J.P. Hirth

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interpret the increasing strength of multilayered metals with decreasing layer thickness when the Hall-Petch model is no longer applicable.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.

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.

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.

Rolling textures in nanoscale Cu/Nb multilayers

Abstract Rolling textures in nanoscale multilayered thin films are found to differ markedly from textures observed in bulk materials. Multilayered thin films consisting of alternating Cu and Nb layers with columnar grains were produced by magnetron sputtering, with individual layer thickness ranging from 4 μm to 75 nm and Cu/Nb interfaces locally satisfying the Kurdjumov–Sachs (K–S) orientation relations. After rolling to 80% effective strain, samples with a larger initial layer thickness develop a bulk rolling texture while those with a smaller initial layer thickness display co-rotation of Cu and Nb columnar grains about the interface normal, in order to preserve the K–S orientation relations. The resulting K–S texture has 〈0u20080u20081〉Nb parallel to and 〈1u20081u20080〉Cu approximately 5° from the rolling direction. A crystal plasticity model based on the Principle of Minimum Shear captures the K–S texture approximately and suggests that Nb drags Cu along in the rotation process.

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 1–10u2009nm, 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.

Mechanism for shear banding in nanolayered composites

Recent studies have shown that two-phase nanocomposite materials with semicoherent interfaces exhibit enhanced strength, deformability, and radiation damage resistance. The remarkable behavior exhibited by these materials has been attributed to the atomistic structure of the bimetal interface that results in interfaces with low shear strength and hence, strong barriers for slip transmission due to dislocation core spreading along the weak interfaces. In this work, the low interfacial shear strength of Cu/Nb nanoscale multilayers dictates a new mechanism for shear banding and strain softening during micropillar compression. Our findings, supported by molecular dynamics simulations, provide insight on the design of nanocomposites with tailored interface structures and geometry to obtain a combination of high strength and deformability. High strength is derived from the ability of the interfaces to trap dislocations through relative ease of interfacial shear, while deformability can be maximized by controlli...

Dislocation structures of Σ3 {112} twin boundaries in face centered cubic metals

High resolution transmission electron microscopy of nanotwinned Cu films revealed Σ3 {112} incoherent twin boundaries (ITBs), with a repeatable pattern involving units of three {111} atomic planes. Topological analysis shows that Σ3 {112} ITBs adopt two types of atomic structure with differing arrangements of Shockley partial dislocations. Atomistic simulations were performed for Cu and Al. These studies revealed the structure of the two types of ITBs, the formation mechanism and stability of the associated 9R phase, and the influence of stacking fault energies on them. The results suggest that Σ3 {112} ITBs may migrate through the collective glide of partial dislocations.

Deformation mechanism maps for polycrystalline metallic multiplayers

Metallic multilayers represent an ideal vehicle for the exploration of length scales in plasticity. They also provide the opportunity to synthesize materials with controlled interfaces and structures for the production of materials close to the theoretical strength. In the present investigation, the authors present a simple analysis that allows us to obtain limiting values of microstructural scales at which these different mechanisms operate. They present the results in the form of two-dimensional maps of layer thickness and grain size ranges over which different deformation mechanisms operate. These maps are intended to be guidelines for interpreting the scale-dependent strengthening or softening mechanisms in multilayers, in the same manner as Ashbys deformation mechanism maps for temperature and stress dependent deformation behavior of metals. An attempt to extend Ashbys deformation mechanism maps to Al thin films by Frost revealed that the predicted strain rates were several orders of magnitude higher than the observed rates due to the higher flow stresses of thin films. Hence, more work is needed to incorporate the fundamental differences in the deformation behavior of thin films and bulk polycrystals to map the mechanisms as a function of stress, temperature and microstructural scale. In this article the authors only considermorexa0» changes in deformation mechanisms with decreasing length scales at constant temperature and strain rates.«xa0less

Pure-Shuffle Nucleation of Deformation Twins in Hexagonal-Close-Packed Metals

The propagation of deformation twins in hexagonal-close-packed metals is commonly described by a conventional glide-shuffle mechanism. The widely accepted convention is that this process is also responsible for twin nucleation, but lacks direct confirmation. Using atomistic simulations, we identify an unconventional pure-shuffle mechanism for the nucleation of (1¯012) twins, which then grow through the conventional glide-shuffle mechanism entailing the glide of twinning disconnections. The pure-shuffle nucleation of twins at grain boundaries can be ascribed to a high-stress concentration and pre-existing grain boundary dislocations.

Nucleation and growth of platelets in hydrogen-ion-implanted silicon

H ion implantation into crystalline Si is known to result in the precipitation of planar defects in the form of platelets. Hydrogen-platelet formation is critical to the process that allows controlled cleavage of Si along the plane of the platelets and subsequent transfer and integration of thinly sliced Si with other substrates. Here we show that H-platelet formation is controlled by the depth of the radiation-induced damage and then develop a model that considers the influence of stress to correctly predict platelet orientation and the depth at which platelet nucleation density is a maximum.