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Progress in the study of dislocation nucleation mechanism of chemically disordered entropy alloys in mechanics
[ Instrument R&D of Instrumentation Network ] The recently emerging multi-primary high/medium entropy alloy subverts the traditional alloy design concept constructed by one or two metal principal elements, because of its high strength, toughness, plasticity, radiation resistance, impact resistance, etc. A series of excellent mechanical properties show broad application prospects in the fields of air, space and energy. However, due to the complex local atomic environment and short chemical processes caused by the random occupation of a variety of metal principal elements in topological periodic lattices, how do dislocations, which are important carriers of plastic deformation, nucleate in high/medium entropy alloys And evolution is still a mystery. Recently, the State Key Laboratory of Non-Linear Mechanics, Institute of Mechanics, Chinese Academy of Sciences has made new progress in this regard.
Taking the ternary Ni-Co-Cr medium-entropy alloy system as the research object, the researchers first used molecular dynamics simulation and combined with the transition state theory to investigate the evolution of atoms in the process of dislocation nucleation and found that the process of dislocation nucleation Along with the local fcc-bcc structural transformation, these bcc-deficient atoms act as precursors for dislocation nucleation, promoting the formation of dislocations. Further research found that such bcc defects are mostly concentrated in Cr atoms.
To further determine this bcc-induced dislocation nucleation mechanism, the researchers used the Monte Carlo method to establish a quasi-random atom model to perform first-principle uniaxial tensile calculations. The results again show that the dislocation nucleation is always accompanied by the generation of bcc atoms, and these bcc atoms are dominated by Cr element, which verifies the key role of bcc clusters in the process of dislocation nucleation.
Furthermore, the electronic origin of the bcc defect structure in the process of dislocation nucleation was determined by electronic structure calculation. The study found that Cr element exhibits obvious electron localization behavior. This localization behavior makes the deformation coordination of Cr atomic bonds lower, and it is easy to produce stress concentration. Moreover, the atomic density around Cr atoms is low, resulting in low resistance to deformation of Cr atomic bonds. The special electronic structure of Cr atoms leads to the collapse of the surrounding electronic structure during the deformation process, thus forming a bcc structure, which further promotes the dislocation nucleation.
This study combines classical molecular dynamics, transition state theory and first-principles calculations to obtain the correlation map between electron-atom-defect cluster-dislocation nucleus, effectively analysing the origin of dislocation nucleus in medium and high entropy alloys The evolution mechanism provides clues for a deep understanding of the micro-mechanism of high/medium entropy alloy plastic deformation and the design of high-performance alloy materials.
The work, titled Novel atomic-scale mechanism of incipient plasticity in a chemically complex CrCoNi medium-entropy alloy associated with inhomogeneity in local chemical environment, was recently published online in the metal material field journal Acta Materiala. Cao Fuhua, the special research assistant of the Institute of Mechanics, is the first author of the paper. Researchers Wang Yunjiang and Dai Lanhong are corresponding authors. This research was supported by major projects of the National Fund Committee, basic research center projects of the National Fund Committee, and strategic pilot projects of the Chinese Academy of Sciences.