Showing papers by "Jeffrey W. Kysar published in 2007"

High strain gradient plasticity associated with wedge indentation into face-centered cubic single crystals: Geometrically necessary dislocation densities

Abstract: Experimental studies on indentation into face-centered cubic (FCC) single crystals such as copper and aluminum were performed to reveal the spatially resolved variation in crystal lattice rotation induced due to wedge indentation. The crystal lattice curvature tensors of the indented crystals were calculated from the in-plane lattice rotation results as measured by electron backscatter diffraction (EBSD). Nye's dislocation density tensors for plane strain deformation of both crystals were determined from the lattice curvature tensors. The least L 2 -norm solutions to the geometrically necessary dislocation densities for the case in which three effective in-plane slip systems were activated in the single crystals associated with the indentation were determined. Results show the formation of lattice rotation discontinuities along with a very high density of geometrically necessary dislocations.

Viscoplastic and granular behavior in films of colloidal nanocrystals.

Abstract: Nanoindentation measurements of electrophoretically deposited films of colloidal CdSe nanocrystals, capped by organic ligands, show the films have an elastic stiffness modulus of approximately 10 GPa and exhibit viscoplasticity. This mechanical response suggests polymeric features that are attributable to the ligands. After particle cross-linking and partial ligand removal, the films exhibit more features of granularity.

Influence of ultrasonic irradiation on the microstructure of Cu/Al2O3, CeO2 nanocomposite thin films during electrocodeposition

Abstract: The agglomeration of nanoparticles during electrocodeposition of nanocomposite thin films is an unresolved issue. In this paper, the effects of ultrasonic irradiation on agglomeration during electrocodeposition of Cu–Al2O3 and Cu–CeO2 nanocomposite thin films on a silicon substrate are investigated. In addition, the effect of electrolyte concentration on agglomeration of nanoparticles is investigated. Irradiation by ultrasound during electrocodeposition yields smaller grain size, improved surface conditions, decreased agglomeration and higher volume fraction of nanosized inert particles within the nanocomposite thin films.

Plastic deformation in nanoscale gold single crystals and open-celled nanoporous gold*

Abstract: The results of two sets of experiments to measure the elastic?plastic behaviour of gold at the nanometre length scale are reported. One set of experiments was on free-standing nanoscale single crystals of gold, and the other was on free-standing nanoscale specimens of open-celled nanoporous gold. Both types of specimens were fabricated from commercially available leaf which was either pure Au or a Au/Ag alloy following by dealloying of the Ag. Mechanical testing specimens of a 'dog-bone' shape were fabricated from the leaf using standard lithographic procedures after the leaf had been glued onto a silicon wafer. The thickness of the gauge portion of the specimens was about 100?nm, the width between 250?nm and 300?nm and the length 7??m. The specimens were mechanically loaded with a nanoindenter (MTS) at the approximate midpoint of the gauge length. The resulting force?displacement curve of the single crystal gold was serrated and it was evident that slip localization occurred on individual slip systems; however, the early stages of the plastic deformation occurred in a non-localized manner. The results of detailed finite element analyses of the specimen suggest that the critical resolved shear stress of the gold single crystal was as high as 135?MPa which would lead to a maximum uniaxial stress of about 500?MPa after several per cent strain. The behaviour of the nanoporous gold was substantially different. It exhibited an apparent elastic behaviour until the point where it failed in an apparently brittle manner, although it is assumed that plastic deformation occurred in the ligaments prior to failure. The average elastic stiffness of three specimens was measured to be Enp = 8.8?GPa and the stress at ultimate failure averaged 190?MPa for the three specimens tested. Scaling arguments suggest that the stress in the individual ligaments could approach the theoretical shear strength.

Study of anisotropic character induced by microscale laser shock peening on a single crystal aluminum

Abstract: The beam spot size used in microscale laser shock peening is of the same order as grain size in many materials. Therefore, the deformation is induced in only a few grains so that it is necessary to treat the material as being anisotropic and heterogeneous. In order to investigate the corresponding anisotropic features, different experimental techniques and three-dimensional finite element simulations are employed to characterize and analyze anisotropic responses for single crystal aluminum under single pulse shock peening at individual locations. X-ray microdiffraction techniques based on a synchrotron light source affords micron scale spatial resolution and is used to measure the residual stress spatial distribution along different crystalline directions on the shocked surface. Crystal lattice rotation due to plastic deformation is also measured with electron backscatter diffraction. The result is experimentally quantified and compared with the simulation result obtained from finite element analysis. The...

