Bernhard Mingler

TEM investigation of multidirectionally deformed copper

Abstract Polycrystalline samples of Cu 99.95% were deformed by equal channel angular (ECA) pressing by rotating the sample after each pass and by subsequent compression or combined compression torsion. The resulting microstructure was analysed by TEM methods. In case of pure ECA deformation the grain sizes remain unchanged with the number of passes; the misorientation of the grains increases and the distribution of grain sizes gets narrower. When applying additional deformations subsequent to ECA pressing both grain sizes and the number of adjacent grains with large misorientations increase. The changes in distribution of grain sizes depend on the mode of deformation following the ECA deformation.

On the Microstructure of HPT Processed Cu under Variation of Deformation Parameters

The evolution of strength characteristics and the microstructure of copper subjected to high pressure torsion (HPT) are studied under variation of strain and hydrostatic pressure. Measurements of Multiple X-ray Bragg Profile Analysis (MXPA) yield microstructural parameters like dislocation density and arrangement, as well as crystallite (domain) size and distribution, and long-range internal stresses. TEM investigations are carried out to analyse the structural elements and to compare them with the results of MXPA. The strength behaviour is studied by microhardness measurements. The investigations are performed within wide ranges of resolved shear strains = 1 to 400 and of applied pressures p = 0.8 to 8 GPa. The onset of the deformation stages IV and V is strongly affected by the hydrostatic pressure i.e. shifted to higher values of stress and strain with increasing pressure. The experimental results indicate the occurrence of recovery effects, which seem to be of static as well as of dynamic nature, and to be responsible for extended ductility in SPD materials.

The potential of isotopically enriched magnesium to study bone implant degradation in vivo

This pilot study highlights the substantial potential of using isotopically enriched (non-radioactive) metals to study the fate of biodegradable metal implants. It was possible to show that magnesium (Mg) release can be observed by combining isotopic mass spectrometry and isotopic pattern deconvolution for data reduction, even at low amounts of Mg released a from slowly degrading 26Mg enriched (>99%) Mg metal. Following implantation into rats, structural in vivo changes were monitored by μCT. Results showed that the applied Mg had an average degradation rate of 16±5μmyear-1, which corresponds with the degradation rate of pure Mg. Bone and tissue extraction was performed 4, 24, and 52weeks after implantation. Bone cross sections were analyzed by laser ablation inductively coupled plasma mass spectrometry (ICP-MS) to determine the lateral 26Mg distribution. The 26Mg/24Mg ratios in digested tissue and excretion samples were analyzed by multi collector ICP-MS. Isotope pattern deconvolution in combination with ICP-MS enabled detection of Mg pin material in amounts as low as 200ppm in bone tissues and 20ppm in tissues up to two fold increased Mg levels with a contribution of pin-derived Mg of up to 75% (4weeks) and 30% (24weeks) were found adjacent to the implant. After complete degradation, no visual bone disturbance or residual pin-Mg could be detected in cortical bone. In organs, increased Δ26Mg/24Mg values up to 16‰ were determined compared to control samples. Increased Δ26Mg/24Mg values were detected in serum samples at a constant total Mg level. In contrast to urine, feces did not show a shift in the 26Mg/24Mg ratios. This investigation showed that the organism is capable of handling excess Mg well and that bones fully recover after degradation. STATEMENT OF SIGNIFICANCE Magnesium alloys as bone implants have faced increasing attention over the past years. In vivo degradation and metabolism studies of these implant materials have shown the promising application in orthopaedic trauma surgery. With advance in Mg research it has become increasingly important to monitor the fate of the implant material in the organism. For the first time, the indispensible potential of isotopically enriched materials is documented by applying 26Mg enriched Mg implants in an animal model. Therefore, the spatial distribution of pin-Mg in bone and the pin-Mg migration and excretion in the organism could be monitored to better understand metal degradation as well as Mg turn over and excretion.

Regionalized quantitative LA-ICP-MS imaging of the biodegradation of magnesium alloys in bone tissue

The spatially and timely resolved degradation of Mg alloys used as implants in bone material was monitored by LA-ICP-MS. Data evaluation and interpretation was accomplished by means of a geospatial information software (ArcGIS) tool to match spatially referenced LA-ICP-MS data to the microscopic images and to create regionalized images of quantitative data in order to describe the biodegradation process. Quantification was accomplished by matrix matched hydroxyapatite standards. Total combined uncertainty budgets were assessed for the entire quantification process. In the study, rat bones were extracted after 2 and 6 months implantation time and were analyzed for the lateral distribution of the alloying elements (Mg, Mn, Zn, Zr and Yb) in bone cross sections. The use of ArcGIS enabled the spatial referencing of data points and the identification of regions of interest by means of chemical and structural parameters. Moreover, ArcGIS allowed for the direct statistical analysis of defined areas (zones of interest) within these regions. Selected zones visualizing the Mg mass fractions at different distances from the pin after 2 and 6 months and within the blank sample were determined by chemometric means using cluster analysis and showed a clear decrease of the Mg content in the bone material surrounding the pin over time. Four meaningful different zones with significantly different Mg mass fractions in hydroxyapatite could be identified ranging from 8 μg g−1 in undisturbed cortical bone to 16 μg g−1 adjacent to the pin.

