Hani E. Elsayed-Ali

Laser-shock processing effects on surface microstructure and mechanical properties of low carbon steel

Abstract The effects of laser-shock processing (LSP) on the microstructure, microhardness, and residual stress of low carbon steel were studied. Laser-shock processing was performed using a Nd:glass phosphate laser with≈600 ps pulse width and up to 120 J pulse energy at power densities above 10 12 W cm −2 . The effects of shot peening were also studied for comparison. Laser-shock induced plastic deformation caused the surface to be recessed by≈1.5 μm and resulted in extensive formation of dislocations. Surface hardness increased by up to 80% after the LSP. The microstructure and mechanical properties were altered up to≈100 μm in depth. The LSP strengthening effect on low carbon steel was attributed to the presence of a high dislocation density. Shot peening resulted in a relatively higher compressive residual stress throughout the specimen than did LSP.

Observation of surface enhanced multiphoton photoemission from metal surfaces in the short pulse limit

Photoelectrons with excess kinetic energy corresponding to several absorbed photons above the work function have been measured from atomically clean Cu(110) and Cu(100) surfaces under ultrahigh vacuum conditions. The power dependence of the photoemission yield does not follow a simple power law dependence corresponding to the number of photons absorbed. This behavior is reminiscent of other above threshold ionization (ATI) or tunnel ionization (TI) processes observed for atoms in the gas phase. The photoelectrons are generated with laser pulsewidths less than 100 fs in duration and peak powers as low as 100 MW/cm2. These intensities are on the order of 105 times lower than that required to observe similar phenomena in the gas phase. The relatively low intensities and correlation with surface roughness suggests a contribution from a surface enhancement mechanism. Thermal heating and space charge effects have been ruled out, and the possibility of electric field enhancement at the surface due to the couplin...

Picosecond reflection high‐energy electron diffraction

Reflection high‐energy electron diffraction with picosecond time resolution is demonstrated. The surface diffraction patterns are obtained using a laser driven picosecond electron gun. Due to the synchronization of the photogenerated electron pulses with the laser source, such a technique provides a picosecond time‐resolved surface structural probe sensitive to the first few monolayers.

Electron pulse broadening due to space charge effects in a photoelectron gun for electron diffraction and streak camera systems

The electron pulse broadening and energy spread, caused by space charge effects, in a photoelectron gun are studied analytically using a fluid model. The model is applicable in both the photocathode-to-mesh region and the postanode electron drift region. It is found that space charge effects in the photocathode-to-mesh region are generally unimportant even for subpicosecond pulses. However, because of the long drift distance, electron pulse broadening due to space charge effects in the drift region is usually significant and could be much larger than the initial electron pulse duration for a subpicosecond electron pulse. Space charge effects can also lead to a considerable electron energy spread in the drift region. Temporal broadening is calculated for an initial electron pulse as short as 50 fs with different electron densities, final electron energies, and drift distances. The results can be used to design electron guns producing subpicosecond pulses for streak cameras as well as for time resolved elec...

Picosecond time-resolved surface-lattice temperature probe

Picosecond reflection high‐energy electron diffraction is used as a time‐resolved surface‐lattice temperature probe. A picosecond laser pulse is split into two beams. The first interacts with the sample. The second activates the cathode of an electron gun creating a collimated and focused electron pulse that is well synchronized with the heating laser pulse. The electron pulse is used to generate a reflection high‐energy electron diffraction pattern of the sample. Since heating results in an intensity reduction of the elastically scattered electrons (Debye–Waller effect), the diffraction pattern provides information on the surface temperature as well as structure. Time‐resolved measurements of the picosecond laser‐heated surface show general agreement with a heat diffusion model.

A picosecond electron gun for surface analysis

Theoretical and experimental investigations for a new design of an ultrashort pulsed laser activated electron gun for time resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time resolved reflection high‐energy electron diffraction in the multiple‐shot operation, are reviewed. Special attention is directed to minimize the photoelectron transit‐time spread from the electron gun, in spite of an unusually long focal length and a small convergence angle of the pulsed electron beam. Both requirements are necessary to use the electron gun for diffraction techniques. The design value for the temporal resolution in the synchroscan operation is 1.3 ps. Based on a thorough theoretical investigation, a new electron gun has been designed, constructed, and tested.

Laser shock‐induced mechanical and microstructural modification of welded maraging steel

The effect of laser‐induced high‐intensity stress waves on the hardness, fatigue resistance, and microstructure in the heat affected zone of welded 18 Ni(250) maraging steel was investigated. Laser‐shock processing increased the hardness and fatigue strength of the weldments. Some melting of the surface was involved during laser‐shock hardening which produced the reverted austenite phase. Microscopic analyses showed an increased dislocation density in the laser‐shocked area.

Modeling of the temperature-dependent spectral response of In1−χGaχSb infrared photodetectors

A model of the spectral responsivity of In1–GaSb p-n junction infrared photodetectors is developed. This model is based on calculations of the photogenerated and diffusion currents in the device. Expressions for the carrier mobilities, absorption coefficient, and normal-incidence reflectivity as a function of temperature are derived from extensions made to Adachi and Caughey-Thomas models. Contributions from the Auger recombination mechanism, which increase with a rise in temperature, are also considered. The responsivity is evaluated for different doping levels, diffusion depths, operating temperatures, and photon energies. Parameters calculated from the model are compared with available experimental data, and good agreement is obtained. These theoretical calculations help us to better understand the electro-optical behavior of In1–GaSb photodetectors, and can be utilized for performance enhancement through optimization of the device structure.

Ultrahigh vacuum picosecond laser-driven electron diffraction system

A laser‐driven picosecond time‐resolved electron diffraction system operating in ultrahigh vacuum is described. A picosecond laser pulse is split into two beams. The first interacts with the sample under study. The second activates the cathode of an electron gun creating a collimated and focused electron pulse that is well synchronized with the heating laser pulse. By spatially delaying the part of the laser pulse that photoactivates the cathode from that which irradiates the sample, the electron pulse can be set to arrive at the sample at a specific time after sample irradiation. When a flat smooth sample is aligned such that the electrons are in grazing incidence on its surface, a reflection high‐energy electron diffraction pattern of its first few atomic layers is generated. Analysis of the diffraction pattern provides information on the surface structure and temperature at a set time lapse between the arrival of the laser and the electron pulse to the sample. Design, characterization, and operation of...

Effects of laser-shock processing on the microstructure and surface mechanical properties of Hadfield manganese steel

The effects of laser-shock processing (LSP) on the microstructure, hardness, and residual stress of Hadfield manganese (1 pct C and 14 pct Mn) steels were studied. Laser-shock processing was performed using a Nd: glass phosphate laser with 600 ps pulse width and up to 120 J/pulse energy at power density above 1012 W/cm2. The effects of cold rolling and shot peening were also studied for comparison. Laser-shock processing caused extensive formation of ε hexagonal close-packed (hep) martensite (35 vol pct), producing up to a 130 pct increase of surface hardness. The surface hardness increase was 40 to 60 pct for the shot-peened specimen and about 60 pct for the cold-rolled specimen. The LSP strengthening effect on Hadfield steel was attributed to the combined effects of the partial dislocation/stacking fault arrays and the grain refinement due to the presence of the ε-hcp martensite. For the cold-rolled and shot-peened specimens, the strengthening was a result of ε-hcp martensite and twins with dislocation effects, respectively. Shot peening resulted in a relatively higher compressive residual stress throughout the specimen than LSP.