Molecular Dynamics Research On Strengthening And Toughening Mechanisms Of Single/Dual-Phase High-Entropy Alloys At Micro/Nano-Scale

High-entropy alloys(HEAs)are a class of disordered alloys discovered in recent years while exploring bulk metallic glasses,which are composed of five or more principle elements,forming a simple solid solution structure.The unique composition and microstructure make HEAs exhibit more excellent mechanical properties,high temperature resistance,corrosion resistance,and other properties compared to conventional alloys,which have broad application prospects in the fields of mechanical manufacturing,aerospace,energy,life sciences.Among the numerous excellent properties of HEAs,the mechanical properties are crucial for their application in the aforementioned fields.HEAs exhibit high work hardening rate and stable plastic deformation ability,which is expected to break the bottleneck of inverted strength-ductility relationship that exists in traditional alloys.The development of nanotechnology has increased the demand for precision mechanical components,and the micro/nano-scale mechanical properties of materials show significant differences from the macroscopic mechanical properties.Therefore,the research on micro/nano-scale mechanical properties and deformation mechanisms of HEAs is of great significance for promoting their application in the field of nanotechnology.Existing experimental conditions are difficult to meet the reasearch requirements for microscopic deformation mechanisms and mechanical properties of HEAs.The complex composition of HEAs makes the existing crystallographic theories inapplicable,which restricts the theoretical research on their mechanical properties.Therefore,the research on the mechanical properties of HEAs is still in the developing stage,and the understanding on the microscopic deformation mechanisms of HEAs is insufficient.To address these issues,based on the molecular dynamics(MD)simulation method,this paper has focused on the strength and ductility of single/dual-phase HEAs to carry out the following research work:(1)Construction of the HEAs atomic models.The basic principles of the MD method were elucidated,the important parameters affecting the accuracy of MD research such as reasonable potential functions,ensembles,and boundary conditions were selected.The construction methods of the HEAs atomic models were analyzed,the random substitution method was used to construct the atom random distribution models,and the Monte Carlo(MC)and MD hybrid method was used to construct the HEAs atomic models with chemical short-range order(CSRO).Additionally,the basic crystallographic theories were summarized,and the post-processing methods for analyzing the simulation results were provided.(2)The research on strengthening and toughening mechanisms of FCC single-phase and FCC/amorphous dual-phase HEAs nanowires(NWs).The tensile and compressive behaviors of single crystal FCC structure HEA NWs were simulated,and the ratio of the tensile yield strength to the compressive yield strength was used to characterized the tension-compression asymmetry.The influence mechanisms of the NW diameter,the element content,the loading direction,the temperature on the strength,the ductility and the tension-compression asymmetry of single crystal FCC structure HEA NWs were revealed.It was found that the Schmid factors of the leading partial dislocations were the main factor influencing the tension-compression asymmetry.The amorphous phase was introduced into single crystal FCC structure HEA NWs,and the tensile and compressive simulations were carried out.The results indicated that the tensile yield strength decreased with the thickness of the amorphous phase increasing.During the tensile process,the amorphous phase deformed uniformly,and as the thickness of the amorphous phase increased,no more stacking faults were generated in the FCC phase.During the compressive process,the amorphous phase deformed uniformly,and stacking faults and twins were generated in the FCC phase,the amorphous-FCC and the FCC-BCC phase transformation also occurred.(3)The research on the influence of CSRO on the strength and ductility of FCC single-phase and FCC/BCC dual-phase HEAs.For single crystal FCC structure HEAs,CSRO increased the energy difference between FCC and BCC phase as well as the stacking energy,which hindered the FCC-BCC phase transformation during stretching along the [001] crystal orientation,and the dislocation nucleation during stretching along [110] and [111] crystal orientations,thereby improved the strength and ductility of single crystal FCC structure HEAs.For FCC/BCC dualphase HEAs,Shockley dislocations nucleated at the FCC/BCC phase interface and slipped to form stacking faults in the FCC phase,leading to yielding.CSRO hindered dislocations from nucleating and slipping in the FCC phase by increasing the stacking fault energy of the FCC phase and reducing the potential energy of the alloy system,thereby improved the strength and ductility of FCC/BCC dual-phase HEAs.(4)The research on the influences of(110)twist grain boundaries and FCC/BCC heterogeneous structure on the strength and ductility of polycrystalline HEAs.Different angle(110)twist grain boundaries were constructed in FCC single-phase HEAs,and the tensile deformation was simulated.It was found that the increase of the grain boundary angle changed the dislocation structure in the grain boundaries and hindered dislocations from nucleating at the grain boundaries and emitting into the grains,which improved the strength of HEAs.The BCC phase was introduced into polycrystalline FCC structure HEAs,through tensile and compressive deformations,it was found that when the volume fraction of the BCC phase was in the range of 33.8%-80.3%,the BCC phase could improve the tensile strength and the tensile ductility.When the volume fraction of the BCC phase was in the range of 16.6%-50%,the BCC phase could improve the compressive strength and the compressive ductility.When the volume fraction of the BCC phase was in the range of 50%-80.3%,the BCC phase improved the compressive strength but decreased the compressive ductility.This article revealed the influence mechanisms of uneven microstructures such as FCC/amorphous heterogeneous structure,CSRO,(110)twist grain boundaries,and FCC/BCC heterogeneous structure on the strength and ductility of HEAs at the atomic scale,which provided a theoretical foundation for designing HEAs with the excellent strength and ductility,and helped broaden the application of HEAs in the fields of micro/nano precision devices,high-speed cutting tool manufacturing,and automobile body manufacturing.