Bulk metallic glasses (BMG) are amorphous metal alloys known for their unique physical and mechanical properties. In the present study, the formation of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on the Zr-based BMGs Zr46Cu46Al8, Zr61Cu25Al12Ti2, Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit105) and Zr57Cu15.4Al10Ni12.6Nb5 (Vit106) was investigated as a function of their different chemical composition. For this purpose, LIPSS were generated on the sample surfaces in an air environment by fs-laser irradiation (& lambda; = 1025 nm, & tau; = 300 fs, frep = 100 kHz). The surface topography was characterized by scanning electron microscopy and atomic force microscopy. Moreover, the impact of LIPSS formation on the structure and chemical surface composition was analyzed before and after fs-laser irradiation by X-ray diffraction and X-ray photoelectron spectroscopy as well as by transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Despite the different chemical composition of the investigated BMGs, the fs-laser irradiation resulted in almost similar properties of the generated LIPSS patterns. In the case of Zr61Cu25Al12Ti2, Vit105 and Vit106, the surface analysis revealed the preservation of the amorphous state of the materials during fs-laser irradiation. The study demonstrated the presence of a native oxide layer on all pristine BMGs. In addition, fslaser irradiation results in the formation of laser-induced oxide layers of larger thickness consisting of an amorphous ZrAlCu-oxide. The precise laser-structuring of BMG surfaces on the nanoscale provides a versatile alternative to thermoplastic forming of BMG surfaces and is of particular interest for the engineering of functional material surfaces.

Upon exposure to oxygen, NiTi forms a Ti-rich surface oxide layer, and mass balance requires that a Ni-enriched zone forms below the oxide. Both the surface oxide layer and the Ni-enriched zone are discussed to affect key properties of NiTi as material for minimally invasive medical application or for actuators, e.g., the release of Ni and the resistance to initiation and propagation of cracks when exploiting the shape memory effect/pseudoelasticity. However, owing to the small extension of the Ni-enriched zone of a few nanometers, little is known about its crystallinity, phase and evolution of composition. We study the formation of the surface layers during annealing at similar to 500 degrees C for up to 10 min, as routinely applied for shape-setting. With an approach employing nano beam electron diffraction in a transmission electron microscope and an in-house software package for phase analysis it is shown for the first time that phase transformations in the Ni-enriched zone, specifically from NiTi to Ni4Ti3, occur already after 2 min of annealing, much earlier than documented in the literature. Furthermore, the phase transformation to the thermodynamically stable Ni3Ti in the Ni-enriched zone is shown to be mediated by metastable Ni4Ti3. In contrast to Ni4Ti3 precipitates in NiTi bulk, the morphology of the crystalline phases is granular. Considering the swift formation of Ni-rich phases below the surface oxide and the observation of a band-like diffraction contrast in the TEM images originating from crystal interfaces, a phase transformation mechanism in the Ni-enriched zone is suggested consisting of short range order rearrangements of atoms.

In this work, attempts have been made to improve the oxidation resistance of Fe-Si alloys with 0.5, 1.0 and 2.0 mass pct Si by a preheating treatment, which was carried out at 1073 K for 24 hours in high purity hydrogen atmosphere. Compared with pure Fe and untreated Fe-Si alloys, the oxidation resistance of the preheated Fe-Si alloys at 673 K in O 2 is drastically increased, in the case of 2 mass pct Si by two orders of magnitude. The improvement is attributed to a uniform, dense and continuous 25 nm pure SiO 2 layer formed on the alloy surface during the preheating treatment in hydrogen atmosphere hindering the diffusion of Fe and O.

Pseudoelastic deformation of NiTi usually results in cracks at the surface. Cracking is promoted by surface oxide layers that form, e.g., during heat treatments required for shape-setting of minimally invasive implants. In connection with the advancing miniaturization of minimally invasive implants, the question arises whether their mechanical integrity may get impaired by such cracks. In the present work, the characteristics of the cracks was investigated in cross section with the help of targeted preparation using focused ion-beams. SEM and TEM on samples after a single cycle of pseudoelastic deformation revealed that cracks extend perpendicular to the loading direction in the surface oxide layer and change to angles between 90° and 45° in the Ni-rich layer below. Pores observed in the surface oxide close to the Ni-rich layer did not prevent the extension of cracks towards the NiTi bulk. When the cracks reach the NiTi, blunting of the crack tip was observed. The crack length essentially corresponds to the thickness of the surface oxide layer and the Ni-rich layer. The findings provide data for estimating crack propagation in according implants in the future.

The present work investigates the universal applicability of glow discharge plasmas for the microstructure representation of different materials taking the example of Ni-Ti alloys, Cu-Zn alloys, and the Ni based alloy "Hastelloy C 276". Results are compared with the results provided by classical etching methods. Microstructures became visible for the previously mechanically polished materials within a few seconds, even without detailed optimization of the excitation conditions of the glow discharge plasma. The results partially significantly outperfomed the results of the classical preparation techniques with respect to the detectability of structural details such as grain and phase boundaries. Due to the demonstrated wide and relatively uncomplicated applicability, the per se established glow discharge technology is expected to have a huge potential for application for a rapid and high-contrast microstructure representation.

The use of focused ion beams, whilst permitting the targetted preparation of thin specimens for Transmission Electron Microscopy, also results in modification of the material to be investigated as a result of energy being transferred into the material. This undesirable effect is normally limited to the surface of the material, which is particularly unfavourably orientated towards the impinging ion beam. If the crystal structure and composition of areas close to the surface of such specimens need to be characterised, protective layers may be used. However, those layers, depending on the applied deposition technique, may interact with the sample surface as well thus affecting the results of the analysis. In the work presented here, the possible interactions which might occur between the various protective coatings of ion-beam deposited Platinum, electron beam deposited Platinum, Silicon Oxide or adhesively bonded Gold foil and the subsequent FIB-preparation of the oxide layers on Ni-Ti alloys are investigated, with respect to and how these might affect the TEM-images obtained of areas close to the surface of such specimens. It is shown that the use of adhesively bonded Gold foil as a protective coating, in particular, permits comprehensive characterisation of the surface, including the use of high-resolution TEM, to be carried out, up to the surface of the Oxide layer itself.