Oral contributions

abstract: 4.1
Where and how are dislocations created during plastic deformation ?
M. DOYAMA, Y. KOGURE, T. NOZAKI, Y. KATO, Teikyo University of Science and Technology, Japan.
In the plastic deformation of single crystals the question``where and how dislocations are created'' has been an important problem to solve lattice defects. Copper rectangular parallelepiped single crystals with and without notch were prepared. The specimens were pulled, compressed and bent, using the molecular dynamics with an embedded atom potential. When a specimen with a notch was pulled, partial dislocations were initiated near the tip of the notch as we expected. When a specimen without notch was pulled partial dislocations were initiated near the grip at the ends of the specimen and then other dislocations were created compensating the bend of the specimen near another grip point. When specimens were compressed, many partial dislocations were uniformly created with or without a notch. For bending, partial dislocations were created near the compression surface, particularly near the wrinkles of the slip planes. It was quite hard to create partial dislocations on a smooth extended slip plane. Stress-strain curves are obtained. Very sharp yield points are observed. On a rough surface, dislocations are easier to be created. Thermal vibrations may start partial dislocations near the surfaces. In case of copper partial dislocations were always created instead of complete dislocations because of the low value of stacking faults in copper. The classical picture of bending by dislocations is shown to be wrong. These are clearly shown using the simulation of X-ray diffraction method. The results by computer simulation are compared with those by X-ray Laue method and Lang method. Short movies will be presented.


abstract: 4.4
Dislocation Modeling for the Microelectronics Industry
K.W. SCHWARZ, IBM Research, USA.
Dislocations are of considerable importance in the microelectronics industry, and this talk will describe several instances in which dislocation modeling is proving concretely useful. First, semiconductor processing involves high temperatures, high stresses, crystalline damage, and other factors favoring the production of dislocations; yet a few dislocations in the wrong place on a wafer containing hundreds of millions of devices can cause failure. In this instance, simulations of the nucleation and glide of single dislocations in actual devices under development are proving useful in diagnosing and perhaps even anticipating and preventing such problems. Secondly, the semiconductor roadmap anticipates the use of strained-silicon layers grown heteroepitaxially on relaxed SiGe layers, which in turn may have Si or SOI substrates. Thus, the plastic response of strained layers and multilayers bears directly on the future of the industry. Detailed comparisons between well-controlled experiments and large-scale simulations of layer relaxation have been carried out. Excellent quantitative agreement is obtained, which in turn validates our large-scale dislocation simulations. Runs of this type are currently being utilized to supplement and (to some extent) replace experiments aimed at finding ways to make better relaxed SiGe substrates. Finally, we discuss the possibility of using the uniquely well-controlled silicon environment to nucleate dislocations at particular dislocations on specific glide planes by constructing quantifiable stress-concentrators. Such experiments not only are expected to provide fundamental information about the dislocation nucleation process, they also raise the possibility of being able to engineer specific dislocation configurations.


abstract: 4.8
Continuum dislocation-based approach to size effects in plasticity
RADAN SEDLACEK, TUM, WKM, Garching, Germany. JAN KRATOCHVIL, CVUT, FS, Dpt. of Physics, Praha, Czech Republik. CORNELIA SCHWARZ, EWALD WERNER, TUM, WKM, Garching, Germany.
Size effects with the general tendency `smaller is harder' have been observed on the micrometer scale in various modes of plastic deformation. Whereas the size dependence of the yield strength of thin films on substrates has been ascribed to channeling dislocations that bow out in the film depositing segments at the impenetrable interfaces, the size effect observed in inhomogeneous deformation of free standing samples, like torsion of thin wires or bending of thin foils, has been attributed to the additional hardening caused by geometrically necessary dislocations that are related to plastic strain gradients. A variety of more or less phenomenological continuum strain gradient theories has been developed that are able to reproduce the size effect not only in torsion and bending, but also in confined plasticity of micro-composites and thin films, nano-indentation and crack-tip plasticity. However, the interpretation of the size effects in terms of plastic strain gradients and geometrically necessary dislocations has not been put on firm physical grounds. The discrepancy between the pure dislocation approaches utilizing the channeling dislocations (Orowan-type size effect) and the nonlocal continuum approaches using the strain gradients and geometrically necessary dislocations is obvious.

