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The CoNi, CoFeNi and CoCr x FeNi (x = 0.2, 0.6, 1) alloys were undercooled and in-situ observed. Sluggish dendrite growth was found for the CoCrFeNi alloy, e.g., the maximum dendrite growth velocity decreases by nearly two orders from the CoNi (78 m s−1) to the CoCrFeNi (0.83 m s−1) alloy. It was shown that it is the Cr element and its adding amount but not the number of components that plays a more decisive role in sluggish dendrite growth. The current work might be helpful for understanding rapid solidification, thereby further controlling the microstructures and modulating the mechanical properties of high entropy alloys. Image 1 • A metastable BCC phase was identified in the undercooled CoCrFeNi alloy. • Sluggish dendrite growth was found with the increase of the number of components for the equiatomic alloy melt. • It is not the components number but the properties and its adding amount that play important roles in sluggish growth. • The sluggish dendrite growth could be ascribed to the interaction between the interface and component atoms. [ABSTRACT FROM AUTHOR]

The CoNi equiatomic alloy was undercooled and spontaneously solidified using the glass fluxing method. The dendrite growth behaviors were recorded by a high-speed high-resolution camera. Three dendrite growth stages (i.e., the power law growth stage when undercooling Δ T <120 K, the plateau growth stage when 120 K<Δ T <135 K and the linear law growth stage when Δ T >135 K) were found and well predicted by a dendrite growth model. For the as-solidified microstructures, the average grain size increases with undercooling, indicating that unique grain "coarsening" instead of common spontaneous grain refinement occurs. Such a grain "coarsening" phenomenon was ascribed to stress accumulation upon rapid solidification. Furthermore, a plenty of sub-grains were observed within the abnormal coarse grains. From the electron back-scattering diffraction results, the angle difference between neighbouring sub-grains was less than 6°. The current work might be helpful for not only theoretically understanding dendrite growth but also practically controlling the non-equilibrium microstructures. • Rapid solidification of CoNi equiatomic alloy was studied using the glass fluxing method. • Three growth stages were observed and well predicted by the dendrite growth model. • Sub-grains within the coarse grains were identified by electron back-scattering diffraction. • A unique grain "coarsening" phenomenon for the as-solidified microstructures was reported. [ABSTRACT FROM AUTHOR]