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Register a new userMolecular dynamics simulation of twin boundary effect on deformation of Cu nanopillars
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Molecular dynamics simulations performed on <110> Cu nanopillars revealed significant difference in deformation behavior of nanopillars with and without twin boundary. The plastic deformation in single crystal Cu nanopillar without twin boundary was dominated by twinning, whereas the introduction of twin boundary changed the deformation mode from twinning to slip consisting of leading partial followed by trailing partial dislocations. This difference in deformation behavior has been attributed to the formation of stair-rod dislocation and its dissociation in the twinned nanopillars.
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Molecular dynamics simulations are performed to investigate the role of a coherent {Sigma}3 (111) twin boundary on the plastic deformation behavior of Cu nanopillars. Our work reveals that the mechanical response of pillars with and without the twin boundary is decisively driven by the characteristics of initial dislocation sources. In the condition of comparably large pillar size and abundant initial mobile dislocations, overall yield and flow stresses are controlled by the longest, available mobile dislocation. An inverse correlation of the yield and flow stresses with the length of the longest dislocation is established, and its extrapolation agrees well with experimental yield stress data. The experimentally reported subtle differences in yield and flow stresses between pillars with and without the twin boundary are thus likely related to the maximum lengths of the mobile dislocations. In the condition of comparably small pillar size, for which a reduction of mobile dislocations during heat treatment and mechanical loading occurs, the mechanical response of pillars with and without the twin boundary can be clearly distinguished. Dislocation starvation during deformation is more clearly present in pillars without the twin boundary than in pillars with the twin boundary because the twin boundary acts as a pinning surface for the dislocation network.
Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from one to five providing twin boundary spacing in the range 8.5 - 2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism, due to which the twin boundaries offers a negligible resistance to slip of twinning partials. This is reflected in the negligible variation of yield stress as a function of twin boundary spacing. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. Further, it has been observed that the curved twin boundary can acts as a source for full dislocation. The occurrence of twin-twin interaction during tensile deformation and dislocation-twin interaction during compressive deformation were presented and discussed.
Molecular dynamics simulations have been performed to understand the variations in deformation mechanisms of Cu nanowires as a function of orientation and loading mode (tension or compression). Cu nanowires of different crystallographic orientations distributed uniformly on the standard stereographic triangle have been considered under tensile and compressive loading. The simulation results indicate that under compressive loading, the orientations close to $<$100$>$ corner deform by twinning mechanism, while the remaining orientations deform by dislocation slip. On the other hand, all the nanowires deform by twinning mechanism under tensile loading. Further, the orientations close to $<$110$>$ and $<$111$>$ corner exhibit tension-compression asymmetry in deformation mechanisms. In addition to deformation mechanisms, Cu nanowires also display tension-compression asymmetry in yield stress. The orientations close to $<$001$>$ corner exhibits higher yield stress in tension than in compression, while the opposite behaviour (higher yield stress in compression than in tension) has been observed in orientations close to $<$110$>$ and $<$111$>$ corners. For the specific orientation of $<$102$>$, the yield stress asymmetry has not been observed. The tension-compression asymmetry in deformation mechanisms has been explained based on the parameter $alpha_M$, defined as the ratio of Schmid factors for leading and trailing partial dislocations. Similarly, the asymmetry in yield stress values has been attributed to the different Schmid factor values for leading partial dislocations under tensile and compressive loading.
In this work, the single-component Cu metallic glass was fabricated by the physical vapor deposition on the Zr (0001) crystal substrate at 100 K using the classical molecular dynamic simulation. The same deposition process was performed on the Cu (1 0 0) and Ni (1 0 0) crystal substrate for comparison, only the Cu crystal deposited layer with the fcc structure can be obtained. When depositing the Cu atoms on the Zr substrate at 300 K, the crystal structure was formed, which indicates that except the suitable substrate, low temperature is also a key factor for the amorphous structure formation. The Cu liquid quenching from 2000 K to 100 K were also simulated with the cooling rate 1012 K/s to form the Cu glass film in this work. The Cu metallic glass from the two different processes (physical vapor deposition and rapid thermal quenching from liquid) revealed the same radial distribution function and X-ray diffraction pattern, but the different microstructure from the coordination number and Voronoi tessellation analysis.
We perform molecular dynamics simulations of friction for atomically thin Xe films sliding on Ag(111). We determine the inverse of the coefficient of friction (i.e. slip time) by direct calculation of the decay of the center of mass velocity after applying an external force, as well as from the velocity autocorrelation function. We find that the slip time exhibits a drop followed by a sharp increase in a range of coverage near one monolayer. The slip time then levels off with further coverage increases in agreement with previously reported experiments. Our simulations suggest that the friction found in this system is dominated by phonon excitations.
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