In this work, a CoCrFeNiMn nano-entropy alloy with an average grain size of 25 nm was prepared for the first time by laser evaporation inert gas condensation. Unlike the nano-high-entropy alloy prepared by SPD, the nano-high-entropy alloy prepared by this method has ultra-high hardness and excellent thermal stability. After annealing at 600 ℃ for 1 hour, the hardness increased from 484 HV to 791 HV unprecedentedly. Even more exciting is that even after annealing at a high temperature of 1100 ℃, the original high hardness can still be maintained. The basic mechanism is attributed to the unique microstructure of high-entropy alloy prepared by laser evaporation inert gas condensation.
Thermal stability is one of the most critical issues of nano-metal materials. As a classic high-entropy alloy with FCC structure, CoCrFeNiMn has been extensively studied due to its good ductility, strength and corrosion resistance. Ultrafine or nanocrystalline high-entropy alloys prepared by high-pressure torsion, equal channel angular compression, rotary die casting and rolling and other severe plastic deformation (SPD) methods further exhibit excellent mechanical strength. However, after heat treatment at more than 800 ℃, its strength drops sharply to the same level as the as-cast alloy, which limits the application of nano-high-entropy alloy at high temperature. Therefore, it is of great significance to find and develop nano-high-entropy alloys with good high-temperature stability as the next-generation high-performance alloys.
Recently, Professor Feng Tao (corresponding author) and Professor Lansi (corresponding author) of Nanjing University of Science and Technology and Academician HorstHahn of Karlsruhe Institute of Technology in Germany have carried out relevant cooperative research on nanostructured CoCrFeNiMn high-entropy alloy. This work used in situ synchrotron high energy X-ray scattering, transmission electron microscope TEM, electron backscatter diffraction EBSD and other characterization methods to study the microstructure evolution and mechanical properties of CoCrFeNiMn nano-entropy alloy prepared by laser evaporation inert gas condensation The relationship between them and the strengthening mechanism, found that its hardness is related to a variety of strengthening mechanisms. The research results were recently published on Scripta Materialia under the title "Ultrahigh hardness with exceptional thermal stability of a nanocrystalline CoCrFeNiMn high-entropyalloy prepared by inert gas condensation".
Paper link:
https://doi.org/10.1016/j.scriptamat.2020.06.042
In the early annealing stage (30-400℃), its strengthening mechanism mainly benefits from the presence of BCC precipitates, the release of grain boundary energy and the decrease in sample porosity. With the increase of annealing temperature (400-600℃), the main mechanism of its hardening is the precipitation of Cr-rich BCC phase and the formation of annealing twins. The hardness after annealing at high temperature (900-1100°C) still maintains a high level, mainly due to the further increase of annealing twins and the contribution of heterostructure formation. The study also found that the grain size of the alloy remained at ultrafine grain (~458 nm) level after annealing at 900 ℃, which is nearly 40 times smaller than the similar high-entropy alloy prepared by SPD method. This is due to its original unique grain size distribution. This research result provides a new method for the preparation of new nanostructured metal materials with ultra-high hardness and excellent thermal stability.
In summary, this work is the first to prepare CoCrFeNiMn nano-entropy alloy by laser evaporation inert gas condensation method. Compared with as-cast samples and CoCrFeNiMn nano-entropy alloy prepared by other methods, the alloy has extremely high hardness and thermal stability. The excellent performance of the alloy prepared by laser evaporation inert gas condensation is due to the combination of its complex microstructure and multiple strengthening mechanisms. In particular, the alloy can maintain high hardness after high temperature annealing. High temperature annealing not only involves precipitation phase hardening, but also involves the formation of twins and the back stress strengthening caused by the heterostructure. This study provides a new nanostructure alloy preparation technology—laser evaporation inert gas condensation method, which has broad application prospects.