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In this work, the dynamic behavior of nano-bubbles near the surface of single crystal iron (Fe) was investigated using molecular dynamics simulations. The cavitation erosion behavior of single crystal Fe and the structural evolution of its eroded surface was examined at different bubble diameters. The results show that nano-bubble diameter is inversely correlated with impact pressure and diameter is positively correlated with nanojet energy. The volume, surface area, and depth of cavitation pits are nearly directly proportional to the bubble diameter with correlation linear fitting coefficients of R2 = 0.9837, R2 = 0.9922, and R2 = 0.9799, respectively. Additionally, cavitation erosion induces the structural evolution of iron atoms from bcc to fcc and hcp structures. The percentage of new phase transformed is related to the bubble diameter and the type of transformed structure, the percentage of fcc and hcp structures of Fe atoms exhibits an increasing trend with the increase in the bubble diameter, and the Fe atoms of fcc structures occur an obvious increase beyond a bubble diameter of 12 nm. [ABSTRACT FROM AUTHOR]

The surface free energy of solids, γ, plays a crucial role in all physical and chemical processes involving material surfaces. For the first time, we obtained γ directly from molecular dynamics simulations using a crystal cleavage method. The approach was successfully realized in a Lennard-Jones system by inserting two movable external walls, each consisting of a single crystal layer, into a bulk crystal to create flat, defect-free surfaces. The cleavage technique designed allowed us to calculate the surface free energy according to its definition and avoid surface premelting. The temperature dependence of γ was determined for the (100) and (110) crystal planes along the whole sublimation line and its metastable extension, up to T = 1.02 · Tm, where Tm is the melting point. Good agreement with indirect values of γ(T) was found. The proposed computational cleavage method can be applied to other solids of interest, providing valuable insight into the understanding of chemical and physical surface processes, and demonstrates the successful import of the cleavage method, traditionally used in technical preparation and study of crystal surfaces, into a modern atomistic simulation. [ABSTRACT FROM AUTHOR]

In ultra-precision machining, material anisotropy has an essential impact on surface generation and subsurface damage. In this work, the molecular dynamics method is applied to create the diamond nano-cutting models of Ni-based single crystal (NBSX) superalloy workpieces with different crystal orientations, and the surface/subsurface damage characteristics, crystal structure changes, cutting force, and atomic temperature and stress are discussed. The results show significant differences in surface/subsurface quality, crystal structure transformation, cutting force, atomic temperature and stress distributions and dislocation density obtained from nano-cutting workpieces along different crystal orientations. In four groups of workpieces, the surface/subsurface quality is used as an evaluation index, which shows that the [110](001) crystal orientation has the best cutting effect, and the [111](1 ‾ 10) crystal orientation is the worst cutting direction. This work reveals the damage generation mechanism of the machined surface/subsurface of NBSX superalloy on an atomic scale, which provides technical support for improving machining quality and optimizing machining parameters. [Display omitted] • Surface/subsurface damage characteristics are formed by different crystal orientations. • The workpiece with the [110](001) crystal orientation has the optimal surface/subsurface quality. • Material stress induces the formation of dislocations and defects, and three deformation zones appear in the tool-to-workpiece contact area. • The temperature drop point is closely related to chips and side flow. [ABSTRACT FROM AUTHOR]

Nickel-based single crystal alloys are widely used in aerospace, national defense and other fields because of their excellent properties (excellent strength, high temperature creep resistance and corrosion resistance). At present, its machining accuracy is very high, and high machining surface quality is required. In the sub nano and near atomic scale (SN-NAS) cutting process, the crystal orientation of workpiece has an important impact on the internal dislocation nucleation and expansion and surface material removal. These changes will affect the evolution process of subsurface defects and have an important impact on the surface quality. The SN-NAS cutting models are established, and the relationship between dislocation emission and energy change is deduced. The cutting models of nickel-based single crystal alloys with different crystal orientations were established by the molecular dynamics method. It is found that dislocations are emitted along the direction perpendicular to the cutting surface. Due to the different crystal orientation of the workpiece, the dislocation slip direction presents anisotropy. The plastic deformation of the workpiece is closely related to dislocation slip and stacking fault propagation. Due to the difference in lattice arrangement, dislocation slip orientation and surface energy, the surface roughness shows strong anisotropy. The work hardening behavior is determined by the change of micro defect structure and dislocation density in the workpiece. At the same time, it is closely related to the movement of material slip system and the self-hardening and latent-hardening of dislocations. This study analyzes the formation mechanism of the subsurface deformation layer and how the change of crystal orientation affects the evolution of the subsurface deformation layer, so as to provide technical support for the development of SN-NAS cutting of nickel-based alloy. The figure shows the model of dislocation emission (a1) and relationship of angle-energy difference(a2) The surface roughness of nickel-based single crystal alloy after cutting (b)Shockley dislocation density(c) self-hardening and latent-hardening between slip systems(d). [Display omitted] 1 From the perspective of theoretical analysis, the relationship between dislocation emission angle and energy difference under different dislocation slip lengths is established. 2 The reasons for the anisotropy of dislocation emission angle and dislocation slip direction are analyzed by the molecular dynamics method. 3 The reasons for the anisotropy of surface roughness are analyzed from the perspectives of surface energy, atomic arrangement, atomic dense arrangement et al. 4 The anisotropy of work hardening is studied from the aspects of dislocation density, micro-defect evolution, dislocation self-hardening and latent-hardening between slip systems. [ABSTRACT FROM AUTHOR]