Xiaoguang Guo

Deformation twinning evolution from a single crystal in a face-centered-cubic ternary alloy

Deformation twinning evolution from a single crystal is conducted by molecular dynamics simulations, to elucidate a twinned face-centered-cubic alloy in an experiment with hardness up to 100 times as that of single crystals, and with ductility simultaneously. Critical twinning stress of cadmium zinc telluride (CdZnTe or CZT) calculated is 1.38u2009GPa. All the twin boundaries are along the (11-1) orientation, except the one with the (-111) plane that supports the indentation, interpreting the unidirectional and boundary-free characteristics, confirmed in the experiment. Three twin thicknesses after unloading are 3.2, 3.5, and 16u2009nm, which is consistent with the experimentally repeated pattern of a lamellar twin with thickness larger than 12.7u2009nm, followed by one or several twins with thicknesses smaller than 12.7u2009nm. An inverse triangle of a twin combining with three twins generate a synergistic strengthening effect through the hardening and softening functions, illuminating the ultrahigh hardness demonstrated in the experiment. Twinning takes place in loading, and detwinning occurs in unloading, which expounds the high ductility observed in the experiment.

Modelling the effects of tool edge radius on micro machining based on smooth particle hydrodynamics simulation method

Smooth particle hydrodynamics (SPH) method is introduced to simulate the oxygen-free copper micro-cutting process. At the same cutting thickness r , several different a / r are employed to investigate the effects of tool edge radius on the micromachining mechanism. The simulation results indicate that the larger a / r was, the greater the transverse cutting force was, the smaller the vertical pressure was, which benefits for material removal and good quality surface generation. And it could also be found that the cutting heat was smaller with the smaller a / r , the flows along the rake face generated a large amount of friction heat and oxygen came into the contact slit easily, which caused chemical corrosion on diamond rake face seriously. When the a / r was bigger, chemical corrosion becomes difficult because the cutting tool contacted the material adequately and oxygen was difficult to enter the contact zone.

Simulation research on diamond cutting of mold steel using SPH method

The theory and application of SPH (Smoothed Particle Hydrodynamics) is presented. A Langrangian SPH model is conducted using the LS-DYNA software. As a recently developed and promising method, particles are used in SPH as a substitute for mesh and large material deformation is easily handled. Material separation is well achieved through defining nodes to surface contact. The cutting process of 4340 steel is simulated with SPH method. The process of chip formation is depicted well through the natural flow of SPH particles. The stress distribution and changes of cutting force displays the constitutive behavior of material during cutting. And the maximum value of Von Mises stress, shear stress and plastic strain are concentrated at primary deformation zone, especially near the tool tip.

Study on the effect of grain boundary on the mechanical properties of polysilicon by molecular dynamics simulation

Three-dimensional molecular dynamics simulations are conducted to investigate the effect of grain boundary on the mechanical properties of polycrystalline silicon. The load-displacement curves and nanohardness curves are obtained. The coordination analysis method is introduced to visualise the motion of the silicon atoms. The diagram about stress distribution is applied to analyse the mechanical behaviour of polysilicon under stress. The results show that when the moving direction of the indenter is almost the same as the grain boundary, grain boundaries become sliding source, which causes the suddenly unloading phenomenon. As a result, the nanohardness is continuously decreasing in the process of sliding. When the moving direction of the indenter is perpendicular to the grain boundary, the dislocations of the polysilicon are limited at grain boundaries, pile-up phenomenon occurs, which means the hardening mechanism is directly related to the nanoscale grain boundaries.

Simulation research on ultra-precision cutting of mould steel with diamond tool

Conventional finite element methods are not able to deal with large deformation problems because of severe mesh distortion. And the smooth particle hydrodynamics (SPH) method can avoid this drawback, which is of mesh-free fashion and has an inherent advantage over the FEM methods in cutting simulations. In this paper, an SPH model of ultra-precision cutting is conducted using the LS-DYNA software. The orthogonal cutting process of AISI 4340 steel is simulated. It is revealed that cutting process is mainly performed by the rounded tool tip. During the cutting process, one part of the cutting layer is pushed forward to form the chip and the other part of cutting layer is extruded back into the bulk to form a smooth and dense surface. The simulation also gives an insight into changes of the temperature filed and the result implies cutting speed has a most notable effect on cutting temperature.