H. Kung

Structure and Mechanical Behavior of Bulk Nanocrystalline Materials

The reduction of grain size to the nanometer range (˜2-100 nm) has led to many interesting materials properties, including those involving mechanical behavior. In the case of metals, the Hall-Petch equation, which relates the yield stress to the inverse square root of the grain size, predicts great increases in strength with grain refinement. On the other hand, theory indicates that the high volume fraction of interfacial regions leads to increased deformation by grain-boundary sliding in metals with grain size in the low end of the nanocrystalline range. Nanocrystalline ceramics also have desirable properties. Chief among these are lower sintering temperatures and enhanced strain to failure. These two properties acting in combination allow for some unique applications, such as low-temperature diffusion bonding (the direct joining of ceramics to each other using moderate temperatures and pressures). Mechanical properties sometimes are affected by the fact that ceramics in a fine-grained form are stable in a different (usually higher pressure) phase than that which is considered “normal” for the ceramic. To the extent that the mechanical properties of a ceramic are dependent on its crystal-lographic structure, these differences will become evident at the smaller size scales. It is uncertain how deformation takes place in very fine-grained nanocrystalline materials. It has been recognized for some time that the Hall-Petch relationship, which usually is explained on the basis of dislocation pileups at grain boundaries, must break down at grain sizes such that a grain cannot support a pileup. Even some of the basic assumptions of dislocation theory may no longer be appropriate in this size regime. Recently considerable progress has been made in simulating the behavior of extremely fine-grained metals under stress using molecular-dynamics techniques. Molecular-dynamics (MD) simulations of deformation in nanophase Ni and Cu were carried out in the temperature range of 300–500 K, at constant applied uniaxial tensile stresses between 0.05 GPa and 1.5 GPa, on samples with average grain sizes ranging from 3.4 nm to 12 nm.

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.

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.

In-situ TEM tensile testing of DC magnetron sputtered and pulsed laser deposited Ni thin films

Two nanocrystalline Ni thin films, one prepared via DC Magnetron Sputtering and the other prepared via Pulsed Laser Deposition, were strained in-situ in the Transmission Electron Microscope. Although the grain sizes were similar, the two films behaved quite differently in tension. The sputtered material was found to behave in a brittle manner, with failure occurring via rapid coalescence of intergranular cracks. Conversely, the laser deposited film behaved in a ductile manner, with failure occurring by slow ductile crack growth. The difference in failure mechanism was attributed to the presence of grain boundary porosity in the sputtered thin film. Both films exhibited pervasive dislocation motion before failure, and showed no conclusive evidence of a change in deformation mode.

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.

Creep of nanocrystalline Cu, Pd, and Al-Zr

Creep tests were carried out over a range of temperatures (0.24 – 0.64 Tm) and stresses on samples of nanocrystalline Cu, Pd, and Al-Zr made by inert gas condensation and compaction. The measured creep rates are two to four orders of magnitude smaller than the values predicted by the equation for Coble creep. At moderate temperatures, the creep rates are comparable or lower than the corresponding coarse-grain rates. Except for the tests performed at the highest homologous temperatures, all creep curves can be fit by the equation for logarithmic creep. The samples in the as-prepared state are highly twinned, and it is believed that the prevalence of low-energy grain boundaries together with inhibition of dislocation activity caused by the small grain sizes are responsible for the low strain rates.

Texture development in IBAD MgO films as a function of deposition thickness and rate

We have examined the effect of film thickness on in-plane texture for ion-beam assisted deposition (IBAD) of MgO films. Plan-view dark-field transmission electron microscopy (TEM) has revealed that texture develops rapidly, reaching its best value at a critical thickness of/spl sim/10 nm. These results have been confirmed by quantifying the in-plane texture of these samples at each thickness with X-ray diffraction /spl phi/-scans. We have also examined the effects of variable deposition rate on texture formation. X-ray diffraction shows that the optimum in-plane texture is achieved at the critical thickness with a rate of 0.2 nm/s. However, TEM imaging has shown that the distribution of well-aligned grains decreases with an increase in rate. As such, deposition at 0.1 nm/s was found to be sufficient for achieving good in-plane distribution values and good surface coverage for subsequent depositions. By combining the results of both of these experiments, we were then able to optimize our deposition process and apply them to the growth of IBAD MgO on metal substrates.

Residual stresses and ion implantation effects in Cr thin films

Abstract The evolution of intrinsic residual stresses in sputtered Cr thin films with substrate bias and post-deposition ion irradiation is investigated. The relaxation of tensile stresses and build up of compressive stresses with increasing ion irradiation dose is studied using ions of different masses and energies such as 110 keV Ar, 33 keV C and 330 keV Xe. The stress evolution is related to the corresponding microstructural changes in the films. The changes in the residual stress during ion irradiation are explained by considering the manner in which the interatomic distances and forces change during irradiation, and the generation of defects during irradiation.