H. Van Swygenhoven

Nanocrystalline electrodeposited Ni: microstructure and tensile properties

The microstructure of commercially available nanocrystalline (nc) electroplated Ni foils is studied by means of Xray diffraction and transmission electron microscopy. It is shown that the microstructure is inhomogeneous and batch-dependent. Tensile properties at strain rates between 10(-5) and 10(3) s(-1) are studied and compared with the results of coarse-grained Ni. Data on strength, strain-rate sensitivity and work hardening are presented. At the highest strain rates, shear banding with local grain growth is observed in the nc structure. It is also suggested that the differences found in nc Ni for 3 and 20 mm tensile specimens are the size effects related to the inhomogeneous microstructure. (K) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

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.

Plastic behavior of nanophase Ni: A molecular dynamics computer simulation

We report molecular dynamics computer simulations of low temperature-high load plastic deformation of Ni nanophase samples with several mean grain sizes in the range of 3–5 nm. The samples are polycrystals nucleated from different seeds, with random location and orientation. Among the mechanisms responsible for the deformation, grain boundary sliding and motion, as well as grain rotation are identified. No dislocation activity is detected, in contrast to the behavior of coarse grain metals. Interpreting the results in terms of grain boundary viscosity, a linear dependence of strain rate with the inverse of the grain size is obtained.

A molecular dynamics study of polycrystalline fcc metals at the nanoscale: grain boundary structure and its influence on plastic deformation

Molecular dynamics computer simulation of nanocrystalline Ni and Cu show that grain boundaries in nanocrystalline metals have the short range structure of most grain boundaries found in conventional polycrystalline materials. The simulations also indicate the presence of a critical grain size below which all plastic deformation is accommodated in the grain boundary and no intra-grain deformation is observed.

Atomistic simulations as guidance to experiments

Abstract Atomistic simulations have provided unprecedented insight into the structural and mechanical properties of nanocrystalline materials. However the extrapolation of this knowledge to the experimental regime requires a clear understanding of the temporal and spatial scales of the modeling technique and a detailed structural characterisation of the simulated samples.

Mechanical and tribological performance of MoS2 co-sputtered composites

Abstract Co-sputtered films of MoS 2 with Au, Ti, Cr or WSe 2 , respectively, have been deposited in order to investigate the relationships between dopant and tribological and mechanical behaviour. The alloying constituents were selected based on possible chemical effects, for example substitution in the MoS 2 structure, intercalation, edge-bond passivation, and with and without simultaneous oxygen gettering of the sputtering environment. The mechanical properties of the composite films were investigated by pin-on-disk sliding wear testing and linear, oscillating wear testing both in 50% relative humidity air, microscratch and indent adhesion characterisation, and nanoindentation hardness as a function of the dopant element(s) and concentration. All of the dopants investigated exhibited superior tribological and mechanical behaviour compared to the pure MoS x films. In tribological tests, results indicate a significant improvement in the lifetime, mean friction coefficient, and friction ‘noise’ of the films. Tribological performance was found to be strongly related to both the dopant element and dopant concentration. The best dopant for improved performance was found with WSe 2 followed by Au, Ti, and Cr.

Crystal rotation in Cu single crystal micropillars : In situ Laue and electron backscatter diffraction

In situ microdiffraction experiments were conducted on focused ion beam machined single crystal Cu pillars oriented for double slip. During deformation, the crystal undergoes lattice rotation on both the primary and critical slip system. In spite of the initial homogeneous microstructure of the Cu pillar, rotation sets in already at yield and is more important at the top of the pillar than at the bottom, demonstrating the inhomogeneous stress state during a microcompression experiment. The rotation results are confirmed by electron backscatter diffraction measurements.

Length scale effects in the simulation of deformation properties of nanocrystalline metals

The observed inverse grain size strain rate dependency in Al columnar structures [Acta Mater 49 (2001) 2713] can be explained by a geometrical argument arising when a dislocation traverses the entire grain. The paper addresses also the differences in dislocation activity in a 2D-columnar and a full 3D-nanostructured geometry.

Defect structure in micropillars using X-ray microdiffraction

White beam x-ray microdiffraction is used to investigate the microstructure of micron-sized Si, Au, and Al pillars fabricated by focused ion beam (FIB) machining. Comparison with a Laue pattern obtained from a Si pillar made by reactive ion etching reveals that the FIB damages the Si structure. The Laue reflections obtained from the metallic pillars fabricated by FIB show continuous and discontinuous streakings, demonstrating the presence of strain gradients.

Atomistic simulation of dislocation emission in nanosized grain boundaries

The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB) in nanocrystalline metals. It is shown that, in a 12 nm grain-size sample, GBs containing grain-boundary dislocations (GBDs) can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free-volume migration. As in previous work, the nucleation occurs at a GBD, which, upon nucleation and propagation, is removed. In the present case, free-volume migration occurs away from the nucleation region both before and after the nucleation event.