Abstract
A series of fine-grained FeCoNi(CuAl)x (x=0, 0.4, 0.6, 0.8, 1.0) medium-entropy alloy (MEA) and high-entropy alloys (HEAs) were fabricated by Mechanical Alloying (MA) and Spark Plasma Sintering (SPS). The effect of Al and Cu content (x) on phase composition, microstructure and mechanical properties of the alloys was investigated. Experimental results show that the crystal structure of FeCoNi(CuAl)x alloy transforms from single FCC phase for x=0 to FCC+BCC duplex phases for x=0.4~1.0, with the fraction of BCC phase gradually increasing with the increase of x. Adding a low content of Al and Cu elements to FeCoNi alloy can significantly hinder the grain growth during sintering process, the average grain size of FCC phase decreases from 0.95 to 0.30 µm at x=0.4. However, the grain sizes of FCC and BCC phases gradually grow up when x increases from 0.4 to 1.0. The variation in grain size indicates that the atomic diffusion rate of sintered alloy may be influenced by the sluggish diffusion effect in HEA as well as the content of Al and Cu with lower melting points. Mechanical properties of the HEAs are mainly affected by the volume fraction of BCC phase. The compressive yield strength and hardness of HEAs are improved at first and then slightly reduced, while the plasticity drops down continuously with the increase of x. The bulky HEA achieved excellent comprehensive mechanical properties with a compressive yield strength of 1467.7 MPa and plastic strain to failure of 24.9 % at x=0.6, due to the fine duplex microstructure.
Highlights
Different from traditional alloy design strategies, the concept of high-entropy alloys (HEAs) was first proposed by Yeh et al in 2004
Tong et al (2019) prepared a precipitationstrengthened FeCoNiCrTi0.2 high-entropy alloy using arc melting and casting methods. It exhibits a great increase in yield strength (σ0.2) and ultimate tensile strength, with little loss in ductility at room temperature compared with the single-phase FeCoNiCr parent alloy; in addition, at cryogenic temperatures, its σ0.2 and σUTS are increased from 700 MPa to 1.24 GPa to 860 MPa and 1.58 GPa, respectively, associated with a ductility improvement from 36% to 46%
After ball milling for 20∼40 h, single face-centered cubic (FCC) phases are identified as basic crystal structure in the mechanically alloyed powders
Summary
Different from traditional alloy design strategies, the concept of high-entropy alloys (HEAs) was first proposed by Yeh et al in 2004. Comparing the exothermic peaks of various FeCoNi(CuAl)x alloy powders, it is found that with the increase of Al and Cu content, the FCC solid solution generated by mechanical alloying becomes more unstable and the precipitation temperature of the second phase is descending.
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