Ductile crystalline–amorphous nanolaminates

Abstract

It is known that the room-temperature plastic deformation of bulk metallic glasses is compromised by strain softening and shear localization, resulting in near-zero tensile ductility. The incorporation of metallic glasses into engineering materials, therefore, is often accompanied by complete brittleness or an apparent loss of useful tensile ductility. Here we report the observation of an exceptional tensile ductility in crystalline copper/copper-zirconium glass nanolaminates. These nanocrystalline-amorphous nanolaminates exhibit a high flow stress of 1.09 ؎ 0.02 GPa, a nearly elastic-perfectly plastic behavior without necking, and a tensile elongation to failure of 13.8 ؎ 1.7%, which is six to eight times higher than that typically observed in conventional crystallinecrystalline nanolaminates (<2%) and most other nanocrystalline materials. Transmission electron microscopy and atomistic simulations demonstrate that shear banding instability no longer afflicts the 5-to 10-nm-thick nanolaminate glassy layers during tensile deformation, which also act as high-capacity sinks for dislocations, enabling absorption of free volume and free energy transported by the dislocations; the amorphous-crystal interfaces exhibit unique inelastic shear (slip) transfer characteristics, fundamentally different from those of grain boundaries. Nanoscale metallic glass layers therefore may offer great benefits in engineering the plasticity of crystalline materials and opening new avenues for improving their strength and ductility.metallic glass ͉ size-dependent plasticity ͉ nanocrystalline materials ͉ amorphous-crystalline interface ͉ tensile ductility A traditional strategy to develop ultrahigh-strength crystalline materials is to limit or inhibit the motion of dislocations required for plastic deformation (1-3) so that a higher applied stress is necessary. Examples of such advanced materials include thin films (4), nanocrystalline metals (5-7), and nanolaminates (8-10). As dislocation motion in high-strength crystalline materials becomes increasingly difficult (11), the ductility, i.e., the ability of a material to change shape without catastrophic failure, is often reduced dramatically (6, 7). In bulk metallic glasses, plastic deformation is not enabled by dislocations (12-21) but rather by clusters of atoms that undergo cooperative shear displacements [shear transformation zones (STZs)] (16); in the extreme limit of homogeneous-toinhomogeneous flow transition, shear bands of nanoscale width form (17,(19)(20)(21). The formation of such shear bands causes large strain softening and abrupt rupture of the metallic glasses. By way of contrast, large compressive plastic strains have been obtained in several bulk metallic glasses (12)(13)(14). Nonetheless, they show nearzero macroscopic ductility when subjected to tensile loading. To our knowledge, there is no experimental evidence currently suggesting that macroscopic metallic glass samples can sustain large tensile plasticity. An interesting question arises whether shear banding remains...

High-Temperature Stability and Grain Boundary Complexion Formation in a Nanocrystalline Cu-Zr Alloy

“…43 Because nanocrystalline metals with grain size above 10 nm plastically deform through grain boundary dislocation emission and absorption, 45 grain boundary complexions should have a major impact on the ductility and toughness of these materials. In fact, this hypothesis is supported by the work of Wang et al 46 on nanocrystalline Cu/Cu-Zr nanolaminates. Although they were not complexions created by thermodynamic driving forces, the thin amorphous layers that were added between nanocrystalline Cu layers were found to increase ductility when compared to a monolithic nanocrystalline Cu film.…”

Smaller critical size and enhanced strength by nano-laminated structure in nickel

“…The effects of length scale (layer thickness) on the strength of bi-metal multilayered laminates have also been studied from the micrometer to the nanometer range [21][22][23][24][25][26][27]. The strengthening effects as a function of layer thickness can be summarized as three mechanisms: (i) the dislocation pile-up mechanism from submicron to micron range; (ii) the confined layer slip mechanism from few nm to a few tens of nm range; (iii) the interface crossing mechanism for about 1-2 nm [21,22].…”

Thickness-Dependent Strain Rate Sensitivity of Nanolayers via the Nanoindentation Technique

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