Phase transformation and microstructure evolution of nanoimprinted NiCoCr medium entropy alloys

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CoCrFeMnNi high-entropy alloy (HEA) has a high degree of thermodynamic stability and excellent ductility, making it a crucial structural material. However, the plastic deformation and microstructural behavior of gradient grain structured CoCrFeMnNi HEA under cutting remain unclear. In this study, the machining properties of gradient nanostructured CoCrFeMnNi HEA under conventional cutting were investigated by molecular dynamics (MD) simulation. The results displayed that the small grain gradient samples exhibited grain size softening. The shear angle and cutting ratio increased with the increase in the grain gradient. The grain boundaries of the low grain gradient samples were damaged and slid during the cutting process. Moreover, the dislocation density increased with the increasing grain gradient. The multi-dislocation nodes and the Lomer-Cottrell junction were produced in the grain coarsening gradient samples, contributing to work hardening. The cutting forces from low to high cutting velocities were 136.70, 147.91, 165.82, and 164.79 nN, which confirmed that the cutting forces increased with increased cutting velocity. This work elucidated the cutting mechanism of the nanostructured CoCrFeMnNi HEA and highlighted the influence of the gradient grain sizes.

Section: Methodologiesmentioning

confidence: 99%

Effects of grain boundary and gradient structure on machining property of CoCrFeMnNi alloys

Lu,

Bui,

Fang

2024

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“…The interactions between Ni-Ni, Co-Co, and Cr-Cr of the NiCoCr MEA specimen in the simulation were studied using the EAM. The EAM potential was provided by previous research [34,35],…”

Section: Methodologiesmentioning

confidence: 99%

“…The stress distribution of the NiCoCr MEA specimens was provided by the atomic von Mises stress. According to the literature [24,36], the atomic von Mises stress (σ v ) of a polycrystalline specimen can be illustrated in following equation: The stress on the atomic scale was used to describe the effects of residual stress in the NiCoCr MEA specimens during the compression process [34,37],…”

Section: Methodologiesmentioning

confidence: 99%

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Buckling instability and compressive deformation of Ni-Co-Cr medium-entropy alloy nanotubes

Lu¹,

Fang²

2022

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The aspect ratio has a significant effect on buckling instability. In this study, we used molecular dynamics (MD) simulation to investigate the buckling instability mechanism of NiCoCr medium-entropy alloy (MEA) nanotubes under the compression process. Based on the compression results, we find that the buckling point and the critical buckling stress decrease with increasing aspect ratio. The critical buckling stress of the MD simulations resembles the Euler and Rayleigh-Ritz theories. The critical aspect ratio of 10 showed instability at NiCoCr nanotubes. In addition, we found that the compressive strength of single-crystalline specimens is much higher than that of polycrystalline specimens. Partial dislocations (PDs) dominated by stacking faults (SFs) and twinning structures are mainly distributed on the grain boundaries of small grain specimens and decrease with increasing grain size. We also found that polycrystalline structures tend to induce buckling instability. The wall thickness specimen of the 7 nm has confirmed similar compressive strength as the nanowire. The compressive strength decreases as the wall thickness decreases. More stress concentration and PDs distributed in the specimen when the wall thickness is reduced to 1 nm, and it is easier to induce buckling instability at NiCoCr MEA nanotubes.

“…σ denotes the equilibrium distance, while r signifies the distance between carbon atoms and substrate atoms. The interaction between the indenter tooltip and substrate is established using LJ potential [42,43], with the specific parameters detailed in table 3.…”

Section: Interaction Between Atomsmentioning

confidence: 99%

Study of nanoindentation behavior of NiCrCoAl medium entropy alloys under indentation process using molecular dynamics

Ngo,

Nguyen,

Fang

2024

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The mechanical properties and deformation behavior of CoCrNiAl medium entropy alloy (MEA) subjected to indentation by an indenter tooltip on the substrate are explored using molecular dynamics (MD) simulation. The study investigates the effects of alloy compositions, temperature variations, and ultra vibration (UV) on parameters, such as total force, shear strain, shear stress, hardness, reduced modulus, substrate temperature, phase transformation, dislocation length, and elastic recovery. The findings indicate that higher alloy compositions result in increased total force, hardness, and reduced modulus, with Ni-rich compositions demonstrating superior mechanical strength. Conversely, increasing alloy compositions lead to reduced von Mises stress (VMS), phase transformation, dislocation distribution, and dislocation length due to the larger atomic size of Ni compared to other primary elements. At elevated substrate temperatures, atoms exhibit larger vibration amplitudes and interatomic separations, leading to weaker atomic bonding and decreased contact force, rendering the substrate softer at higher temperatures. Additionally, higher initial substrate temperatures enhance atom kinetic energy and thermal vibrations, leading to reduced material hardness and increased VMS levels. Increasing vibration frequency enlarges the indentation area on the substrate's surface, concentrating shear strain and VMS with vibration frequency. Higher vibration amplitude and frequency amplify force, shear strain, VMS, substrate temperature, and dislocation distribution. Conversely, lower vibration amplitude and frequency result in a smaller average elastic recovery ratio. Moreover, increased amplitude and frequency values yield an amorphous-dominated indentation region and increased proportions of HCP and BCC structures. Furthermore, this study also takes into account the evaluation of a material's ability to recover elastically during the indentation process, which is a fundamental material property.