Michael Dahms

The effect of ultrafine grained microstructures on the hot-workability of intermetallic/ceramic composites based on γ-TiAl

Abstract Intermetallic/ceramic composites are prepared by mechanical alloying and subsequent powder consolidation. The bulk material of the composition Ti–45Al–2.4Si (at.%) exhibits a homogeneous microstructure of equiaxed γ-TiAl grains (dγ=196 nm) with a volume fraction of about 10% ξ-Ti5(Si,Al)3-silicides finely dispersed along the grain boundaries. Large fully dense specimens (m=500 g) allow for the demonstration of the easy hot-workability of ultrafine grained materials by industrial processing techniques like isothermal forging and extrusion. The temperatures for isothermal forging can be kept between 800 and 900°C for maximum strain rates in the range of 10−4 s−1. Deformations up to 75% at stresses below 160 MPa are achieved. Due to the stabilizing effect of the Ti5Si3-silicides, only slight grain coarsening is observed. Taking into account the high strain rate sensitivity of the material, extrusion tests ( e =1.5 s−1) are performed at temperatures between 1025 and 1250°C. While significant coarsening of both the γ-TiAl matrix and the silicide dispersion is observed at high deformation temperatures, extrusion at 1025°C retains the fine grained and homogeneous microstructure.

On the Determination of Magnesium Degradation Rates under Physiological Conditions

The current physiological in vitro tests of Mg degradation follow the procedure stated according to the ASTM standard. This standard, although useful in predicting the initial degradation behavior of an alloy, has its limitations in interpreting the same for longer periods of immersion in cell culture media. This is an important consequence as the alloy’s degradation is time dependent. Even if two different alloys show similar corrosion rates in a short term experiment, their degradation characteristics might differ with increased immersion times. Furthermore, studies concerning Mg corrosion extrapolate the corrosion rate from a single time point measurement to the order of a year (mm/y), which might not be appropriate because of time dependent degradation behavior. In this work, the above issues are addressed and a new methodology of performing long-term immersion tests in determining the degradation rates of Mg alloys was put forth. For this purpose, cast and extruded Mg-2Ag and powder pressed and sintered Mg-0.3Ca alloy systems were chosen. DMEM Glutamax +10% FBS (Fetal Bovine Serum) +1% Penicillin streptomycin was used as cell culture medium. The advantages of such a method in predicting the degradation rates in vivo deduced from in vitro experiments are discussed.

Properties of Sintered Mg Alloys for Biomedical Applications

In addition to the use as light weight construction material, magnesium alloys are also very suitable for future orthopaedic and traumatology applications. Common permanent implant materials such as titanium or stainless steel still suffer from stress shielding problems, causing bone resorption and implant loosening. In contrast, magnesium alloys provide elastic moduli and strengths matching those of cortical bone. In order to support osseointegration and vascularisation, an open porous surface structure of an Mg-implant is advantageous. The powder metallurgical processing route of Mg-alloys enables the generation of such parts. Powder blends with different sintering behaviour were produced via mixing pure Mg-powder with different Ca containing master alloy powders (MAP). As a result, sintering of these Mg alloy powders and blends became feasible. Sintered parts were investigated in view of shrinkage, porosity, grain size using SEM, EDX and XRD. In addition, compression tests were performed revealing ultimate compression strength up to 328 MPa, plastic compressibility of 22 % and compressive yield strength up to 90 MPa. Hence, the PM-route enables the production of parts with mechanical properties matching those of cortical bone.

In vitro biodegradation testing of Mg-alloy EZK400 and manufacturing of implant prototypes using PM (powder metallurgy) methods

The study is focussing towards Metal Injection Moulding (MIM) of Mg-alloys for biomedical implant applications. Especially the influence of the sintering processing necessary for the consolidation of the finished part is in focus of this study. In doing so, the chosen high strength EZK400 Mg-alloy powder material was sintered using different sintering support bottom plate materials to evaluate the possibility of iron impurity pick up during sintering. It can be shown that iron pick up took place from the steel bottom plate into the specimen. Despite the fact that a separating boron nitrite (BN) barrier layer was used and the Mg-Fe phase diagram is not predicting any significant solubility to each other. As a result of this study a new bottom plate material not harming the sintering and the biodegradation performance of the as sintered material, namely a carbon plate material, was found.

Achievements in Deep Drawing of Magnesium Alloy Sheets

Magnesium alloy sheets bear significant potential in replacing conventional materials such as aluminium and steels in ultra lightweight designs. High specific strength and stiffness, combined with the lowest density of all structural metals make magnesium alloy sheets candidates to face the challenges of reducing vessel weight in the transportation industry and thus, green house gas emissions. For forming components from sheet metal, deep drawing is a well established and commonly applied process. Due to the limited formability of magnesium sheets at room temperature, deep drawing processes have to be conducted at elevated temperatures. In the present study, hot deep drawing experiments on an industrial scale hydraulic press were successfully conducted. Forming was done at moderately low temperatures from 150°C to 250°C. Sheets of the magnesium alloy AZ31B (Mg-3Al-1Zn-Mn) were drawn to symmetrical cups according to Swift. For AZ31, distinct basal type textures are formed during hot rolling. The influence of texture on earing is displayed. The microstructural evolution of the material is dominated by the formation of twins and dynamic recrystallisation. By optimising the process, a drawing ratio of 2.9 was achieved for AZ31 sheet, outperforming conventional materials at ambient temperature.