The purpose of the present contribution is twofold. First, we show that, within the rigorous continuum-mechanics framework, the models using channeling dislocations lead to plastic strain gradients as well [Sedlacek and Forest, phys. stat. sol. (b) 221 (2000), 583.; Sedlacek and Werner, Phys. Rev. B, accepted]. Second, even the size effects in inhomogeneous plastic deformation of free standing samples can be ascribed to bowing of dislocations endowed with line tension. For instance, the role of the impenetrable interface in bending is played by the neutral plane [Sedlacek, Scripta Mater., submitted]. In this way, a kind of a unified approach to size effects in plasticity on the micrometer scale can possibly be developed.


abstract: 4.10
Discrete Dislocation Modeling of Frictional Resistance
VIKRAM S. DESHPANDE, ALAN NEEDLEMAN, Brown University, Division of Engineering, USA; ERIK VAN DER GIESSEN, University of Groningen, Department of Applied Physics, The Netherlands.
Predicting the effective frictional resistance of solid surfaces in contact involves two distinct issues: (i) determining the actual area of contact and (ii) determining the resistance to sliding at a single contact. Here, we focus on issue (ii) for a planar single crystal substrate in contact with a flat indenter. Plastic deformation in the substrate is modeled using discrete dislocation plasticity while the adhesion between the indenter and the substrate is characterized by a shear traction versus sliding displacement cohesive relation. We focus on the value of shear stress needed to initiate sliding of the contact, termed the friction stress. The results show that for sufficiently small contact sizes adhesion dominates and the friction stress is equal to the cohesive strength, while for sufficiently large contacts plasticity dominates and the friction stress is approximately equal to the flow strength. For intermediate contact sizes, the friction stress is contact-size dependent. The effects of superposed normal pressure, cohesive strength and dislocation source density on the value of the friction stress and on its size-dependence are investigated. Representative results include: (i) that a normal pressure less than or equal to twice the tensile yield strength has a negligible effect on the friction stress and (ii) that the main effect of the cohesive strength is to set the value of the small contact size upper-shelf in the friction stress versus contact size relationship. Predictions of the scaling behavior in the size-dependent regime are also presented.


abstract: 4.14
Modelling the motion of twinning dislocations in the HCP metals
A. SERRA, Universitat Politecnica de Catalunya, Barcelona, Spain; D. J. BACON, The University of Liverpool, Liverpool, U.K.

Deformation twinning plays an important role in the plasticity of the HCP metals, but little is known about the dynamics of the mechanisms that control twin boundary motion. A method has been developed to simulate a step with dislocation character in a boundary with full periodicity in the boundary plane, i.e. along both the direction of the line of the defect and its direction of motion. It may be used for investigating the properties of such interfaces as the defects in them move over large distances. With the exception of the 11-21 twin, atomic shuffles are required for glide of twinning dislocations and hence boundary movement, and the efficiency of shuffles may be temperature dependent. In the present paper, we first demonstrate the nature of the method and apply the static variant (T = 0K) to determine the critical stress, i.e. Peierls stress, for motion of twinning dislocations in the 10-12 twin boundary of titanium. The influence of temperature and applied stress is being studied with the dynamic variant of the method and is illustrated here by the mechanism and velocity of motion of twinning dislocations in the 11-22 boundary at 300K and 600K.


abstract: 4.15
Dislocation substructure and surface relief in fatigued metals
JAROSLAV POLAK, MARTIN PETRENEC, JIRI MAN, Institute of Physics of Materials, Academy of Sciences of the Czech Republic.

Initiation of fatigue cracks in crystalline materials is usually preceded by the formation of the pronounced surface relief. Persistent slip markings (PSMs) consisting of extrusions and intrusions arise in locations where the intensive slip bands ``persistent slip bands (PSBs)'' emerge on the surface. The geometry of the PSMs is related to the dislocation structure of the corresponding PSBs. We have studied in detail the geometry of the PSMs on the fatigued specimens of two stainless steels, one with f.c.c. structure, the other with b.c.c. structure using atomic force microscopy (AFM) and high resolution scanning electron microscopy (SEM-FEG). The shape and the height of extrusions and intrusions and the kinetics of their evolution in cyclic straining based on the direct observation of the metallic surface and/or plastic of the replicas is reported. Simultaneously, the internal dislocation structure of the fatigued specimens was studied using transmission electron microscopy. The foils were oriented using electron diffraction and Kikuchi lines. The characteristic features of the dislocation structure in both materials with different crystalline structures are reported. The bands of the dislocation structure corresponding to cyclic strain localization were identified in both materials. Recent mechanisms of the localized cyclic plastic straining based on the real dislocation structure of the PSBs, interaction of dislocations, creation of point defects and their migration to sinks during localized cyclic plastic straining were discussed and compared with the resulting surface relief produced in constant plastic strain amplitude cyclic straining.


