R.G. Hoagland

Efficient Annealing of Radiation Damage Near Grain Boundaries via Interstitial Emission

Preventing Radiation Damage Inside a nuclear reactor, long-term exposure to radiation causes structural damage and limits the lifetimes of the reactor components. Bai et al. (p. 1631; see the Perspective by Ackland) now show, using three simulation methods able to cover a wide range of time and length scales, that grain boundaries in copper can act as sinks for radiation-induced defects. The boundaries are able to store up defects, in the form of interstitials, which subsequently annihilate with vacancies in the bulk. This recombination mechanism has a lower energy barrier than the bulk equivalent, and so provides a lower-cost route for the copper to self-heal. Simulations show that grain boundaries store and annihilate radiation-induced defects in copper. Although grain boundaries can serve as effective sinks for radiation-induced defects such as interstitials and vacancies, the atomistic mechanisms leading to this enhanced tolerance are still not well understood. With the use of three atomistic simulation methods, we investigated defect–grain boundary interaction mechanisms in copper from picosecond to microsecond time scales. We found that grain boundaries have a surprising “loading-unloading” effect. Upon irradiation, interstitials are loaded into the boundary, which then acts as a source, emitting interstitials to annihilate vacancies in the bulk. This unexpected recombination mechanism has a much lower energy barrier than conventional vacancy diffusion and is efficient for annihilating immobile vacancies in the nearby bulk, resulting in self-healing of the radiation-induced damage.

On the strengthening effects of interfaces in multilayer fee metallic composites

Abstract The slip behaviour in coherent and semicoherent metallic bilayer composites is examined by atomic simulation in the Cu/Ni and Cu/Ag systems. The coherent interface in Cu/Ni, although energetically unfavourable relative to the semicoherent interface in thick layers, reveals several interesting phenomena. Linear elastic predictions of lattice strains to achieve coherency (removing the 2.7% lattice mismatch) are found not to satisfy equilibrium. The cause is nonlinearity in the elastic response. The application of stresses needed for glide dislocations to cross the interface or to escape from the interface exacerbates the nonlinearities in the elastic response of the system. Koehler forces, arising from elastic mismatch, are in some cases observed to have the wrong sign relative to linear elastic predictions. Core structures of misfit dislocations in semicoherent interfaces are observed to be quite different in the cube-on-cube oriented Cu/Ni and Cu/Ag systems with interfaces parallel to (010). In the former case, the (α/2){110) misfit dislocations have very narrow cores in the plane of the interface but dissociate into Lomer-Cottrell locks out of the interface towards the Cu side. The dissociation is enhanced by the application of tensile stresses and can lead to reactions that form continuous stacking-fault structures. Such structures are shown to be potent barriers to slip. The stability of such structures are analysed and, within the approximations used, we find that such structures may be more stable than the usual two-dimensional flat grid of misfit dislocations. The misfit dislocations at Cu-Ag interfaces, on the other hand, are wide and so fairly mobile in the interface plane. Reactions between misfit dislocations and glide dislocations are discussed.

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.

High-strength sputter-deposited Cu foils with preferred orientation of nanoscale growth twins

Bulk Cu foils have been synthesized via magnetron sputtering with an average twin spacing of 5 nm. Twin interfaces are of {111} type and normal to the growth direction. Growth twins with such high twin density and preferred orientation have never been observed in elemental metals. These Cu foils exhibited tensile strengths of 1.2 GPa, a factor of 3 higher than that reported earlier for nanocrystalline Cu, average uniform elongation of 1%-2%, and ductile dimple fracture surfaces. This work provides a route for the synthesis of ultrahigh-strength, ductile pure metals via control of twin spacing and twin orientation in vapor-deposited materials. (c) 2006 American Institute of Physics.

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...

On the role of weak interfaces in blocking slip in nanoscale layered composites

Layered composites of Cu/Nb achieve very high strength levels when the individual layer thicknesses are 1–10 nm, attributable to the interfaces acting as barriers to slip. Atomistic models of Cu/Nb bilayers were used to explore the origins of this resistance. The models clearly show that dislocations placed near an interface experience an attraction toward the interface, regardless of the sign of the Burgers vector or the material in which it is placed. This attraction is caused by shear of the interface induced by the stress field of the dislocation. Furthermore, the dislocation, upon reaching the interface, is absorbed by it in the sense that the core spreads within the interface. We develop a model, using a fractional dislocation approach, which provides an estimate of the strength of the attraction as a function of distance from the interface and also the dependence of the interaction on the type of dislocation. A screw dislocation is much more effective in shearing the interface, and the resulting attractive forces on screws are larger than for edge dislocations.

Microstructures and strength of nanoscale Cu–Ag multilayers

Abstract Cu–Ag multilayers were found to have lower peak hardness than Cu–Ni in spite of lower misfit dislocation spacing that is expected to increase the resistance of interfaces to glide dislocation transmission. This is attributed to misfit dislocation core spreading in the interface plane in Cu–Ag.

Thermal stability of self-supported nanolayered Cu/Nb films

We report the development of thermally stable nanoscale layered structures in sputter deposited Cu/Nb multilayered films with 75 nm individual layer thickness, vacuum annealed at temperatures of 800°C or lower. The continuity of the layered structure was maintained and layer thickness unchanged in the annealed films. The nanolayers were observed to be offset by shear at the triple-point junctions that had equilibrium groove angles and were aligned in a zigzag pattern. A mechanism is proposed for the evolution of this ‘anchored’ structure that may be resistant to further morphological instability.