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    Investigation of the effects of conventional sintering and spark plasma sintering methods on the microstructural properties and hardness of nanostructured CoCrFeNi high entropy alloy
    (Gazi Univ, Fac Engineering Architecture, 2024) Baloglu, Ali Riza; Tekin, Mustafa; Kotan, Hasan
    Graphical/Tabular CoCrFeNi HEAs were synthesized by mechanical alloying and consolidated via conventional sintering and spark plasma sintering, and microstructural properties and hardness were investigated as a function of sintering type and temperature. The result are shown in Figure A. The findings showed that the as -milled single-phase face centered cubic (fcc) crystal structure retained after conventional sintering at 1000 and 1100 degrees C whereas spark plasma sintering yielded additional Cr-rich carbide (Cr 7 C 3 ) phases at the same temperatures. Figure A. XRD and hardness results of the HEAs correlated with the microstructures Purpose: To investigate the effect of sintering type and temperature on the microstructural properties and hardness by using X-ray diffraction (XRD), focused ion beam microscopy (FIB), transmission electron microscopy (TEM), and microhardness test. Theory and Methods: High energy mechanical alloying was used to synthesize the equiatomic CoCrFeNi HEAs in nanocrystalline structure by SPEX 8000D shaker mill. The as -milled powders were consolidated by conventional sintering and spark plasma sintering methods. Results: The as -milled grain size of 10 nm increased to 450 nm and 1.5 mu m after conventional sintering at 1000 and 1100 degrees C, respectively, which shows that nanocrystalline CoCrFeNi alloy does not remain thermally stable after long temperature exposures at elevated temperatures. After consolidation of as -milled powders by SPS at 1100 degrees C, the grain size of the HEA was retained around 353 nm. This striking thermal stability of HEA is correlated with high heating rates and lower sintering duration by SPS, and the Zener pinning of the boundaries by nano -sized Cr-rich carbide phases. Accordingly, the as -milled hardness of the CoCrFeNi HEA reduced from 4.6 GPa to 2.1 GPa after conventional sintering at 1100 degrees C due to the significant grain growth, while the enhance hardness of 3.6 GPa was maintained after consolidation with SPS at 1100 degrees C. Conclusion: The findings suggest that as -milled CoCrFeNi HEA is not thermally stable particularly at high homologous processing temperatures. Consolidation with spark plasma sintering technique at 1100 degrees C provided higher density and higher thermal stability with a retarded grain growth of around 353 nm.