Nano-grained (NG) metals are believed to be strong but intrinsically brittle: Free-standing NG metals usually exhibit a tensile uniform elongation of a few percent. When a NG copper film is confined by a coarse-grained (CG) copper substrate with a gradient grain-size transition, tensile plasticity can be achieved in the NG film where strain localization is suppressed. The gradient NG film exhibits a 10 times higher yield strength and a tensile plasticity comparable to that of the CG substrate and can sustain a tensile true strain exceeding 100% without cracking. A mechanically driven grain boundary migration process with a substantial concomitant grain growth dominates plastic deformation of the gradient NG structure. The extraordinary intrinsic plasticity of gradient NG structures offers their potential for use as advanced coatings of bulk materials.

https://science.sciencemag.org/content/sci/early/2011/02/16/science.1200177.full.pdf

Revealing Extraordinary Intrinsic Tensile Plasticity in Gradient Nano-Grained Copper

Deformation twinning in nanocrystalline materials

http://nanomechanics.mit.edu/sites/default/files/documents/2007_Acta_nc_Overview.143.pdf

Toward a quantitative understanding of mechanical behavior of nanocrystalline metals

http://physics.gmu.edu/~ymishin/Coupling.pdf

Coupling grain boundary motion to shear deformation

We report significant back stress strengthening and strain hardening in gradient structured (GS) interstitial-free (IF) steel. Back stress is long-range stress caused by the pileup of geometrically necessary dislocations (GNDs). A simple equation and a procedure are developed to calculate back stress basing on its formation physics from the tensile unloading–reloading hysteresis loop. The gradient structure has mechanical incompatibility due to its grain size gradient. This induces strain gradient, which needs to be accommodated by GNDs. Back stress not only raises the yield strength but also significantly enhances strain hardening to increase the ductility.Impact Statement: Gradient structure leads to high back stress hardening to increase strength and ductility. A physically sound equation is derived to calculate the back stress from an unloading/reloading hysteresis loop.

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2016.1153004

Back stress strengthening and strain hardening in gradient structure

There have been long-standing concerns about the stability of the internal structure of nanocrystalline metals. In this letter we examine grain growth in nanocrystalline Cu under the microhardness indenter, examining the influence of temperature of indentation and sample purity. Surprisingly, it is found that grain coarsening is even faster at cryogenic temperatures than at room temperature. Sample purity is seen to play an important role in determining the rate of grain growth. Fast grain coarsening can affect the outcome of mechanical tests, especially if they involve large stresses and high-purity samples.

Rapid stress-driven grain coarsening in nanocrystalline Cu at ambient and cryogenic temperatures

Microhardness measurements have been carried out on high purity Cu samples with average grain sizes ranging from ∼10 to ∼200nm, over temperatures from liquid nitrogen to ambient, and dwell-times of the indenter in the sample from 5 s to 39 h. The Vickers hardness diminishes approximately linearly with the logarithm of the dwell-time. At short dwell-times the hardness increases significantly with decreasing grain size and with decreasing temperature, but the influence of these variables substantially diminishes at longer times. Investigation by transmission electron microscopy shows that rapid grain growth under the indenter most likely is responsible for the strong and long-lasting indentation creep.

Kai Zhang

Papers

Rapid stress-driven grain coarsening in nanocrystalline Cu at ambient and cryogenic temperatures

The influence of time, temperature, and grain size on indentation creep in high-purity nanocrystalline and ultrafine grain copper