scalability, and component repair, albeit with oxidation risks for reactive metals. Vacuum-based L-DED synergizes these two approaches; it suppresses oxidation akin to L-PBF while retaining the efficiency and scalability of L-DED [5], enabling the robust production of oxidation-prone HEA systems (e.g., Ti/Al/Mg alloys) with tailored microstructures and enhanced performance for demanding applications.

Strength and plasticity are critical indicators for the mechanical properties of alloys. Simultaneously achieving high strength and plasticity has long been a significant focus in materials science research. Traditional severe plastic deformation processes such as high-pressure torsion and equal-channel angular extrusion can refine grains, thereby enhancing both the strength and plasticity of an alloy. However, these methods have equipment limitations, making them unsuitable for processing large workpieces and enabling large-scale production. Consequently, alloying remains an effective approach for optimizing the properties of HEAs by altering their phases and microstructural characteristics. The CoCrFeNi HEA, which has a single face-centered cubic (FCC) structure, offers good ductility but limited strength, prompting the application of strategies to introduce body-centered cubic (BCC) strengthening phases via alloying, such as Ti and Al [6], which have the same function.

As the Ti or Al content increases gradually, the crystal structure changes from a single FCC structure to a duplex FCC + BCC structure, and then to a single BCC structure in CoCrFeNi HEAs. A single FCC structure for 0 ≤ x ≤ 0.375, duplex FCC + BCC structure for 0.50 ≤ x ≤ 0.75, and single BCC structure for 0.875 ≤ x ≤ 2.00 in AlxCoCrFeNi HEAs have been reported [7–8]. However, LAM CoCrFeNiTi(Al) HEAs containing BCC phases suffer from extensive thermal cracking problems [9–12], which cannot be eliminated by adjusting the process parameters. These microcracks are believed to be related to the expansion of the solid-state crystallization temperature range, which is associated with the increase in the Ti or Al content [10,13]. A literature review and exploratory experiments [9–12] indicated that for LAM CoCrFeNiTi(Al)x HEAs, the value of x should be controlled to lower than 0.3 to avoid the formation of microcracks.