Effects of cutting tool geometry on material removal of a gradient nanograined CoCrNi medium entropy alloy.

CoCrNi medium-entropy alloys (MEAs) have attracted extensive attention and research because of their superior mechanical properties, such as higher ductility, strength, and toughness. This study uses molecular dynamics (MD) simulations to investigate the cutting behavior of a gradient nanograined (GNG) CoCrNi MEA. Moreover, it explores the influence of relative tool sharpness and rake angle on the cutting process. The results show that an increase in the average grain size of the GNG samples leads to a decrease in the average resultant cutting force, as predicted by the Hall-Petch relationship. The deformation behavior shows that grain boundaries are crucial in inhibiting the propagation of strain and stress. As the average grain size of the GNG sample increases, the range of shear strain distribution and average von Mises stress decreases. Moreover, the cutting chips become thinner and longer. The subsurface damage is limited to a shallow layer at the surface. Since thermal energy is generated in the high grain boundary density, the temperature of the contact zone between the substrate and the cutting tool increases as the GNG size decreases. The cutting chips removed from the GNG CoCrNi MEA substrates will transform into a mixed structure of face-centered cubic and hexagonally close-packed phases. The sliding and twisting of grain boundaries and the merging of grains are essential mechanisms for polycrystalline deformation. Regarding the cutting parameters, the average resultant force, the material accumulation, and the chip volume increase significantly with the increase in cutting depth. In contrast to sharp tools, which mainly use shear deformation, blunt tools remove material by plowing, and the cutting force increases with the increase in cutting-edge radius and negative rake angle.

Investigation of the deformation behavior and mechanical characteristics of polycrystalline chromium-nickel alloys using molecular dynamics.

In this study, the mechanical properties and plastic deformation responses of nanocrystalline Cr-Ni alloy were investigated via tensile tests by molecular dynamics (MD) simulation. The effect of various compositions, various grain sizes (GSs) from 4.7 to 11.0 nm, and various temperatures from 300 to 1500 K is analyzed. The results indicate that the yield strength of the polycrystalline Cr-Ni alloy decreases as decreasing GS, which shows the inverse Hall-Petch relation in the metal softening as reducing GS.  Young's modulus (E) increases in the order of the increasing GSs and single crystalline. E rises as raising the percent of Ni from 5 to 15% and then decreases as increasing %Ni to 20%. Besides, E is the linear decrease function with increasing temperature. The maximum stress decreases as increasing temperature and increasing %Ni from 5 to 15%. But that decreases as increasing %Ni from 15 to 20%. The maximum stress value of single crystalline is smaller than that of polycrystalline. The high shear strain zones depend on the GS and alloy composition. The shear strain zones focus on the grain boundary at a low temperature and disperse over the entire specimen when the specimen works at a high temperature. The reason is that the grain boundary helps release stresses to prolong the plastic deformation period to prevent rapid specimen destruction.

Anisotropic crack propagation and self-healing mechanism of freestanding black phosphorus nanosheets.

In this study, an indentation simulation is employed to study the anisotropic crack propagation and re-forming mechanism of freestanding black phosphorus (FBP) nanosheets by molecular dynamics simulation. The results indicate that the size of the FBP nanosheet decides the crack direction as well as the von Mises stress concentration. It is found that crack directions are not influenced by temperature. With increasing specimen size, the crack propagation rate is nearly the same as at the first stage of crack formation, while in the later stage, cracking develops very quickly in larger specimens. Especially, small FBP nanosheets almost re-form in a short time at ambient temperature. However, after being destroyed, the larger specimen has no possibility of recovery. Besides, when increasing the number of layers of FBP, the energy stored by the top layer and the system undergoing deformation increases. In addition, the specimen with two fixed edges is less stable, leading to increased stress and decreased Young's modulus compared with the specimen with four fixed edges.