Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect

Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.

https://www.pnas.org/content/pnas/112/47/14501.full.pdf

Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility.

Mechanical behavior of high-entropy alloys

Overview on micro- and nanomechanical testing: New insights in interface plasticity and fracture at small length scales

Mechanistic investigation of a low-alloy Mg–Ca-based extrusion alloy with high strength–ductility synergy

Size effects in Al nanopillars: Single crystalline vs. bicrystalline

Emergence of enhanced strengths and Bauschinger effect in conformally passivated copper nanopillars as revealed by dislocation dynamics

We constructed and developed an in-situ cryogenic nanomechanical system to study small-scale mechanical behavior of materials at low temperatures. Uniaxial compression of two body-centered-cubic (bcc) metals, Nb and W, with diameters between 400 and 1300 nm, was studied at room temperature and at 165 K. Experiments were conducted inside of a Scanning Electron Microscope (SEM) equipped with a nanomechanical module, with simultaneous cooling of sample and diamond tip. Stress-strain data at 165 K exhibited higher yield strengths and more extensive strain bursts on average, as compared to those at 298 K. We discuss these differences in the framework of nano-sized plasticity and intrinsic lattice resistance. Dislocation dynamics simulations with surface-controlled dislocation multiplication were used to gain insight into size and temperature effects on deformation of nano-sized bcc metals.

Cold-temperature deformation of nano-sized tungsten and niobium as revealed by in-situ nano-mechanical experiments

Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress–strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material model based on a viscoplastic solid with plastic non-normality and a hardening–softening–hardening plastic flow relation. The hardening slope in compression generated by the model was directly related to the stiffness gradient along the sample height, and hence to the local relative density. These results demonstrate that a microstructural figure-of-merit, the effective relative density, can be used to quantify and predict the mechanical response.

Local relative density modulates failure and strength in vertically aligned carbon nanotubes.

We report the mechanical behavior of vertically aligned carbon nanotube films, grown on Si substrates using atmospheric pressure chemical vapor deposition, subjected to in situ large displacement (up to 70 μm) flat-punch indentations. We observed three distinct regimes in their indentation stress–strain curves: (i) a short elastic regime, followed by (ii) a sudden instability, which resulted in a substantial rapid displacement burst manifested by an instantaneous vertical shearing of the material directly underneath the indenter tip by as much as 30 μm, and (iii) a positively sloped plateau for displacements between 10 and 70 μm. In situ nanomechanical indentation experiments revealed that the shear strain was accommodated by an array of coiled carbon nanotube “microrollers,” providing a low-friction path for the vertical displacement. Mechanical response and concurrent deformation morphologies are discussed in the foam-like deformation framework with a particular emphasis on boundary conditions.

/pdf/compressive-response-of-vertically-aligned-carbon-nanotube-4n2vcl7sun.pdf

Julia R. Greer

Papers

Size effects in Al nanopillars: Single crystalline vs. bicrystalline

Emergence of enhanced strengths and Bauschinger effect in conformally passivated copper nanopillars as revealed by dislocation dynamics

Cold-temperature deformation of nano-sized tungsten and niobium as revealed by in-situ nano-mechanical experiments

Local relative density modulates failure and strength in vertically aligned carbon nanotubes.

Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations