Two-step hydrogen reduction of oxides for making FeCoNiCu high entropy alloy: Part I – Process and mechanical properties

Iron-based 3d-transition group high-entropy alloys (HEAs), with remarkable wear resistance, high hardness, and excellent mechanical properties, are usually produced by a multiple-step high-energy-consumption smelting process. This study offers a novel process for fabricating FeCoNiCu HEAs by two-step hydrogen reduction using a mixture of Fe2O3, CoO, NiO, and CuO, after which a nearly full dense crack-free HEA was successfully produced. The alloy has a face-centered cubic (FCC) structure and an oxygen level as low as 0.10 wt%. After the first hydrogen reduction at 500 °C for 2 h, the oxygen level decreased from 23.30 wt% to 2.05 wt% and the powder obtained consisted of the elemental metal powder with some minor alloying. A second hydrogen reduction occurred during sintering at temperatures above 800 °C, in which near-full densification (> 99% relative density) was achieved at temperatures as low as 900 °C. All the compositions were homogeneously distributed in the alloy when the temperature was >900 °C, and a Cu-enriched FCC-structured secondary phase with two types of shapes was precipitated in the alloy. The final alloy showed excellent mechanical properties at room temperature, with a hardness of ∼150 HV, compression strength up to 2.2 GPa, compression strain of >12%, tensile strength of 475 MPa, and tensile elongation of 7.9%.