abstract: 4.19
Criteria for Dislocation Mechanisms in Plastic Deformation of Crystalline Solids: Deformation of Dislocation-free Thin Foil Specimens and High-speed Deformation of Bulk Specimens
YOSHITAKA MATSUKAWA, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A. and Academic Frontier Research Center for Ultra-high-speed Plastic Deformation, Hiroshima Institute of Technology, Hiroshima, Japan. MASAO KOMATSU, MASAO KOMATSU, KAZUFUMI YASUNAGA, TAMAE TAWARA, MICHIO KIRITANI, Academic Frontier Research Center for Ultra-high-speed Plastic Deformation, Hiroshima Institute of Technology, Hiroshima, Japan.
The mechanism by which dislocation under extreme deformation conditions remain unclear due to lack of definitive experimental data. In the present paper we present experimental data obtained in two extreme conditions, deformation of dislocation-free crystals and high-speed deformation at the strain rates up to 107/s. When thick gold sheets were subjected to in-situ transmission electron microscope (TEM) straining experiments, the specimens gradually decreased in thickness by necking, and then became transparent for TEM observation before fracture. Dislocations were operating while the foil thickness was thick; however, those dislocations escaped to the foil surface, and the thin foil portion became dislocation-free. Thus, subsequent straining of the thin foil produced deformation in a dislocation-free crystal. No dislocations were confirmed while the thin foil portion was still undergoing deformation. Instead of dislocations, vacancy clusters were produced at high density during the plastic deformation. Elastic strain involved in the deformation was evaluated by dynamic observation of diffraction patterns during rupture. The observed elastic strain was 12%, which corresponds to a stress level comparable to the theoretical strength of crystals. These results indicate that the abnormal evolution of microstructure during the plastic deformation of the dislocation-free foils is associated with high stress comparable to the theoretical strength.

The same feature, vacancy clusters at anomalous high density, was confirmed in bulk specimens deformed by high-speed impact deformation. The number of vacancy clusters produced by deformation depended on strain rate, with the vacancy concentration 10-4 in gold for the highest strain rate, 107/s. The critical strain rate for the production of vacancy clusters in bulk specimens was around 104/s, but only $10^{-1}$/s in deformation of dislocation-free thin foils. The difference in the strain rate dependence is likely attributed to the difference of the specimens initial condition. Also, since the maximum velocity of dislocations is generally believed to be above the sound velocity, which corresponds to a strain rate around 108/s, the abnormal deformation microstructure is not simply attributed to the dislocation velocity but must involve other factors, such as the internal stress level during deformation.


abstract: 4.25
Shearing experiments under confining pressure conditions of AlPdMn single quasicrystals
MICHAEL TEXIER, JOËL BONNEVILLE, ANNE PROULT, LUDOVIC THILLY, JACQUES RABIER, Univerité de Poitiers - LMP - Chasseneuil Futuroscope - France.

Quasicrystalline alloys have recently addressed stimulating questions in the field of physical metallurgy in particular since, like in periodic lattices, dislocations have been observed by transmission electron microscopy in these non-periodic structures. Several studies have already been dedicated to identify the microscopic mechanisms by which dislocations move in these structures, but the situation is complex and presently none of the proposed model has been entirely accepted.

Quasicrystalline AlPdMn specimens are known to be extremely brittle at low and intermediate temperatures. Typically, plastic deformation only takes place at temperatures above 950 K for a deformation rate of $10^{-5}$ s$^{-1}$. Techniques commonly used to investigate the plastic behaviour of brittle materials below their brittle-to-ductile transition (BDT) temperature are micro-indentation and deformation under confining pressure. In the present work, we have used an original technique which consists to apply to a thin specimen both a shear stress and a confining pressure. The technique has been successfully applied to plastically deform AlPdMn single quasicrystal specimens with a confining pressure higher than 0.3 GPa at 750K and 960K, i.e. below and above the BDT temperature respectively. The microstructures have been examined, prior to and after plastic deformation, with a JEOL 200CX microscope operating at 200 kV.

