Julia R. Weertman

Deformation in nanocrystalline metals

I t i s n o w p o s si bl e t o s y n t h e si z e p ol y c r y s t alli n e m e t al s m a d e u p of g r ai ns that average less than 100 nm in size. Such nanocrystalline metals contain a significant volume fraction of interfacial regions separated by nearly perfect crystals. The small sizes involved limit the conventional operation of dislocation sources and thus a fundamental question arises: how do these materials deform plastically? We review the current views on deformation mechanisms in nanocrystalline, face-centered cubic metals based on insights gained by atomistic computer simulations. These insights are discussed with reference to recent striking experimental observations that can be compared with predictions made by the simulations.

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.

NANOPHASE NI PARTICLES PRODUCED BY A BLOWN ARC METHOD

Nanophase Ni particles (<10 nm in diameter) were produced by a blown arc method. A helium gas stream directed at the arc reduces the Ni vapor concentration and increases the quench rate. The helium gas velocity is the predominant factor influencing the size of the Ni particles. Gas velocities of 20 m/s and 56 m/s (at 26.6 kPa total helium pressure) resulted in Ni particle sizes of 13 nm and 7 nm, respectively.

Retaining the Nano in Nanocrystalline Alloys

Judicious alloying can prevent grain growth in nanocrystalline metals, preserving their useful properties at high temperatures. When nanocrystalline metals first became available, their remarkably high strength seemed to open up many interesting design applications, but several adverse properties were soon noted. A particularly vexing problem was a microstructural instability that decreases strength over time. The small crystal grain size creates a large grain boundary area; the associated high interfacial energy drives coarsening (increase in grain size) that leads to softening. For many advanced alloys, the coarsening problem is accelerated by the prolonged high temperatures associated with processing. Attempts to prevent grain growth in a nanocrystalline metal by alloying have been hit and miss, and in general they have only retarded coarsening. On page 951 of this issue, Chookajorn et al. (1) present a solution to this microstructural instability problem. They performed thermodynamic calculations on a series of candidate binary alloys to determine which are stabilized by grain boundary segregation, not only against coarsening but also against phase separation. A rational method is made available to design nanocrystalline alloys that meet operational requirements, even at elevated temperatures.

Effect of process variables on the structure, residual stress, and hardness of sputtered nanocrystalline nickel films

Nanocrystalline nickel films of about 0.1 mm thickness grown by sputtering with andwithout substrate bias possessed average grain sizes of 9–25 nm. Variation in substratebias at room and liquid nitrogen temperature of deposition strongly affected grainstructure and size distribution. Qualitative studies of film surfaces showed variation inroughness and porosity level with substrate bias and film thickness (maximum of8 mm). The films had tensile residual stress, which varied with deposition conditions.The hardness values were much higher than those of coarse-grained nickel butdecreased with an increase in the film thickness because of grain growth.I. INTRODUCTIONNanocrystalline metals have generated a great deal ofinterest in recent years because they demonstrate impres-sive mechanical behavior characterized by very highstrengths.

In Situ Study of Deformation Mechanisms in Sputtered Free-Standing Nanocrystalline Nickel Films

Nickel films of 1.5-10-μm thickness, produced by dc magnetron sputtering and with disperse grain size distributions peaking in the 30-60-nm range, were subject to in situ tensile straining in a transmission electron microscope. The deformation was stopped frequently, while keeping the load applied, for transmission electron microscopy observation of the internal structure. Contrast changes occurred in many of the grains between strain increments. Ample evidence was seen of dislocation activity, which appears to be the major mechanism for deformation of the samples. Dislocations were seen in grains as small as 20 nm. Parallel arrays of roughly equally spaced dislocations were observed, spaced about 5-10-nm apart. Intergranular nanovoids were found to form and grow with accompanying strain relief in neighboring grains. The results of the current study are generally consistent with previous in situ investigations and contribute to the understanding of deformation mechanisms in free-standing thin films, which may differ somewhat from those in bulk nanocrystalline materials or in films attached to a substrate.

Structural variations in nanocrystalline nickel films

Nanocrystalline nickel films of technological importance have been grown on various liquid nitrogen-cooled substrates by magnetron sputtering with and without a substrate bias. The atomic structural and chemical studies have unveiled variations in inter- and intragranular structures under the different process conditions. The origin and the development of the crystallization process with and without the substrate bias voltage have been inferred from the results.

Probing structures of nanomaterials using advanced electron microscopy methods, including aberration-corrected electron microscopy at the angstrom scale

Structural and compositional studies of nanomaterials of technological importance have been carried out using advanced electron microscopy methods, including aberration‐corrected transmission electron microscopy (AC‐TEM), AC‐high angle annular dark field scanning TEM (AC‐HAADF‐STEM), AC‐energy filtered TEM, electron‐stimulated energy dispersive spectroscopy in the AC‐(S)TEM and high‐resolution TEM (HRTEM) with scanning tunneling microscopy (STM) holder. The AC‐EM data reveal improvements in resolution and minimization in image delocalization. A JEOL 2200FS double‐AC field emission gun TEM/STEM operating at 200 kV in the Nanocentre at the University of York has been used to image single metal atoms on crystalline supports in catalysts, grain boundaries in nanotwinned metals, and nanostructures of tetrapods. Joule heating studies using HRTEM integrated with an STM holder reveal in situ crystallization and edge reconstruction in graphene. Real‐time in situ AC‐HAADF‐STEM studies at elevated temperatures are described. Dynamic in‐column energy filtering in an AC environment provides an integral new approach to perform dynamic in situ studies with aberration correction. The new results presented here open up striking new opportunities for atomic scale studies of nanomaterials and indicate future development directions. Microsc. Res. Tech., 2011.

12 – Mechanical Behavior of Nanocrystalline Metals

Publisher Summary This chapter discusses the mechanical behavior of nanocrystalline metals. The high hardness and strength values that have been obtained in nanocrystalline metals indicate the potential of these materials. Occasional examples of extensive deformation in tension reveal that elimination of flaws and other defects lead to nanocrystalline metals with acceptable ductility for many applications is possible. An important goal is to devise an internal structure that produces adequate strain hardening to prevent early plastic instabilities. Synthesis of quality material in useful quantities and at a competitive cost is a top priority. From the scientific point of view, considerable progress has been made in understanding the deformation mechanisms in nanocrystalline metals. Molecular dynamics simulations have been especially helpful in explaining these mechanisms down to the atomic level. The predictions of the simulations have helped to guide the design of a number of exciting experiments.

Effect of annealing on microstructure, residual stress, and hardness of Al-Ti multilayered films

Al–Ti multilayered films (12 at.% Ti) with bilayer period of 16 nm were deposited by magnetron sputtering. The films were annealed in vacuum at 350 or 400 °C between 2 and 24 h. During annealing, a diffusion-controlled chemical reaction between Al and Ti layers led to Al3Ti precipitation. Differential thermal analysis studies showed an exothermic reaction associated with Al3Ti formation, taking place between 320 and 390 °C, depending on the heating rate. The evolution of microstructure with annealing was examined with transmission electron microscopy and x-ray diffraction. The hardness and residual stress of the films in the as-deposited and annealed conditions were studied in relation to the microstructural changes on annealing.