Deformation and fracture behavior of electrocodeposited alumina nanoparticle/copper composite films

Abstract: Electrocodeposition of alumina nanoparticles and copper thin film on silicon wafers was performed. The volume fraction of the nanoparticle is about 5% and the size is about 50 nm. Comparison between the static tensile behaviors of specimens with and without nanoparticles reveals that the Young’s modulus is significantly increased by incorporating nanoparticles into the copper film. However, the ultimate tensile strength of the nanocomposite (235 MPa) is slightly lower than that of the pure copper reference specimen (250 MPa). For the nanocomposite, the strain at failure is 7.8%, which is lower than that of the pure copper film (10.5%). Distinct microscale deformation mechanisms are observed: the main deformation mechanism of the pure copper film is slip followed by strain hardening, whereas for the nanocomposite, multistage failure behaviors are found due to the debonding at the nanoparticle/copper interface. Notched specimens were also tested and compared with the unnotched specimens. In addition, cyclic loading tests on the nanocomposite were conducted to show its hardening behavior.

Response of thin films and substrate to micro-scale laser shock peening

Abstract: Micro-scale laser shock peening ({mu}LSP) can potentially be applied to metallic structures in microdevices to improve fatigue and reliability performance. Copper thin films on a single-crystal silicon substrate are treated by using {mu}LSP and characterized using techniques of X-ray microdiffraction and electron backscatter diffraction (EBSD). Strain field, dislocation density, and microstructure changes including crystallographic texture, grain size and subgrain structure are determined and analyzed. Further, shock peened single crystal silicon was experimentally characterized to better understand its effects on thin films response to {mu}LSP. The experimental result is favorably compared with finite element method simulation based on single-crystal plasticity.

Microscale laser peen forming of single crystal: dynamic deformation and anisotropy

Abstract: Microscale laser peen-forming (μLPF) is an innovative process in which compressive residual stress is induced into the treated surface to improve fatigue life of the micro scale metallic parts and meanwhile shape the parts with a controlled bending deformation. In this work, dynamic deformation in μLPF of single crystal aluminum is investigated. A finite element model based on meso scale crystal plasticity integrated with consideration of dynamic effects is implementated to simulate the shock wave propagation in aluminum single crystal and to predict the deformation, lattice rotation and the induced surface residual stress distributions in μLPF. The crystal elastic moduli are pressure dependent to capture the large volume strains properly and to enable evolution of shocks from steep pressure gradient. Experimentally, local plastic deformation as well as residual stress distributions on both top and bottom surfaces measured by X-ray micro-diffraction, is compared with the results obtained from FEM simulation.

Micro-scale laser peen forming of single crystal

Abstract: As the result of quickly increased requirement in many industrial products resulting from microtechnology, laser thermal micro-forming and micro surface treatment (microscale laser shock peening (μLSP)) have been well studied. By combining the beneficial effects of these two processes with a controlled bending deformation, microscale laser peen forming (μLPF) attracts more attention recently since it not only improves the fatigue life of the material but also shapes micro scale metallic parts at the same time. In the present study, μLSP of single crystal aluminum was presented to study anisotropic material response. Local plastic deformation was characterized by lattice rotation measured through electron backscatter diffraction (EBSD). Residual stress distributions of both sides of a peened sample, characterized by x-ray micro-diffraction, were compared with the results obtained from FEM simulation. μLPF anisotropic behavior was investigated in three effective slip systems via both the anisotropic slip line theory and numerical method. Also, the work hardening effect resulted from self-hardening and latent hardening was analyzed through comparing the results with and without considering hardening.As the result of quickly increased requirement in many industrial products resulting from microtechnology, laser thermal micro-forming and micro surface treatment (microscale laser shock peening (μLSP)) have been well studied. By combining the beneficial effects of these two processes with a controlled bending deformation, microscale laser peen forming (μLPF) attracts more attention recently since it not only improves the fatigue life of the material but also shapes micro scale metallic parts at the same time. In the present study, μLSP of single crystal aluminum was presented to study anisotropic material response. Local plastic deformation was characterized by lattice rotation measured through electron backscatter diffraction (EBSD). Residual stress distributions of both sides of a peened sample, characterized by x-ray micro-diffraction, were compared with the results obtained from FEM simulation. μLPF anisotropic behavior was investigated in three effective slip systems via both the anisotropic slip li...

Grain Boundary Response of Aluminum Bicrystal Under Micro Scale Laser Shock Peening

Abstract: Micro scale laser shock peening (lLSP) is a process in which compressive residual stresses are induced in a material surface to improve fatigue life and wear resistance under cyclic loading. Since the diameter of the laser spot used during the process is the same order of magnitude as grain size, the effects of anisotropy and heterogeneity have to be explicitly taken into account in any model of the process. In this study experimental and numerical studies have been performed in order to investigate the response of an aluminum bicrystal under laser shock peening. The grain boundary is shocked to investigate heterogeneity, and single crystals are shocked to study the effect of anisotropy in the absence of heterogeneity. The orientations of the crystals in the bicrystal as well as the reference single crystals have been chosen such that an approximate plane strain condition is achieved. A finite element model which accounts for the anisotropy, heterogeneity and inertia has also been developed based on single crystal micromechanics. Simulation results are compared with experimental findings. The potential benefit of lLSP as a surface treatment for improvement of fatigue life is also discussed.