Date Published: July 07, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Oleg Kovalenko, Christian Brandl, Leonid Klinger, Eugen Rabkin.
Some metal alloys subjected to irreversible plastic deformation can repair the inflicted damage and/or recover their original shape upon heating. The conventional shape memory effect in metallic alloys relies on collective, or “military” phase transformations. This work demonstrates a new and fundamentally different type of self‐healing and shape memory in single crystalline faceted nano and microparticles of pure gold, which are plastically deformed with an atomic force microscope tip. It is shown that annealing of the deformed particles at elevated temperatures leads to nearly full recovery of their initial asymmetric polyhedral shape, which does not correspond to global energy minimum shape. The atomistic molecular dynamic simulations demonstrate that the shape recovery of the particles is controlled by the self‐diffusion of gold atoms along the terrace ledges formed during the particles indentation. This ledge‐guided diffusion leads to shape recovery by the irreversible diffusion process. A semiquantitative model of healing developed in this work demonstrates a good agreement with the experimental data.
Self‐healing materials are able to recover their structural integrity after inflicted mechanical damage. The design of artificial smart self‐healing materials is inspired by the numerous examples of biological systems exhibiting the ability of self‐healing after being wounded. While most of the research on biomimetic self‐healing materials is focused on polymers and polymer‐based nanocomposites,1 several concepts of self‐healing in metallic materials have recently emerged.2 These concepts include the stress‐ and capillary‐driven segregation and precipitation of the second phase in the creep induced cavities or at the crack tips,3, 4 incorporation of the shape memory alloy (SMA) wires in the metal matrix composite,5 and the use of encapsulated solders.6 All these concepts rely on some kind of phase transformation (i.e., melting, precipitation, diffusionless martensitic transformation) in the multicomponent, multiphase composite materials. Yet no comparable experimental observations of self‐healing in pure metals are available in the literature; although the theoretical possibility of crack self‐healing in pure metal through diffusionless disclination–crack interaction is predicted by molecular dynamics (MD) simulation.7
We demonstrated that the localized plastic deformation of faceted single crystalline Au nano and microparticles produced by sharp diamond tip can be almost fully recovered by annealing the deformed particles at the temperature of ≈0.65Tm, where Tm is the melting point of Au. The deformed particles restore their initial anisotropic, polyhedral shape which is different from the thermodynamically equilibrium shape of Au crystal of the same volume. Our atomistic MD simulations and kinetic model demonstrated that the mechanism of the observed shape memory and self‐healing effects is related to the accelerated diffusion of Au atoms along the terrace ledges formed by the dislocations egressing at the free surface of the particle during plastic deformation. This allows us to formulate the conditions necessary for observing the shape memory and self‐healing effects in metal nano and microparticles: The plastic deformation of the particles has to be initiated by dislocation nucleation in a defect free crystal without subsequent dislocation storage in the bulk, since all plastic flow of material should be accommodated by the surface slip lines.22, 33, 34 Therefore, the effects observed in the present work are limited to the deformation of faceted metal particles, and annealing below the surface roughening temperature.The full shape equilibration and achieving the thermodynamic equilibrium crystal shape should be suppressed, i.e., by the high energy barrier associated with the island nucleation on the facets.20 This indicates a restricted window of parameters (annealing time, temperature, and particle size) for self‐healing.
Experimental Details: The Au particles were obtained employing solid state dewetting of 30 nm thick Au film deposited by the electron‐beam deposition technique on the c‐plane oriented polished sapphire substrate of 2 in. in diameter (Gavish Inc.). The substrate was ultrasonically cleaned in acetone, ethanol, isopropanol, and deionized (DI) water and lithographically patterned prior to the deposition. The photolithography procedure included standard process steps: vapor prime with hexamethyldisilazane, spin coating of resist, soft bake, contact printing exposure using mercury lamp source mask aligner (KARL SUSS MA‐6), post bake, development, and hard bake. The liftoff procedure was performed in 70 °C 1‐methyl‐2‐pyrrolidone for 3 min followed by rinsing in acetone, ethanol, isopropanol, and DI water. The samples were annealed in the tube resistance furnace in ambient air for 24 h at 1173 K, resulting in agglomeration of the patterned film and formation of the faceted Au particles.
The authors declare no conflict of interest.