Ermüdungsverhalten von hochdruck-tordiertem Kupfer

Im Rahmen dieser Diplomarbeit ist es gelungen, Lebensdauerkurven (S/N-Kurven) von hochdruck-tordiertem, ultafeinkörnigem Kupfer (HPT-Cu) aufzunehmen. Durch Einsatz eines Ultraschallresonanzsystems zur Wechselbelastung mit einer Frequenz von 20 kHz war es möglich, bis zu sehr hohen Zyklenzahlen (10 hoch 9) zu messen. Neben hochreinem und kommerziell reinem, HPT-Cu wurde bei gleicher Geometrie auch hochreines, geglühtes sowie kommerziell reines, ECAP-verformtes Kupfer untersucht. Zusätzlich zu den Lebensdauermessungen wurde die ermüdete Mikrostruktur sowie der Rissverlauf untersucht

Phase transformation pathways in a Ti-5.9Cu alloy modified with Fe and Al

Titanium alloys have been gaining importance in various industries due to their advantageous combination of strength, low density, excellent corrosion/oxidation resistance, and superior mechanical properties at elevated temperatures. Recently, eutectoid Ti–Cu alloys have been explored as promising candidates for advanced processes. This work investigates the effects of Fe and Al on a Ti-5.9Cu alloy using multi-scale characterization techniques. While Fe acts as a β-stabilizing element (despite being a sluggish eutectoid former), Al acts as an α-stabilizer. This work focuses on the effects of combined addition of these elements, studied in different heat treatment conditions. The results show that a fine, equiaxed microstructure is obtained in the binary Ti-5.9Cu alloy, whereas the addition of 2 wt% Fe, or 2 wt% Fe combined with 2 wt% Al to the Ti-5.9Cu alloy deteriorates the effect of grain refinement and coarse, columnar grains result and a small amount of β-phase is retained. Further, the microstructure resulting from the eutectoid decomposition is altered dramatically from a lamellar pearlitic in the binary alloy to a lath-like α-phase with diverse decomposition products in the ternary and quaternary alloys accompanied by increasing hardness values. Evaluation of the α misorientation suggests that a substantial amount of non-Burgers α is present in the Ti-Cu alloy in contrast to the results of the ternary and quaternary alloys. The observed Cu-rich intermetallic compound was identified as Ti$_2$Cu phase with off-stoichiometric composition. Results obtained explain how adding either Fe or Fe and Al leads to substantial hardening

Exceptional Strengthening of Biodegradable Mg-Zn-Ca Alloys through High Pressure Torsion and Subsequent Heat Treatment

In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45 Tm led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied

Surface Analysis of Biodegradable Mg-Alloys after Immersion in Simulated Body Fluid

Two binary biodegradable Mg-alloys and one ternary biodegradable Mg-alloy (Mg-0.3Ca, Mg-5Zn and Mg-5Zn-0.3Ca, all in wt%) were investigated. Surface-sensitive X-ray photoelectron spectroscopy analyses (XPS) of the alloy surfaces before and after immersion in simulated body fluid (SBF) were performed. The XPS analysis of the samples before the immersion in SBF revealed that the top layer of the alloy might have a non-homogeneous composition relative to the bulk. Degradation during the SBF immersion testing was monitored by measuring the evolution of H2. It was possible to evaluate the thickness of the sample degradation layers after the SBF immersion based on scanning electron microscopy (SEM) of the tilted sample. The thickness was in the order of 10-100 µm. The typical bio-corrosion products of all of the investigated alloys consisted of Mg, Ca, P and O, which suggests the formation of apatite (calcium phosphate hydroxide), magnesium hydrogen phosphate hydrate and magnesium hydroxide. The bioapplicability of the analyzed alloys with regard to surface composition and degradation kinetics is discussed

Advanced Immersion Testing of Model Mg-Alloys for Biomedical Applications

The acceleration of developing magnesium alloys for biomedicine requires the advancement of experimental methods evaluating their performance. We have been developing an advanced immersion testing method for the assessment of biomedical Mg alloy degradation in aqueous environments. It is based on the combination of isothermal calorimetry with pressure measurement in the reaction cell. Such a combination allows in situ quantitative analysis of chemical reactions based on both the enthalpy (heat) of the process itself and hydrogen gas generated as one of the reaction products. Here, we analyze the evolution of the degradation rate of a ternary Mg–5.0Zn–0.3Ca intended for biomedical applications and two model binary Mg–5.0Zn and Mg–0.3Ca alloys (in as-cast and solutionized states) in 0.9% NaCl water solution and a simulated body fluid (SBF). The results obtained using the novel method are critically compared to more traditional immersion testing with hydrogen collection

The effects of severe plastic deformation and/or thermal treatment on the mechanical properties of biodegradable mg-alloys

In this study, five MgZnCa alloys with low alloy content and high biocorrosion resistance were investigated during thermomechanical processing. As documented by microhardness and tensile tests, high pressure torsion (HPT)-processing and subsequent heat treatments led to strength increases of up to 250%; as much as about 1/3 of this increase was due to the heat treatment. Microstructural analyses by electron microscopy revealed a significant density of precipitates, but estimates of the Orowan strength exhibited values much smaller than the strength increases observed. Calculations using Kirchner’s model of vacancy hardening, however, showed that vacancy concentrations of 10−⁵ could have accounted for the extensive hardening observed, at least when they formed vacancy agglomerates with sizes around 50‒100 nm. While such an effect has been suggested for a selected Mg-alloy already in a previous paper of the authors, in this study the effect was substantiated by combined quantitative evaluations from differential scanning calorimetry and X-ray line profile analysis. Those exhibited vacancy concentrations of up to about 10−3 with a marked percentage being part of vacancy agglomerates, which has been confirmed by evaluations of defect specific activation migration enthalpies. The variations of Young’s modulus during HPT-processing and during the subsequent thermal treatments were small. Additionally, the corrosion rate did not markedly change compared to that of the homogenized state