A. Misra

Structure and mechanical properties of Cu-X (X = Nb,Cr,Ni) nanolayered composites

Sputtered Cu/Cr and Cu/Nb multilayers have polycrystalline microstructures with nanometer scale grain sizes, while Cu/Ni multilayers evaporated on single crystal Cu or NaCl were single crystalline. The hardness of the multilayers for layer thicknesses (h) > 50 nm is explained by Hall-Petch model with grain boundaries and interfaces as barriers. At h < 50 nm, a deviation from the Hall-Petch behavior is observed for all three composites. In this regime, plastic flow is believed to occur by single dislocations gliding on closely spaced planes with flow stress proportional to (1/h)ln(h/b). High hardnesses in nanolayered composites result from a combination of increased yield strength and increased work hardening rate at low h.

Single-dislocation-based strengthening mechanisms in nanoscale metallic multilayers

Abstract A breakdown from the dislocation-pile-up-based Hall-Petch model is typically observed in metallic multilayers when the layer thickness (one half of the bilayer period) is of the order of a few tens of nanometres. The multilayer strength, however, continues to increase with decreasing layer thickness to a few nanometres. A model based on the glide of single dislocations is developed to interpret the increasing strength of multilayered metals with decreasing layer thickness when the Hall-Petch model is no longer applicable. The model is built on the hypothesis that plastic flow is initially confined to one layer and occurs by the motion of single ‘hairpin’ dislocation loops that deposit misfit-type dislocations at the interface and transfer load to the other, elastically deforming layer. The composite yield occurs when slip is eventually transmitted across the interface, overcoming an additional resistance from the interface dislocation arrays. In a lower-bound estimate, the stress for the initial glide of ‘hairpin’ dislocation loops, confined to one layer, is similar to the classical Orowan stress. In the upper-bound estimate, the interaction of the glide loop with the existing misfit dislocation arrays at the interface is also considered in deriving the Orowan stress. The effect of in-plane residual stresses in the layers on the Orowan stress calculation is also considered. The model predictions compare favourably with experimentally measured strengths on Cu-based multilayers. When the layer thickness is decreased to a couple of nanometres, the strength reaches a plateau and, in some cases, drops with decreasing layer thickness. The single-dislocation model developed here predicts strengthening with decreasing layer thickness and, therefore, does not explain the deformation behaviour in this regime. In the regime of several nanometres, the deformation behaviour can be explained by dislocation transmission across the interface followed by glide of loops that span several layer thicknesses.

Deformability of ultrahigh strength 5 nm Cu/Nb nanolayered composites

In this work, micropillar compression testing has been used to obtain stress-strain curves for sputter-deposited Cu–Nb nanolaminate composites with nominal bilayer thickness of 10nm. In addition to the extremely high flow strength of 2.4GPa, the 5nm Cu∕5nm Nb nanolaminate exhibits significant ductility, in excess of 25% true strain.

Nanoscale-twinning-induced strengthening in austenitic stainless steel thin films

Magnetron-sputter-deposited austenitic 330 stainless steel (330 SS) films, several microns thick, were found to have a hardness ∼6.5 GPa, about an order of magnitude higher than bulk 330 SS. High-resolution transmission electron microscopy revealed that sputtered 330 SS coatings are heavily twinned on {111} with nanometer scale twin spacing. Molecular dynamics simulations show that, in the nanometer regime where plasticity is controlled by the motion of single rather than pile-ups of dislocations, twin boundaries are very strong obstacles to slip. These observations provide a new perspective to producing ultrahigh strength monolithic metals by utilizing growth twins with nanometer-scale spacing.

