Jiapeng Sun

Length-dependent mechanical properties of gold nanowires

The well-known size effect is not only related to the diameter but also to the length of the small volume materials. It is unfortunate that the length effect on the mechanical behavior of nanowires is rarely explored in contrast to the intensive studies of the diameter effect. The present paper pays attention to the length-dependent mechanical properties of 〈111〉-oriented single crystal gold nanowires employing the large-scale molecular dynamics simulation. It is discovered that the ultrashort Au nanowires exhibit a new deformation and failure regime-high elongation and high strength. The constrained dislocation nucleation and transient dislocation slipping are observed as the dominant mechanism for such unique combination of high strength and high elongation. A mechanical model based on image force theory is developed to provide an insight to dislocation nucleation and capture the yield strength and nucleation site of first partial dislocation indicated by simulation results. Increasing the length of the nanowires, the ductile-to-brittle transition is confirmed. And the new explanation is suggested in the predict model of this transition. Inspired by the superior properties, a new approach to strengthen and toughen nanowires-hard/soft/hard sandwich structured nanowires is suggested. A preliminary evidence from the molecular dynamics simulation corroborates the present opinion.

Orientation-dependent mechanical behavior and phase transformation of mono-crystalline silicon

We perform a large-scale molecular dynamics simulation of nanoindentation on the (100), (110), and (111) oriented silicon surface to investigate the orientation-dependent mechanical behavior and phase transformation of monocrystalline silicon. The results show both the remarkable anisotropic mechanical behavior and structure phase transformation of monocrystalline silicon. The mechanical behavior of the (110) and (111) oriented surfaces are similar (has a high indentation modulus, low critical indentation depth for the onset of plastic deformation) but quite different from the (100) oriented surface. The mechanical behavior is carefully linked to the phase transformation. The formation of crystalline bct5 phase and β-Si phase is the fundamental phase transformation mechanism for (100) oriented surface. But, a large number of amorphous silicon can be found beneath the indenter for (110) and (111) oriented surface beside the bct5 phase and β-Si phase. The β-Si phase region is relatively small for (110) and ...

Pressure-induced amorphization in the nanoindentation of single crystalline silicon

Large-scale molecular dynamics simulations of nanoindentation on a (100) oriented silicon surface were performed to investigate the mechanical behavior and phase transformation of single crystalline silicon. The direct crystalline-to-amorphous transformation is observed during the nanoindentation with a spherical indenter as long as the applied indentation strain or load is large enough. This amorphization is accompanied by a distinct discontinuity in the load–indentation strain curves, known as “pop-in”. Herein, we have demonstrated the pressure-induced amorphization processes via direct lattice distortion. Moreover, the combination of large shear stress and associated hydrostatic pressure facilitates this crystalline-to-amorphous transformation. The structural characteristics, phase distribution, and phase transformation path have also been discussed in this study. The present results provide a new insight into the mechanical behavior and phase transformation of monocrystalline silicon.

Influence of normal load on the three-body abrasion behaviour of monocrystalline silicon with ellipsoidal particle

Currently, monocrystalline silicon has been widely applied in micro-electro-mechanical systems (MEMSs). It is of importance to reveal the wear behavior of the MEMS and evaluate the planarization of silicon surface in chemical mechanical polishing (CMP). In this study, molecular dynamics simulation was used to investigate the nano three-body abrasion of monocrystalline silicon with a diamond ellipsoidal particle sandwiched between two silicon specimens. The normal load acting on the ellipsoidal particle was varied from 80 nN to 240 nN. Results indicate that the movement pattern of the particle changes from rolling to sliding when the normal load becomes greater than 160 nN. Using the criterion of particle movement pattern by comparing the value of e/h and coefficient of friction, the particle movement pattern can be accurately predicted. The evolution of force in the abrasion process depicts both friction force and normal load fluctuations in sinusoid-like curve for the rolling ellipsoidal particles, whereas the front cutting of particle results in an increase in the friction force, making it greater than the normal force for sliding particles under high velocity. The plastic deformation of monocrystalline silicon is attributed to the phase transformation, which is clearly impacted by the movement pattern of the particle. The rolling of the particle causes substrate deformation with periodical inhomogeneous characteristics, while sliding helps produce a high-quality surface and improves efficiency in the CMP process.

Movement patterns of ellipsoidal particles with different axial ratios in three-body abrasion of monocrystalline copper: a large scale molecular dynamics study

In three-body abrasion, the abrasive particle shape has a major impact on the movement patterns. These consist of sliding or rolling relative to the abraded surfaces. It has been recognized that the movement patterns of the particles dominate the wear mechanism of the materials in three-body abrasion. In this paper, the movement patterns of monocrystalline diamond ellipsoidal particles, which are sandwiched between monocrystalline copper workpieces, were investigated by large-scale molecular dynamics (MD). During the simulations, the axial ratio of the ellipsoidal particle varied from 0.90 (an approximate sphere) to 0.50 (a flattened ellipsoid). It has been found that there is a transition of the movement patterns between rolling and sliding. The particle slides when the axial ratio is smaller than 0.83, and it rolls when the axial ratio is larger than 0.83. Normal load and friction force curves were also obtained relative to the wear time. It has been shown that the average friction coefficient of rolling particles is lower than that of sliding particles. If the ratio of two-moment arms, such as the driving and resistant force moment arms of the particle, is defined as e/h, the curves for the friction coefficient and value e/h can determine the movement patterns of particles at the nanoscale, the same as at the macroscale. When the friction coefficient is higher than e/h, rolling of the particle occurs, whereas the particle slides if the friction coefficient is smaller than e/h. By comparing with macroscale three-body abrasion, a particle at the nanoscale has a strong tendency to roll because of its significant elastic recovery. When the particle rolls, the defect depth, groove depth and dislocation length are all increased relative to particle sliding, resulting in more severe subsurface defects of the monocrystalline copper.

