Abstract
Plasticity is ubiquitous and plays a critical role in material deformation and damage; it inherently involves the atomistic length scale and picosecond time scale. A fundamental understanding of the elastic-plastic deformation transition, in particular, incipient plasticity, has been a grand challenge in high-pressure and high-strain-rate environments, impeded largely by experimental limitations on spatial and temporal resolution. Here, we report femtosecond MeV electron diffraction measurements visualizing the three-dimensional (3D) response of single-crystal aluminum to the ultrafast laser-induced compression. We capture lattice transitioning from a purely elastic to a plastically relaxed state within 5 ps, after reaching an elastic limit of ~25 GPa. Our results allow the direct determination of dislocation nucleation and transport that constitute the underlying defect kinetics of incipient plasticity. Large-scale molecular dynamics simulations show good agreement with the experiment and provide an atomic-level description of the dislocation-mediated plasticity.
Highlights
Dynamic compression waves propagating into solids can cause irreversible plastic deformation where the deviatoric stresses exceed yield strengths
A 600-fs-long bunch length of the relativistic electrons (~220 μm in diameter) makes it possible to probe the ultrafast microstructural evolution during plastic deformation. Using this time-resolved electron diffraction technique, our experiment demonstrates a complete history of incipient plasticity in single-crystal materials
Experimental setup and static diffraction signal. This experiment was performed on the mega-electronvolt ultrafast electron diffraction (MeV-UED) instrument of the Linac Coherent Light Source (LCLS)[16,17]
Summary
To further understand the elastic–plastic deformation process, we calculate the longitudinal elastic strain, εel , and the transverse elastic strain, εet , from the corresponding diffraction peak shifts (Materials and methods are available as supplementary materials). Plastic deformation is interrupted by the rarefaction wave originating from the sample free surface These experimental results of the temporal evolution of εel and εet and their dependence on pump fluence are well reproduced by our MD simulations performed with the experimental conditions. MeV-UED combined with laser-driven compression provides unprecedentedly the ultrahigh temporal and spatial resolutions, and the broad reciprocal space sampling to resolve the structural dynamics at the very onset of plastic deformation These results resolve unambiguously the associated 1D to 3D lattice deformation processes and provide a complete history of Mobile dislocation density ρm (x 1011 cm-2)
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