Papers by Author: Peter J. Uggowitzer

Abstract: In this paper the role of vacancies in the aging of Al-Mg-Si alloys is examined and novel concepts to improve their aging behavior are presented. It has been proposed that the technologically favored fast nucleation of the major hardening phase during artificial aging requires quenched-in vacancy assisted diffusion. The well-known interdependence of natural aging and subsequent artificial in Al-Mg-Si alloys can be understood in terms of quenched-in vacancy trapping in Mg/Si-clusters formed during natural aging. Diffusion during artificial aging is then determined by the dissolution of these vacancy-containing Mg/Si-clusters. This simple concept can guide the development of strategies to avoid the negative effect of natural aging. It is shown that the aging behavior of Al-Mg-Si alloys can be improved not only by processing related measures, but also by compositional interventions, which apply the following recipe: (i) avoid the trapping of vacancies in Mg/Si-clusters, (ii) prevent the vacancy annihilation during RT, and (iii) make them available for diffusion during artificial aging. It is shown that this strategy can be executed in Al-Mg-Si alloys by adding defined trace amounts of elements with an attractive binding energy to vacancies and sufficient solubility in the aluminum matrix.

Abstract: Cooling of age-hardening Al-alloys after solution annealing is a critical step with respect to distortion and residual stresses. In order to predict their extent by simulation models, the mechanical behaviour must be known in a wide range of conditions and compositions. Therefore, experimental data is needed both for calibration and validation of the mechanical model. It is known for Al-Mg-Si alloys that supersaturation of the solid solution leads to a significant increase of strength during cooling. In order to understand the influence of single alloying elements on the strengthening effect, the mechanical properties of different binary alloys are investigated experimentally. The precipitation behaviour during cooling was investigated by Differential Scanning Calorimetry in a wide cooling rate range. A methodology to determine the degree of supersaturation of the solid solution based on the calorimetric results is presented. This approach is compared to atom probe tomography data. The mechanical behaviour of the alloys after various heat treatments was analysed by mechanical tests performed in a quenching and deformation dilatometer. Flow curves with high resolution at small strains (< 3 %) were measured at different temperatures. The results of the different experimental techniques are discussed in comparison and with respect to their testing limitations.

Abstract: The influence of the microstructure, particularly the morphology of the β phase, on the corrosion of Mg alloys has been studied using AZ91 as a model alloy and compared with the corrosion of pure magnesium, used as a standard for comparison. The concentration of the impurity element Fe was below the limit evaluated from theoretical phase diagram construction. Corrosion was measured using hydrogen evolution measurements and some polarization measurements. Corrosion behaviour was characterized for four different microstructures produced by heat treatment of as-cast AZ91: namely (i) as-cast, (ii) homogenization anneal (for 5h and 10h at 380°C), (iii) solid solution and (iv) solution treated and aged. The influence of microstructure can be understood from the interaction of the following three factors: (i) the surface films, (ii) micro-galvanic corrosion acceleration dependant on the amount and distribution of the second phase (the  phase in AZ91) and (iii) the second phase can act as a corrosion barrier and hinder corrosion propagation in the matrix, if the second phase is in the form of a continuous network. It is expected that these factors are important for all multi-phase Mg alloys because all known second phases have corrosion potentials more positive than that of the -phase. The electrochemical measurements did not give good values for the corrosion rate in agreement with the literature.

Abstract: The aim of this article is to describe the design strategy deployed in developing new bioabsorbable Mg–Y–Zn alloys. The development approach is based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the characteristics of the new alloys using metal-physical experiments, thermodynamic calculations, and TEM analysis. Our results show that after extrusion the alloys have very fine grains (<10m), exhibit high ductility (uniform elongation: 17–20%) at considerable strength (ultimate tensile strength: 250–270 MPa), and reveal the presence of finely distributed intermetallic particles, which are stable upon annealing. Due to an attractive combination of mechanical, electrochemical and biological properties, the new alloys are very promising not only for applications in medicine but also in other fields.

Abstract: Magnesium has become an interesting candidate in the field of bioabsorbable implant materials; it is an essential element in the human body, biocompatible and degradable due to its low corrosion resistance in a pH range below 11.5. However, in the human body (pH  7.4) a magnesium implant might degrade too quickly and lose its mechanical strength before the tissue has fully healed. However, the corrosion resistance can be improved for example by the choice of a suitable Mg alloy containing corrosion-inhibiting elements such as yttrium or by the deployment of surface heat treatments at high temperatures causing protective oxide layers to form. We studied the bio-degradation behaviour of a Mg–Y–RE alloy in different heat treatment states by electrochemical impedance spectroscopy and immersion testing in simulated body fluid. The heat treatments caused a change in microstructure and also the formation of a thermal oxide layer on the sample surface, which consisted mainly of Y2O3 and which slowed the degradation and increased the polarisation resistance significantly compared to the polished state. However, in some specimens localised corrosion attacks occurred which drastically weakened the protective effect of the oxide. Because the implant might be deformed during implantation resulting in the oxide cracking, we intentionally strained the samples and investigated the degradation performance. These cracks led to a decrease in polarisation resistance compared to the non-strained oxidised state, but in comparison to the polished state they still perform better. Macroscopically, the degradation process occurred in a homogeneous way without localised corrosion attacks. Microscopically, corrosion attacks started at the cracks and undermined the oxide layer with time.

Abstract: The bearing steel 100Cr6 in the forged and hardened condition is of great importance in industrial use. Escaping the geometry restrictions of conventional forging, the application of semi-solid metalworking (SSM) offers significantly increased design freedom. Using conventionally available rolled feedstock material with carbide banding, however, results in a higher segregation tendency during thixoforging, and thus special attention was paid to the feedstock’s “quality”. To achieve a fine-grained, globular microstructure in the semi-solid state, castings with and without the addition of 100 ppm titanium were compared with the hot rolled material. With its inherent nitrogen Ti forms TiN particles, which reduce grain-growth in austenite. The results indicate that TiN precipitates strongly affect grain growth during solid state processing, but the grain size in the semi-solid state can only be influenced for short process times. Generally the cast feedstock materials possess smaller globulites in the semi-solid state compared to forgings, so that a reduction of the sponge effect and a minimization of the segregation in produced components are expected. Since the cast material already showed a fine-grained, globulitic microstructure, the use of TiN is not recommended because of the possible negative influence of TiN on the dynamic mechanical properties.

Abstract: To meet the requirements of weight-saving and low-cost production of components for future transport vehicles, the concept of multi-material mix is of increasing importance. In this context aluminum-iron compounds produced by means of compound casting are considered to be of particular importance. An essential and critical aspect of such compound castings is the formation of intermetallic phases (IMP) at the Al-Fe interface. Both the nature and the kinetics of potential IMPs are not well understood and require a systematic investigation. In this paper we document the interface formation of pure Al and binary Al-alloys on a mild steel substrate by means of isothermal wetting experiments. Tests were carried out employing the sessile droplet method in a controlled atmosphere. Using pure Al and Al7Si, Al7Cu, and Al7Zn alloys the interface reactions were investigated by quantitative metallography (LOM, SEM/EDX). Special attention was paid to the influence of the alloying elements on the type and sequence of IMPs at the interface.