Nanoindentation and deformation behaviors of silicon covered with amorphous SiO2: a molecular dynamic study

A fundamental understanding of the mechanical properties and deformation behaviors of surface modified silicon during chemical mechanical polishing (CMP) processes is difficult to obtain at the nanometer scale. In this research, MD simulations of monocrystalline silicon covered with an amorphous SiO2 film with different thickness are implemented by nanoindentation, and it is found that both the indentation modulus and hardness increase with the growing indentation depth owning to the strongly silicon substrate effect. At the same indentation depth, the indentation modulus decreases shapely with the increase of film thickness because of less substrate influence, while the hardness agrees well with the trend of modulus at shallow depth but mismatches at larger indentation depth. The observed SiO2 film deformation consists of densification and thinning along indentation direction and extension in the deformed area due to the rotation and deformation of massive SiO4 tetrahedra. The SiO2 film plays an important role in the onset and development of silicon phase transformation. The thinner the SiO2 film is, the earlier the silicon phase transformation takes place. So the numbers of phase transformation atoms increase with the decrease of SiO2 film thickness at the same indentation depth. It is suggested that the thicker film should be better during CMP process for higher material removal rate and less defects within silicon substrate.

Precipitation behavior of 14H LPSO structure in single 18R phase Mg–Y–Zn alloy during annealing at 773 K

Abstract The microstructural evolution of a 18R single phase (S18) alloy during annealing at 773 K for 100 h was investigated in order to reveal the formation mechanism of 14H phase. The results showed that the as-cast S18 alloy was composed of 18R phase (its volume fraction exceeds 93%), W particles and α –Mg phase. The 18R phase in S18 alloy was thermally stable and was not transformed into 14H long period stacking ordered (LPSO) phase during annealing. However, 14H lamellas formed within tiny α –Mg slices, and their average size and volume fraction increased with prolonging annealing time. Moreover, the 14H phase is nucleated within α –Mg independently on the basis of basal stacking faults (SFs). The broadening growth of 14H lamellas is an interface-controlled process which involves ledges on basal planes, while the lengthening growth is a diffusion-controlled process and is associated with diffusion of solute atoms. The formation mechanism of 14H phase in this alloy could be explained as α –Mg→ α –Mg+14H.

Hot Workability of the as-Cast 21Cr Economical Duplex Stainless Steel Through Processing Map and Microstructural Studies Using Different Instability Criteria

To develop a fundamental understanding of the flow behavior and optimal hot workability parameters of this material, the hot workability and deformation mechanisms of the as-cast 21Cr EDSS were studied using processing map technology combined with microstructure analysis and isothermal hot compression over the temperature range of 1000–1150xa0°C and strain rate range of 0.01–10xa0s−1. The processing maps and constitutive equation of peak stress were developed based on Prasad’s and Murty’s criteria. The results show that the processing maps exhibit a stable domain at 1000–1150xa0°C and 0.01–1xa0s−1. The instability domain is exhibited at high strain rates (≥1xa0s−1). This implies that Murty’s criterion can predict the unstable domain with high reliability. The detailed deformation mechanisms are also studied by microstructure observation, showing that the flow localization and microcracking are responsible for the flow instability.

Microstructure and corrosion resistance of yellow MAO coatings

ABSTRACT Yellow ceramic coatings were prepared on ultra-fine grained (UFG) AZ91 magnesium alloy using micro-arc oxidation (MAO) process in the alkaline-silicate electrolyte with different KMnO4 addition. Microstructure and phase composition were investigated by SEM, EDS (energy-dispersive spectrometer) and XRD (X-ray diffraction). With 2u2005gL−1 KMnO4 addition, the extreme sparking discharge was inhibited and the coating exhibited more compact and uniform morphology. However, adding 3u2005gL−1 KMnO4 in electrolyte extremely woke the discharge intensity and caused deleterious influence such as micro-crack. The generation of complex oxide Mg6MnO8 phase resulted in the yellow colour of the coatings. Electrochemical measurements including potentiodynamic polarisation and electrochemical impedance spectroscopy tests were carried out in 3.5% NaCl solution. The UFG alloy with the MAO coating produced with 2u2005gL−1 KMnO4 addition exhibited superior corrosion resistance, which was owing to the more compact and uniform microstructural features.

Rebuilding the Strain Hardening at a Large Strain in Twinned Au Nanowires

Metallic nanowires usually exhibit ultrahigh strength but low tensile ductility, owing to their limited strain hardening capability. Here, our larger scale molecular dynamics simulations demonstrated that we could rebuild the highly desirable strain hardening behavior at a large strain (0.21 to 0.31) in twinned Au nanowires by changing twin orientation, which strongly contrasts with the strain hardening at the incipient plastic deformation in low stacking-fault energy metals nanowires. Because of this strain hardening, an improved ductility is achieved. With the change of twin orientation, a competing effect between partial dislocation propagation and twin migration is observed in nanowires with slant twin boundaries. When twin migration gains the upper hand, the strain hardening occurs. Otherwise, the strain softening occurs. As the twin orientation increases from 0° to 90°, the dominating deformation mechanism shifts from slip-twin boundary interaction to dislocation slip, twin migration, and slip transmission in sequence. Our work could not only deepen our understanding of the mechanical behavior and deformation mechanism of twinned Au nanowires, but also provide new insights into enhancing the strength and ductility of nanowires by engineering the nanoscale twins.