The quasi-static and cyclic properties of bulk glassy Zr 52.5 Cu 17.9 Al 10 Ni 14.6 Ti 5 alloy (Vitreloy 105) were investigated under three-point bending conditions for two different shot-peened surface states. Residual stress analysis and nanoindentation measurements revealed the presence of compressive residual stresses and an enhanced hardness in the near surface layer after shot peening. Further investigations of the longitudinal cross-sections of the mechanically tested specimens by optical and scanning electron microscopy showed small cracks propagating along shear bands in the vicinity of the fracture surface. The results are in accordance with the improved plasticity of the shot-peened states under quasi-static loading conditions compared to the as-cast reference state. All mechanical testing was carried out with the aim to find a material's state with improved mechanical properties with a special focus on the improvement of the fatigue lifetime and the endurance limit of Vitreloy 105 bulk metallic glass.

No abstract

Micro hardness determination on rough surfaces is a topic of interest e.g. in industrial applications where the component surface is of functional relevance. In most approaches, hardness on a rough surface is determined by including profile roughness parameters like R a (e.g., [1][2][3][4][5]) to adjust measured hardness values or to get the minimum indentation depth value where the influence of the surface topography is assumed to become negligibly small. In the present study, local surface topography data were used instead to enable precise micro hardness measurements on a sample with arbitrary surface topography. Samples made of 1.457 1 stainless steel with different surface states were face milled by using an end mill with different feed rates. Instrumented indentation tests were performed on these samples as well as on comparative samples with polished surfaces. From the resulting load-indentation depth (F-d)-curves the averaged indentation hardness was calculated for all surface states. A method was applied to manipulate and to average the F-d-curves to eliminate deviations, occurring at the beginning of the indentation. The indentation hardness was calculated from these modified F-d-curves and compared to the indentation hardness from the actually measured F-d-curves of the polished samples with feasible results. Using surface topography measurements is considered to enable deriving more accurate indentation hardness values directly and to put the investigations to another level. The surface topography of the samples was evaluated by confocal microscope measurements before and after the indentation tests. From the surface topography data at the location of indentation, four parameters were calculated: volume, projected contact area and depth of the indentation mark, as well as the curvature of the surface topography before indentation. These parameters were correlated with the hardness value from the respective indentation and compared to the indentation hardness of the polished sample. The results of the present study are the basis for combining optical imaging techniques like confocal microscopy or white light interferometry and indentation testing equipment to broaden the field of application.

Due to the constant need for better functionalized surfaces or smaller, function integrated components, precise and efficient manufacturing processes have to be established. Micro milling with micro end mills is one of the most promising processes for this task as it combines a high geometric flexibility in a wide range of machinable materials with low set-up costs. A downside of this process is the wear of the micro end mills. Due to size effects and the relatively low cutting speed, the cutting edge is especially subjected to massive abrasive wear. One possibility to minimize this wear is coating of micro end mills. This research paper describes the performance of eight different hard coatings for micro end mills with a diameter <40 µm and discusses some properties for the best performing coating type. With this research, it is therefore possible to boost the possibilities of micro milling for the manufacture of next generation products.

Metallographic methods which are typically comprising chemical etching processes are applied to reveal grain boundaries in sections of metallic materials. In the case of an optically anisotropic material, grain boundaries can also be detected suing polarized light microscopy (PLM). To this end, the identification of grain boundaries, an automated image processing procedure, based on PLM, was developed. In this context, two different α Ti-based alloys were examined: a cast, non-porous rod material and a material obtained by powder metallurgy which, owing to its high porosity, places specific demands on the image processing system. The present work outlines the underlying metallographic and computer-based methods.

No abstract

Surfaces of technical components rarely appear in perfectly smooth condition. During fatigue loading, stress concentrations at surface asperities cause localized plastic deformation that can lead to crack initiation. Therefore, we have established a computer-aided method based on material ratio curves to investigate the possibility to predict the crack initiation site in fatigue tests by using detailed information on the local surface topography. The present study shows the results of investigations on the mutual influence of the average grain size and the surface condition on the fatigue behavior of commercially pure Titanium (cp-Ti) miniature specimens. Three cp-Ti states were investigated: two types of coarse-grained cp-Ti Grade 2 with 35 µm and with 100 µm average grain size and one ultrafine-grained cp-Ti Grade 4 state with less than 2.5 µm average grain size. Confocal microscopy provided the surface topography data of all specimens and data post-processing was applied to the topography in order to locate critical areas where crack initiation may preferentially occur. These areas were compared with the actual crack initiation areas in fatigue test. Finally, scanning electron microscopy (SEM) images of the fracture surfaces were studied to analyze fatigue crack initiation site and crack path of the three microstructural states.

To describe the material behaviour of the microstructure of commercially pure (cp-) titanium with the Finite Element (FE) Method, a crystal plastic material model is used. The anisotropic plastic deformation is considered by specifying the slip systems of the hexagonal-closed-packed (hcp) crystal structure. To circumvent the difficulties associated with possible nonuniqueness of the set of active slip systems, a rate-dependent formulation is used. Because of the volume preserving plastic deformation, locking effects can arise, which are dealt with by a modified F-bar deformation gradient. To investigate the material behaviour of cp-titanium small scale tensile test simulations are performed. Electronic backscatter diffraction (EBSD) investigations of the sample provides the specific grain orientations of the microstructure to perform realistic simulations.