Thermal stability of sputtered Cu films with nanoscale growth twins

We have investigated the thermal stability of sputter-deposited Cu thin films with a high density of nanoscale growth twins by using high-vacuum annealing up to 800 °C for 1 h. Average twin lamella thickness gradually increased from approximately 4 nm for as-deposited films to slightly less than 20 nm after annealing at 800 °C. The average columnar grain size, on the other hand, rapidly increased from approximately 50 to 500 nm. In spite of an order of magnitude increase in grain size, the annealed films retained a high hardness of 2.2 GPa, reduced from 3.5 GPa in the as-deposited state. The high hardness of the annealed films is interpreted in terms of the thermally stable nanotwinned structures. This study shows that nanostructures with a layered arrangement of low-angle coherent twin boundaries may exhibit better thermal stability than monolithic nanocrystals with high-angle grain boundaries.

Design of Radiation Tolerant Materials Via Interface Engineering

A novel interface engineering strategy is proposed to simultaneously achieve superior irradiation tolerance, high strength, and high thermal stability in bulk nanolayered composites of a model face-centered-cubic (Cu)/body-centered-cubic (Nb) system. By synthesizing bulk nanolayered Cu-Nb composites containing interfaces with controlled sink efficiencies, a novel material is designed in which nearly all irradiation-induced defects are annihilated.

Suppression of the shear band instability during plastic flow of nanometer-scale confined metallic glasses

The shear band instability that occurs during plastic deformation of metallic glasses limits the application of these high-strength materials. We show that this instability can be suppressed in nanometer-scale metallic glasses constrained by ultrafine crystalline layers. Free-standing Cu/amorphous Pd0.77Si0.23 multilayers consisting either of 10∕90nm glass/Cu or 100∕100nm glass/Cu were deformed to layer thickness reductions greater than 75% by cold rolling or bending, respectively. Transmission electron microscopy showed uniform reduction in the layer thickness with no shear band formation in the amorphous layers. The mechanisms that allow homogeneous codeformation of metallic glasses with nanoscale crystalline layers at high stresses are discussed.

Influence of interfaces on the storage of ion-implanted He in multilayered metallic composites

We studied ion beam mixing and He accumulation in Cu∕Nb multilayer thin films after 33keV He implantation at room temperature to a dose of 1.5×1017atoms∕cm2. Multilayered thin films consisting of alternating Cu and Nb layers were produced by magnetron sputtering. Two types of samples, one with an individual layer thickness of 4nm and another with 40nm were examined. The Cu∕Nb samples were analyzed in the as-deposited state, after He ion implantation, as well as after post-implantation annealing. The ion beam mixing of the interface structure was monitored by Rutherford backscattering spectrometry and cross-section transmission electron microscopy imaging. Elastic recoil detection analysis was performed to examine the helium concentration depth distribution. Scanning electron microscopy was employed to investigate He blister formation upon annealing. A comparison of the results deduced from the methods listed above reveals a very high morphological stability of the nanolayered structure. The nanolayered st...

Formation of nanoporous noble metal thin films by electrochemical dealloying of PtxSi1−x

We demonstrate the synthesis of nanoporous Pt thin films on Si by electrochemical dealloying. Amorphous PtxSi1−x films (∼100–250nm thick) are formed by electron beam codeposition and dealloyed in aqueous HF solutions at an electrochemical potential sufficient to selectively remove Si while allowing self-assembly of Pt into a nanoporous structure. The Pt nanoporous layers have a pore size of 5–20nm, ligament thickness ∼5nm, a surface area enhancements >20 times, and an ultrafine grain polycrystalline microstructure.

Are nanoporous materials radiation resistant

The key to perfect radiation endurance is perfect recovery. Since surfaces are perfect sinks for defects, a porous material with a high surface to volume ratio has the potential to be extremely radiation tolerant, provided it is morphologically stable in a radiation environment. Experiments and computer simulations on nanoscale gold foams reported here show the existence of a window in the parameter space where foams are radiation tolerant. We analyze these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (i) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically ∼5 nm for primary knock on atoms (PKA) of ∼15 keV in Au), while (ii) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKAs energy and dose rate used in this study). In between these dimensions, (i.e., ∼100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose-rate. We conclude that foams can be tailored to become radiation tolerant.