The obtained stress-strain curves are similar to those of conventional compression tests and exhibit the yield point. Plastic strains larger than 10% are achieved, but strain markers in the specimens indicate that shearing is not uniformly distributed all over the specimen thickness. At 750 K, the upper yield stress is close to 1.6 GPa. Such a high value, which can only be achieved under confining pressure conditions, is expected from high temperature data extrapolation. The deformed specimen exhibits numerous microcracks and microstructural observations confirm the heterogeneous nature of plastic deformation. Dominant features of low temperature deformation are phase transformations and nano-grains, while high temperature deformation microstructure is characterised by subgrain boundaries, which formation mainly results from dislocation climb processes. The present results confirm the recent idea that glide mechanisms are difficult to initiate in icosahedral quasicrystals.


abstract: 4.28
HVEM Observation of Crack-Tip Dislocations in Silicon Crystals
MASAKI TANAKA, KENJI HIGASHIDA, Kyushu University, Fukuoka Japan.

Crack-tip dislocations in silicon crystals have been observed by high voltage electron microscopy (HVEM). Fine slip bands with nano-scale step heights formed around a crack-tip have been also examined by atomic force microscopy (AFM). Their shielding effect on fracture toughness is discussed based on the calculation of the local stress intensity factor modified by the dislocations.

Cracks are introduced into $\{001\}$ and $\{011\}$ silicon wafers at room temperature by a Vickers indenter. The specimens indented were heated up to high temperatures (873K - 973K) to activate dislocation sources around the crack tip under the residual stress due to the indentation. Selected areas around the crack tip were thinned by focused ion beam (FIB). This method enables us to observe characteristic dislocation structures at the very beginning of the dislocation emission from the crack tip. In addition to the usual observation, plan-view observations have been also carried out, where the crack plane lies parallel to the specimen foil surface so that a wide area around the crack front is observable. Fine slip bands around the crack tip were observed by AFM, and they were in a good agreement with those of the plastic zone expected from dislocation structures observed by HVEM. Based on these observations, the contribution of dislocations to the local stress intensity factor was calculated by using 3-D Bueckner-Rice weight function theory, and it was clarified that the dislocations observed were shielding-type which causes the increase of facture toughness in the brittle-to-ductile transition of silicon crystals.


abstract: 4.30
Microstructural Evolution at High Strain Rates in Solution-Hardened Interstitial Free Steels.
A. UENISHI, LPMTM-CNRS, University of Paris 13, 93430 Villetaneuse, France / Nippon Steel Corporation, Futtsu, Chiba, 293-8511, Japan; C. TEODOSIU, LPMTM-CNRS, University of Paris 13, 93430 Villetaneuse, France; E. V. NESTEROVA, CRISM Prometey, St. Petersburg 193015, Russia.

Recently, the high strain rate properties of steels have drawn an increased attention, due to the strong demand for improving the vehicle crash safety. The mechanical properties at high strain rates might be significantly affected by previous changes in strain rate and/or strain path. In order to master such properties, it is often helpful to understand and model the evolution of the dislocation microstructure.

In the present study, comprehensive TEM observations have been conducted for solution hardened steels deformed at high (1,000 s$^{-1}$) and low (0.001 s$^{-1}$) strain rates, in order to clarify the effects of the strain rate history, eventually involving jumps in strain rate, on the evolution of the microstructure and its association with the mechanical response.

It was revealed that the different types of microstructure, which are observed even within the same specimen, are associated to the corresponding grain orientations, while the evolution of the microstructures with progressive deformation greatly depends on their types. The quantitative analysis of the micrographs provided the changes of the dislocation density and the characteristic length of dislocation patterning, such as the spacing of dislocation boundaries and the cell size, with progressing deformation.

At high strain rates, the dislocation density increases especially at low strains and the onset of dislocation organization is delayed. This result corroborated the mechanical behaviour at high strain rates after compensation for the cross-sectional reduction and the temperature increase of the tensile specimens. The higher flow stress at high strain rate results from the higher work hardening rate, which could be connected to a delay of dislocation organization.

