Improved Performance of Sintered Alloys by Spark Plasma Sintering of Gas- atomized CoCrFeNi-Si High-entropy Alloy Powders with Ball Milling

Abstract

In this study, to improve the hardness of CoCrFeNi-Si alloys, CoCrFeNi-Si alloy powders prepared by gas atomization (Co_28.5Cr_15.5Fe_28.5Ni_15.5Si₁₂, Co₂₈Cr₁₅Fe₂₈Ni₁₅Si₁₄, and Co₂₂Cr₂₂Fe₂₂Ni₂₂Si₁₂) were processed by ball milling (BM) (t = 0 h, 25 h and 50 h) followed by spark plasma sintering (SPS, 1073 K, 50 MPa, holding for 10 min). BM 0 h of Co_28.5Cr_15.5Fe_28.5Ni_15.5Si₁₂, Co₂₈Cr₁₅Fe₂₈Ni₁₅Si₁₄, and Co₂₂Cr₂₂Fe₂₂Ni₂₂Si₁₂ powders identified the main fcc phase, and silicide Co₂Si by X-ray diffraction (XRD). The XRD results of the BM 25 h and 50 h powders of Co_28.5Cr_15.5Fe_28.5Ni_15.5Si₁₂, Co₂₈Cr₁₅Fe₂₈Ni₁₅Si₁₄, and Co₂₂Cr₂₂Fe₂₂Ni₂₂Si₁₂ reveal that a single fcc phase was formed. The BM also caused the Co₂Si compound to dissolve, and all elements were completely solid- soluted in the fcc matrix. According to the XRD results of the sintered samples after the SPS process, the sintered samples of Co_28.5Cr_15.5Fe_28.5Ni_15.5Si₁₂, Co₂₈Cr₁₅Fe₂₈Ni₁₅Si₁₄, and Co₂₂Cr₂₂Fe₂₂Ni₂₂Si₁₂ exhibited an fcc phase and contained FeSi and Co₂Si compounds. The hardness results showed that the Co₂₈Cr₁₅Fe₂₈Ni₁₅Si₁₄ sample was the hardest of the three sintered samples, because the higher the mixing entropy ΔS_mix (closer to the isoatomic composition), the higher the Si content, the higher the increase in hardness. Furthermore, the hardness of the three alloys increased with BM time; in this case, the increase in hardness can be attributed to the introduction of more transitions rather than the BM plastic working.