Microstructure Evolution And Mechanical Properties Regulation Of High-entropy Alloys Under Deep Undercooled Rapid Solidification

In recent years,High Entropy Alloys(HEAs)characterized by multi-principal-element have been extensively studied.Its four core effects make it tend to form a simple solid solution phase and lead to excellent comprehensive properties.So far,the research on high entropy alloys mainly focuses on the selection of alloys system and the processing process,the understanding of deformation mechanism and the regulation of properties.In this work,deep-undercooling rapid solidification of high entropy alloys with different particle sizes was accomplished,and the microstructure evolution and its influence on mechanical properties were studied.The spherical droplets of Fe26.7Co26.7Ni26.7Si8.9B11.0 high entropy alloy and AlCoCrFeNi2.1 high entropy alloy were prepared by high vacuum drop tube equipment.The phase composition and microstructure evolution of spherical droplets with different sizes were studied by X-ray diffraction and scanning electron microscopy.The change of mechanical properties was studied by Vickers hardness testFirstly,the author build the required drop-tube equipment platform.In order to ensure that the under-cooled droplets with rational specifications could be continuously prepared by the device,Fe26.7CO26.7Ni26.7Si8.9B11.0 HEAs is selected as the sample for the theoretical thermodynamic and kinetic calculation in this paper.Combined with the calculation results,the argon environment of 5×104Pa was selected as the protective gas for the drop tube pre-experiment,and the reasonable experimental parameters were summarized through the pre-experiment.Secondly,Fe26.7Co26.7Ni26.7Si8.9B11.0 high-entropy droplets are composed of coarse primary FCC dendrites,(Fe,Co)2B intermetallic compounds and a small amount of Ni31 Si12 phase,which is consistent with the microstructure of the as-cast alloy.The difference indicates that deep undercooling can induce the transformation from a regular eutectic matrix to an abnormal eutectic matrix.As the droplet size decreases,the volume fraction of fcc dendritic structure and eutectic structure gradually decreases and increases,respectively.These microstructural changes should be associated with the local re-melting of the primary eutectic structures due to the recalescence effect and the subsequent decoupled growth during the deeply undercooling solidification.These microstructure changes should be related to the local remelting of primary eutectic structure caused by the regenerative effect during the deep undercooled solidification and the subsequent decoupling growth.As the droplet size decreases from 2400?m to 100?m,the Vickers hardness of the sample increases from 451±8Hv to 483±10Hv,which is higher than that of the as-cast 450 HV,and the Vickers hardness of the droplet sample was linearly related to the volume fraction of the eutectic matrix.The corresponding strengthening mechanism is attributed to solid solution strengthening,fine grain strengthening and second phase strengthening.Finally,AlCoCrFeNi2.1 high-entropy droplets are composed of fcc phase and bcc phase.The microstructure gradient of primary dendrite phase and anomalous eutectic phase? irregular eutectic? regular lamellar eutectic appeared in the large size samples from 2800?m to 1500?m.As the size decreases,the cooling rate of the droplets increases and the conventional eutectic structure(such as regular eutectic and irregular eutectic)gradually disappear.With the further decreases,the sizes of primary dendrites and eutectic structures become smaller.When the size is less than 520?m,the primary dendritic phase disappears.There is a gradient distribution of Vickers hardness inside the droplets from 2800?m to 1500?m,and the Vickers hardness increases from the center of the sample to the periphery,which is related to the gradient distribution of the internal microstructure.There is no positional correlation in the Vickers hardness of droplets of other sizes.The Vickers hardness increased linearly with the decrease of droplet sizes,from 307±6Hv of the 1200?m sample to 324±11Hv of the sample with a diameter of 100?m The corresponding strengthening mechanism should be attributed to fine-grain strengthening.