An investigation of abnormal grain growth in Zr doped CoCrFeNi HEAs through in-situ formed oxide phases

Abnormal grain growth (AGG) in nanocrystalline (CoCrFeNi)100-xZrx (x = 1 and 4 at. %) HEAs, prepared through high energy mechanical alloying, was comprehensively investigated upon annealing. Transmission electron microscopy (TEM), including high angle annular dark field imaging (HAADF) and energy dispersive spectroscopy (EDS) mapping, focused ion beam microscopy (FIB), and X-ray diffraction experiments (XRD) were utilized to investigate the microstructures as a function of added Zr content and temperature exposures. The results showed that nanocrystalline grains of the as-milled HEAs did not increase significantly upon annealing up to 700 °C as the nanocrystalline grain sizes were retained. However, grain growth was observed in (CoCrFeNi)99Zr1 after annealing at 900 °C, which turned into AGG after annealing at a higher temperature of 1100 °C, disrupting the equiaxed grain structures observed at 900 °C. Although the increased amount of Zr doping reduced the average grain size in (CoCrFeNi)96Zr4, bimodal grain structure existed in the microstructure composed of a matrix with 255 nm grain size and abnormally grown grains up to 3 μm. The observed AGG was attributed to the pinning effect of in-situ formed secondary oxide phases. The microstructural evolution as a function of Zr doping and annealing temperatures was also correlated with the microhardness test results. The AGG and bimodal grain structure reported for the Zr doped CoCrFeNi HEA may open a new avenue to produce HEAs with the enhanced strength-ductility combination due to the incorporation of larger grains and in-situ formed oxide phases in a fine-grained matrix.