A jump in strain rate increases the dislocation density inside an already organized structure. The difference in flow stress between a constant strain rate experiment and a jump in strain rate could be understood by considering the strain-rate dependency of the microstructural evolution. The high work hardening rate immediately following a jump in strain rate is generally due to the increase of the density of uniformly distributed dislocations inside the preformed structures.


abstract: 4.33
Atomic-scale details of dislocation - stacking fault tetrahedra interaction
YU.N. OSETSKY, Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, TN 37831-6158 USA; D.RODNEY, GPM2-INPG, 101 rue de la physique BP46, F38402 Saint Martin d'Hères, France; D.J. BACON, Materials Science and Engineering, Department of Engineering, The University of Liverpool, Liverpool L69 3GH, UK; R.E.STOLLER, Materials Science and Engineering, Department of Engineering, The University of Liverpool, Liverpool L69 3GH, UK.
Stacking fault tetrahedra (SFTs) are formed under irradiation of fcc metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, taken into account when estimating mechanical properties changes of irradiated materials. The central issue is describing the individual interaction between a moving dislocation and an SFT, which is characterized by a very fine size scale on the order of a few to one-hundred nanometer. This scale is amenable to both in-situ TEM experiments and large-scale atomic modelling. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with SFTs of different sizes at different temperatures and strain rates. The results are compared with observations from in-situ deformation experiments in which interactions between moving dislocations and SFTs were observed. It is demonstrated that in some cases the simulations and experimental observations are quite similar, suggesting a reasonable interpretation of experimental observations. Other cases, when modelling does not reproduce experimental observations, are also discussed and the importance of SFT size, strain rate, dislocation nature and specimen surface effect are discussed.


abstract: 4.44
The effect of grain size on the mechanical properties of nanonickel examined by nanoindentation
BO YANG, HORST VEHOFF, University of Saarland, Germany.
Nanocrystalline nickel produced by pulsed electrolysis was heat-treated to produce grain sizes from nanoscale to micro-nanoscale. Micro- and instrumented nanohardness of these specimens were examined. A NI-AFM (Nanoindenting AFM) was used to measure the interaction between grain boundaries and dislocations. Nanoindentation was performed always in the center of the grains. When the size of the indent was kept constant (constant strain) it could be shown, that the hardness scales with the dislocation density within the grains. However, when the size of the indent approached the grain size, grain boundary sliding and a decrease in hardness was observed.These experiments showed, the increase in hardness could be directly related to the dislocation density within the indented grains whereas the decrease in hardness was related to grain boundary sliding.


abstract: 4.45
Modification of Grain Boundary Structure by Introduction of Lattice Dislocations for Optimal Sliding-Controlled Superplasticity
TADAO WATANABE, Tohoku University, Japan; SHIGEAKI KOBAYASHI, Yokohama National University, Japan; SADAHIRO TSUREKAWA, Tohoku University, Japan.
Superplasticity of polycrystalline materials is well known as collective behavior of grain boundary sliding which strongly depends on the type and structure of grain boundary. High angle random boundaries can slide easily, while low-angle and low-sigma coincidence boundaries are difficult, as demonstrated by eary works on metal bicrystals [1-3]. Moreover, the mechanism of grain boundary sliding ia also known to be the movement of lattice dislocations or grain boundary structural dislocations along the boundary. It is predicted that there is the optimal grain boundary microstructure which can produce the extent of grain boundary sliding suitable for superplasticity and restrict the occurrence of intergranular fracture caused by grain boundary sliding at random boundaries [4]. The present paper will discuss that the modification of the type and structure of grain boundary by the introduction of lattice dislocations is a key issue to produce superplasticity, by engineering the optimal grain boundary microstructure defined by the grain boundary character distribution (GBCD) and the grain boundary connecxtivity. The introduction of new lattice dislocations by strain rate increment during deformation can modify the previously existing sliding grain boundaries and also generate new low-angle dislocation boundaries to produce a new grain boundary microstructure. This was confirmed experimetally on the bsis of the observations of the grain boundary microstructure in aluminium-lithium alloy by using the modern FE-SEM-EBSD/OIM [5].

