Fast mechanical synthesis, structure evolution, and thermal stability of nanostructured CoCrFeNiCu high entropy alloy

Abstract A powder of equiatomic CoCrFeNiCu high entropy alloy (HEA) was prepared by short-term (120 min) high energy ball milling (HEBM). Our structural and chemical analysis showed that microsized particles of fcc CoCrFeNiCu with a grain size of 8 nm were obtained after 120 min of HEBM at 694/1388 rpm. The structural/phase evolution of CoCrFeNiCu HEA powder and its thermal stability were explored by high-temperature XRD at 600 °C, 800 °C and 1000 °C, by DSC up to 1500 °C, through the consolidation by SPS at 800 °C and 1000 °C, and characterized using XRD, SEM and EDX analyses. In-situ HT XRD analysis during 5.5 h of annealing showed the involvement of transient phases: the bcc phase that appeared in 1 h of annealing at 600 °C and disappeared at higher temperatures; and the fcc1 phase (Cu-rich) arising in 2 h of annealing at 800 °C and disappearing at 1000 °C in 3 h of annealing. SPS consolidation at 1000 °C and annealing at 1000 °C for 5.5 h were found to result in the formation of single-phase fcc2 CoCrFeNiCu alloy with a lean amount of Cu. The melting points for Cu-rich and Cu-depleted HEAs were found as 1118 °C and 1288 °C (Calphad calculations) and 1115 °C and 1365 °C (DSC measurements), respectively. SPS consolidation at 1000 °C under a pressure of 50 MPa yielded the single-phase fcc CoCrFeNiCu0.5 alloy that turned thermodynamically more favorable than the equiatomic one. Thus, we can suppose thereupon, that the equiatomic fcc phase that appears after 120 min of HEBM is metastable because of the excess of Cu atoms. During annealing in the temperature range 800–1000 °C, the Cu-rich fcc1 phase precipitates from the initial single-phase alloy, while the “mother phase” transforms into the more stable Cu-depleted fcc2 phase. The chemical compositions of Cu-depleted and Cu-rich phases for the SPS-consolidated HEA CoCrFeNiCu alloy (at 800 °C) were determined from TEM–EDX analyses. Optimal combination of short-term HEBM and SPS consolidation can be recommended as a facile route to fabrication of single-phase fcc equiatomic CoCrFeNiCu HEA powders and bulk materials with good structural/elemental homogeneity.