Structure and Fatigue Properties of the Mg Alloy AM60 Processed by ECAP

This paper reports on the microstructures and fatigue properties of ultrafine-grained (UFG) AM60 magnesium alloy processed by equal channel angular pressing (ECAP) at various temperatures. After ECAP processing, samples exhibited an increase in fatigue endurance limit, which correlates well with a decrease in grain size. In case of lowest ECAP temperature, the mean grain size is as small as 1 2m which leads to an increase in fatigue endurance limit by 70 % in comparison to coarse-grained alloy. The temperature of ECAP not only governs the grain size and misorientation angles of grain boundaries but also the volume fraction of precipitates, thus affecting the probability of twinning and grain growth after fatigue treatment.

TEM Investigations of Titanium Processed by ECAP Followed by Cold Rolling

Conventional coarse grained (CG) commercial pure (CP) Ti Grade 2 was studied after cold rolling (CR) at room temperature, and after equal channel angular pressing (ECAP) at 450° C followed by CR, by transmission electron microscopy (TEM) methods. CR of the CG material leads to a microstructure showing initially twins with (0112) type and later subgrains separated by lowangle grain boundaries. CR carried out after ECAP yields the fragmentation of fine grains (300 – 800 nm) mostly bounded by high-angle boundaries into elongated subgrains (~ 100 nm). It was shown with in-situ annealing experiments in the TEM that this microstructure is thermally stable up to a temperature of 450° C. Tensile tests showed that the combination of ECAP with CR has the potential to produce at the same time high strength (941 MPa) and high ductility (16.7%).

Equal Channel Angular Pressing (ECAP) of hollow profiles made of titanium

Cylindrical tubes made of commercially-pure titanium (CP-Ti) filled with various mandrels (metallic as well as non-metallic materials) were processed by Equal Channel Angular Pressing (ECAP). Different temperatures (500°C down to room temperature), routes (Bc, B120, C), tools and number of passes (1-6) were applied depending on the mandrel material. For reasons of comparison, solid bolts made of metallic mandrel materials were processed at the same ECAP conditions. The mechanical properties of as-received as well as ECAP tubes and core materials were characterised by hardness mappings (in order to reveal the homogeneity) and tensile tests. The results can be summarised as follows: i) It is feasible to process tubes by ECAP; ii) Using appropriate mandrels, tubes made of CP-Ti can be processed by ECAP at significantly lower temperatures, even at room temperature, as compared to solid bolts; iii) Mechanical properties of tubes and mandrel materials after ECAP are similar to or even better than those of their solid counterparts; iv) Tubes after ECAP at low temperatures show higher strength than what can be achieved in bulk material; v) Excellent homogeneity of microhardness is achieved at least when metallic mandrels and a sufficient number of ECAP passes are used.

FRAGMENTATION IN LARGE STRAIN COLD ROLLED ALUMINIUM AS OBSERVED BY SYNCHROTRON X-RAY BRAGG PEAK PROFILE ANALYSIS (SXPA), ELECTRON BACK SCATTER PATTERNING (EBSP) AND TRANSMISSION ELECTRON MICROSCOPY (TEM)

In the last decade investigations of plastic deformation have focussed on the large strain ranges, i.e. stage IV and V of deformation [1–5]. Although several models have been developed to explain the work hardening behavior in these stages [2, 6–8], the existing experimental findings are not sufficient to identify the real microstructural processes governing stage IV and V hardening. This situation arises mainly from two facts: (i) traditional methods were not convenient to measure dislocation densities, local internal stresses and misorientation of the substructure, and (ii) most of microstructural investigations were done without relation to the specific mechanical properties. Two new methods, X-ray Bragg Profile Analysis (XPA) [9–12] using a rotating anode and/or synchrotron radiation, and Electron Back Scatter Patterning (EBSP) [13, 14], are effectively for studying microstructures induced by large strains. The XPA-method implemented with a rotating anode generator allows investigation of microstructural evolution within one or more grains by using a focal spot size of several tenth of a mm [15]. Using synchrotron radiation with intensities up to 1012 photons/mm/s allows a reduction in the footprint of the beam on the sample to an order of a few tens of microns. This allows the investigation of the microstructural evolution within a single grain [16, 17]. The EBSP method evaluates the Kikuchi-line pattern from back scattered electrons in a Scanning Electron Microscope (SEM) with a spatial resolution down to 0. 5 μm. This is done by computer support in a very efficient way so that a very large number of different lattice sites can be studied in short time.