References:
1. T.Watanabe, M.Yamada and S.Karashima; Phil.Mag.,A40(1979),667-683.
2. H.Kokawa, T.Watanabe and S.Karashimna; Phil.Mag.,A44(1981),1239-1254.
3. T.Watanabe; Met.Trans., 14A(1983), 531-545.
4. T.Watanabe; Mater.Sc.Forum.,304-306(1999),421-430.
5. S.Kobayashi, T. Yoshimura, S.Tsurekawa and T.Watanabe;Mater.Trans.,44 (2003),1469-1479


abstract: 4.52
Influence of dislocation distribution and boundary conditions on plastic response under reverse loading.
DANIEL M. WEYGAND, PETER GUMBSCH, IZBS, University of Karlsruhe, Germany and Insitut für Werkstoffmechanik, FhG, Freiburg, Germany.

A discrete dislocation dynamics model has been use to study the plastic response of a single grain under various boundary conditions and with various (initial) dislocation distributions under consecutive loading and unloading. The employed method allows for handling physical boundary conditions, e.g. displacement or traction controlled, as described in [1]. The influence of image forces and the increasing inhomogeneity of the stress field with increasing deformation, which is a consequence of the dislocation movement through the volume, are taken into account. This inhomogeneity may lead to the activation of secondary slip planes as larger deformations are reached. It is important to note here that this does not rely on rules for dislocation dislocation interaction and the formation or dissolution of locks. This process is uniquely determined by the elastic interaction between the dislocations.

The main interest is to explore the evolution of the dislocation microstructure starting from various initial conditions and grain diameters. These conditions include the boundary conditions, the character of the initial dislocation source distribution, e.g. the number of sources, spacing between occupied glide planes and the initial length of the Frank-Read sources, leading to different distributions of the critical resolved shear stresses to activate these sources. The elastic/plastic response during unloading is discussed with respect to the type of the employed boundary conditions (e.g. traction free or partially displacement controlled) and the question of the reversibility of the plastic flow will be addressed with respect to the localization of slip.

1. D.M. Weygand, L.H. Friedman, E. Van der Giessen, A. Needleman, Mod. Sim. Mater. Sci 10 (2002) 437-468.


abstract: 4.54
An Investigation Of Plastic Zone Deformation Substructures Under The Nanoindentation in an alpha / beta Ti Alloy
GOPAL B VISWANATHAN, EUNHA LEE, The Ohio State University, Columbus, Ohio, USA. SRIKUMAR BANERJEE, BARC, Mumbai, India. DENNIS MAHER, HAMISH L FRASER, The Ohio State University, Columbus, Ohio, USA.

Hardness measurements by indentation is a simple and rapid method for gathering information about the mechanical properties of materials such as elastic modulus, yield stress etc. With its ability to probe a small volume of material, nanoindentation technique is best suited for obtaining the mechanical properties of single crystal alloys from polycrystalline material. In this study this technique has been applied to evaluate the mechanical properties of Ti-Al based alloys containing an alpha/beta two-phase microstructure. Specifically, nanohardness measurements were obtained from individual alpha and beta phases in these alloys. The indentations were placed within the desired grains. The precise individual orientations of the grains were obtained through Orientation Imaging Microscopy (OIM). TEM samples were prepared from the plastic zone beneath the indentation by Focussed Ion Beam (FIB) technique for analyzing the deformation structures. Attempts have been made to correlate the trends that are seen in hardness estimates to various microstructural features such as orientation of individual grains, grain size supported by TEM analysis of the deformation structures. Results indicate that there exist certain relationships between the hardness of the individual grains and their individual orientations. Near the [0001] stress axis the hardness values were the highest and lie in the range 6.3-6.9 GPa. Hardness values decrease as the stress axis deviates from the [0001] orientation. Hardness values were in the medium range i.e. 5.1-5.8 for the grains in the middle of the stereogram and the values were in the lower range i.e. 4.5-5.1, near the [0110] or [1210] orientations. Dislocation analyses indicate that the deformation in individual grain mostly conforms to the Schmid factor type analysis where the slip occurs on those slip systems where CRSS value is high. Additional slip systems are also observed and they are speculated to be present to accommodate the arbitrary shape. An increase in hardness with decreasing depth was observed (especially at depths $<$ 100 nm) in most experiments and it was again depended on the orientations of the individual grains. Based on the TEM dislocation analysis this increase in hardness as been attributed to strain gradient plasticity effects where the geometrically necessary dislocations contribute to higher hardness at smaller depths.