Michael Störmer

Magnesium alloys as implant materials--principles of property design for Mg-RE alloys.

Magnesium alloys have attracted increasing interest in the past years due to their potential as implant materials. This interest is based on the fact that magnesium and its alloys are degradable during their time of service in the human body. Moreover magnesium alloys offer a property profile that is very close or even similar to that of human bone. The chemical composition triggers the resulting microstructure and features of degradation. In addition, the entire manufacturing route has an influence on the morphology of the microstructure after processing. Therefore the composition and the manufacturing route have to be chosen carefully with regard to the requirements of an application. This paper discusses the influence of composition and heat treatments on the microstructure, mechanical properties and corrosion behaviour of cast Mg-Gd alloys. Recommendations are given for the design of future degradable magnesium based implant materials.

Corrosion Properties of Supersaturated Magnesium Alloy Systems

The range of applications for magnesium alloys is still limited due to their relatively poor corrosion behavior. In recent years, various new magnesium alloys were developed, some of them with improved corrosion properties, thus opening new fields of application. However, the number of alloying elements for the use in conventional cast processes is limited due to their interaction with liquid magnesium, other alloying elements or large differences in the melting temperatures. The possibilities for grain refinement by post-processing are also restricted. PVD techniques can help to produce supersaturated precipitation free and microcrystalline magnesium layers. Using ion beam and magnetron sputtering, binary or ternary Mg-Al, Mg-Ti and Mg-Sn alloy systems as well as standard alloys (AM50, AZ91 and AE42) were deposited on silicon and on magnesium substrates. The effect of the microstructure on the corrosion properties was studied by comparing as cast material and PVD coatings using potentiodynamic polarization, linear polarization resistance, and electrochemical impedance techniques.

Interpretation of Glancing Angle and Bragg–Brentano XRD Measurements for CoCr Alloy and Austenitic Stainless Steel After PIII Nitriding

X-ray diffraction data taken in Bragg-Brentano and glancing angle geometries are compared for stainless steel AISI 304 and CoCr alloys HS188 after nitriding by plasma immersion ion implantation. A direct inferring of the nitrogen depth profile from the line shape for expanded austenite is precluded as additional stress is present. Furthermore, conventional stress analysis is rather complicated as highly textured material is present and only a very much reduced set of reflections is available for evaluation. Nevertheless, a strong dependence of the lattice expansion and the resulting line shape on the detailed process conditions was observed.

The formation of Sr6.33Mg16.67Si13 in magnesium alloy AM50 and its effect on mechanical properties

The increasing use of magnesium castings for automotive components and the number of newly developed alloys raise the question of suitable recycling processes. Remelting offers a high potential of energy saving and thereby improves the live cycle balance of magnesium components. Effective recycling processes are likely to involve the mixing of different alloys but little is known about the interaction of alloying elements. In order to approach this issue, the influence of strontium, silicon and calcium on phase formation and mechanical properties of magnesium alloy AM50 has been investigated. After strontium addition, X-ray diffraction demonstrated the formation of the Al4Sr and the Mg17Sr2 phases. However, after simultaneous alloying with strontium, silicon and calcium the ternary Zintl phase Sr6.33Mg16.67Si13 was detected. This phase forms preferably instead of Al4Sr, Mg17Sr2 and Mg2Si. Compared to the two strontium-containing phases, precipitates of the ternary Zintl phase exhibit a rather compact morphology. This results in a higher elongation-at-fracture under tensile stress.