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Employing optical phase-contrast microscopy we study the morphology of ultrashort pulsed laser-induced modifications in bulk fused silica and in other optical transparent materials for different conditions of irradiation. The influence of the input pulse energy, focusing depth, and number of pulses per site is investigated in order to establish optimal irradiation conditions for direct writing of waveguiding elements. The results obtained suggest that an increase of the refractive index is systematically accompanied by a region of lower index of refraction along the optical axis, presumably due to micro-explosion in the high energy-exposed regions, and the locations of this region is material dependent. Nevertheless, the creation of an hypothetical void in the interaction zone does not allow a complete explanation of the modifications observed and other mechanisms have to be invoked to explain the presence of regions with higher index of refraction. Different interaction regimes with respect to the input energy and to the number of shots per site can be established from these observations, emphasizing the role of nonlinear pulse propagation and plasma generation. To complement the experimental observations we simulate the propagation of an ultrashort laser pulse in bulk silica using the non-linear Schrödinger equation and determine the distribution of energy along the propagation axis. The results are in good agreement with the experimental observations and provides insights into the mechanisms controlling the interaction between ultrashort laser pulses and transparent materials.

Titanium aluminides find application in modern light-weight, high-temperature turbines, such as aircraft engines, but suffer from poor plasticity during manufacturing and processing. Huge forging presses enable materials processing in the 10-GPa range, and hence, it is necessary to investigate the phase diagrams of candidate materials under these extreme conditions. Here, we report on an in situ synchrotron X-ray diffraction study in a large-volume press of a modern (α 2 + γ) two-phase material, Ti-45Al-7.5Nb-0.25C, under pressures up to 9.6 GPa and temperatures up to 1686 K. At room temperature, the volume response to pressure is accommodated by the transformation γ → α 2 , rather than volumetric strain, expressed by the apparently high bulk moduli of both constituent phases. Crystallographic aspects, specifically lattice strain and atomic order, are discussed in detail. It is interesting to note that this transformation takes place despite an increase in atomic volume, which is due to the high ordering energy of γ. Upon heating under high pressure, both the eutectoid and γ-solvus transition temperatures are elevated, and a third, cubic β-phase is stabilized above 1350 K. Earlier research has shown that this β-phase is very ductile during plastic deformation, essential in near-conventional forging processes. Here, we were able to identify an ideal processing window for near-conventional forging, while the presence of the detrimental β-phase is not present under operating conditions. Novel processing routes can be defined from these findings.

Laser peening without coating (LPwC) involves irradiating materials covered with water with intense laser pulses to induce compressive residual stress (RS) on a surface. This results in favorable effects, such as fatigue enhancement; however, the mechanism underlying formation of the compressive RS is not fully understood. In general, tensile RS is imparted on the surface of the material due to shrinkage after heating by laser irradiation. In this study, we assessed the thermo-mechanical effect of single laser pulse irradiation and introduce a phenomenological model to predict the outcome of LPwC. To validate this model, RS distribution across the laser-irradiated spot was analyzed using X-ray diffraction with synchrotron radiation. In addition, the RS was evaluated across a line and over an area, following irradiation by multiple laser pulses with partial overlapping. Large tensile RSs were found in the spot irradiated by the single pulse; however, compressive RSs appeared around the spot. In addition, the surface RS state shifted to the compressive side due to an increase in overlap between neighboring laser pulses on the line and over the area of irradiation. The compressive RSs around a subsequent laser spot effectively compensated the tensile component on the previous spot by controlling the overlap, which may result in compressive RSs on the surface after LPwC.