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Published Online: October 22 2006 | ss20061000a1
Keywords: PHYSICS, MATERIALS | large-scale molecular dynamics (MD) simulations | face-centred-cubic (fcc) lattice | copper | dislocation flow | high strain-rate deformation | hydrostatic relaxation constant

How small can be a dislocation?

Lin PU
PHYSICS, MATERIALS: The large-scale MD simulation fully sketched a time-resolved relaxing picture of copper single crystals during the initial phase of shock compression beyond the elastic limit. Despite providing a measure of 3D hydrostatic relaxation constant, the product of the mobile dislocation density and average dislocation velocity that is important for understanding the dislocation dynamics upon high strain-rate deformation, the study really reveals an underlying physics: how small can be a dislocation for certain material?

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Nature materials 2006 doi: 10.1038/nmat1735 | Abs . Full
AOP online 20060917




Past calculation on X-ray diffraction patterns predicted that the lattice compression of perfect crystal was essentially uniaxial (1D) [ 1 ], whereas the experiment unambiguously revealed that the deformation or relaxation was hydrostatic (3D) [ 2 ] This contradiction has recently been removed by E. M. BRINGA and the colleagues with molecular dynamics (MD) simulations that are large enough to access the spatial and temporal scales of the experiment [ 3 ].
  

The high strain-rate shocks were generated along the [001] direction of face-centred-cubic (fcc) lattice of 352 million copper atoms decorated with pre-existing defects in the form of prismatic dislocation loops. This large-scale MD simulation fully sketched a time-resolved relaxing picture of copper single crystals during the initial phase of shock compression beyond the elastic limit, that is, above the homogeneous dislocation nucleation threshold for copper. Notably, the metal can achieve a nearly fully relaxed 3D hydrostatic state where the authors expected a larger contribution to the lateral relaxation due to longer shock-transit time.

 

ScideaNews.com-Lin PU: Shock 2006 

shock 2006
Credit: Scidea Art 2006 Source: ScideaNews.com


 
For ambient conditions of dislocations distribution, the loaded shock is indeed super-critical. The generated dislocation density (ρm) by a 35 GPa shock is so high to screen the contribution of pre-existing defects to the final simulated microstructure. The density increased to the maximum in less than 20 ps and saturated after ~70 ps at a steady-state value of ~3x1013 cm-2, i.e., the separation between two neighboring dislocations is 1.826 nm for square lattice, or 1.962 nm for hexagonal lattice. This parameter is nearly beyond the lower limit of the core dimension of the conventional dislocation [ 4 ]. 

As to this point, despite providing a time-resolved measure of 3D hydrostatic relaxation constant (ρmvd), the product of the mobile dislocation density and average dislocation velocity that is important for understanding the dislocation dynamics upon high strain-rate deformation, the study really reveals an underlying physics: how small can be a dislocation for certain material?

  

* Lin Pu is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.

 

 

 

References


 

1

Rosolankova K. et al. in Shock Compression of Condensed Matter-2003 (eds Furnish M. D., Gupta Y. M. & Forbes J. W.) 1195–1198 (AIP, Melville, New York, 2004).
2

Loveridge-Smith A. et al. Anomalous elastic response of silicon to uniaxial shock compression on nanosecond time scales. Phys. Rev. Lett. 86, 2349–2352 (2001). | Full |

3Bringa E. M., Rosolankova K., Rudd R. E., Remington B. A., Wark J. S. , Duchaineau M., Kalantar D. H., Hawreliak J. & Belak J. Shock deformation of face-centred-cubic metals on subnanosecond timescales. Nature Mater. 5, 805–809 (2006). | Abs | Full |
4Hirth J. P. & Lothe J. Theory of Dislocations (Wiley, New York, 1982).

 

 

 

 

Citation
L. PU 

 

Lin PU. How small can be a dislocation? Scidea Sketch 1 (1), ss20061000a1 (2007). 

 

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