Papers by Author: Babette Tonn

Abstract: As part of a study devoted to the effect of trace elements on graphite degeneracy, near-eutectic ductile iron melts were prepared to which minute amounts of lead and of both lead and cerium were added. The melts were cast into an insulated Y4 mould, giving a solidification time of about 1 hour and a cooling time to room temperature of about 15 hours. In the thermal centre of the Pb containing sample graphite spheroids as well as intergranular lamellar graphite have been found. At the same location of the casting containing both Pb and Ce, exploded as well as chunky graphite could be observed, while the formation of intergranular lamellar graphite has been suppressed. Deep etching of the samples allowed reaching the following conclusions: i) intergranular graphite in the SG-Pb sample often, if not always, originates on graphite nodules and extends towards the last to freeze areas; ii) in one location of the SG-PbCe sample, chunky graphite strings were observed to originate on an exploded nodule, thus confirming the close relationship between these two forms of graphite. Because of the over-treatment in cerium of the SG-PbCe sample, other unusual degenerate graphite was observed which appears as coarse aggregates of "porous" graphite after deep etching.

Abstract: Austempered Ductile Iron (ADI) is characterized by high tensile strength with acceptable ductility. Steel, as a large competitor to ADI, also meets the tensile and yield strength. Nevertheless, the main advantages of ADI compared to steel are the lower density (7.2 g/cm3 to 7.85 7.87 g/cm3) for weight reduction and lower manufacturing costs because of less energy consumption during the production. One of the main problems of producing ADI is the quenching process during heat treatment of thick-walled castings. The inner part of a massive casting – in contrast to the outer part – cools down more slowly, resulting in a heterogeneous microstructure with parts of pearlite and ferrite embedded in austenite before reaching the isothermal transformation temperature. Molybdenum is, besides nickel, copper and manganese, one of the possible alloying elements that postpone the transition point of ferrite and/or pearlite. To investigate the influence of molybdenum in thick-walled castings experiments with different molybdenum contents were performed. In dependence on the molybdenum content, different austenisation and ausferritisation temperatures and times are examined in order to investigate the transformation points, fraction and morphology of different phases. The mechanism of molybdenum in ADI has been investigated by means of dilatometer tests, microstructure analysis and mechanical tests.

Abstract: High life expectancy of cast components and good material performance at dynamic load are a prerequisite to cater for future trends in wind energy generators. To remain competitive in this ever evolving sector challenges reside in alloy development. In this work fractional factorial design has been applied to ferritic ductile iron with varying contents of silicon (1.6‑2 wt%), nickel (0‑1 wt%), cobalt (0‑3 wt%) and copper (0‑0.2 wt%). The minimum criteria the new alloy should meet were a minimum yield strength of 240 MPa and an impact work of minimal 8 J at a temperature of -20 °C for wall thicknesses of 60‑200 mm. To obtain these mechanical properties thick-walled castings with additional insulation were produced to achieve a higher thermic module. They provided the material for test specimens to perform static tensile tests, Charpy impact tests at varying temperatures and a microstructure analysis. With these results, a sweet spot plot has been created. That way, an optimum alloy composition could be found and has been proven by validation experiment.The optimum alloy for thick-walled castings is composed of Si = 1.6 wt%, Cu = 0.2 wt%, Ni = 0 wt% and Co = 0 wt%. It offers an enhancement in yield strength and acceptable impact work at low temperatures for massive castings in as cast state. The heat treated, full ferritic material could even improve these results.

Abstract: The mechanical and fatigue properties of Cu - Al - Mn shape memory alloys with different phase fractions at room temperature were investigated. The specimens with different chemical compositions (Al: 8.9 - 12.5 wt. % and Mn: 3.3 - 9.3 wt. %) were tensile loaded with 10-3 s-1 tensile strain rate. Austenitic specimens have the highest tensile strength and fracture strain. Yield strength, tensile strength and elongation of martensitic alloys were lower compared with austenitic alloys. Fracture strain of martensitic alloys depend only little on the chemical composition. Specimens of martensitic, austenitic and three different multiple phase specimens were tested in the high cycle fatigue range at room temperature. The Woehler curves for multiple specimens depend on the phase fraction at testing temperatures. Different elements as Co, Ni, Fe and Si were alloyed to CuAl11.6Mn5. All decreased the ductility of the specimens, and their fatigue properties. Maxima could be detected in the strain amplitude dependence of damping for multiple phase specimen. These maximum are shifted to lower damping and to higher strains with increasing number of mechanical cycles, compared to the as cast condition for not cycled specimen. The strain amplitude dependence of damping in martensitic and austenitic Cu – Al – Mn shape memory alloys does not change much during mechanical cycling.

Abstract: The strain amplitude dependent internal friction at room temperature and the transition temperatures of CuAlMn-shape memory alloys with Al contents from 8.9 wt.% to 12.7 wt.% and Mn contents from 4.7 wt.% to 9.3 wt.% were investigated. The investigated strain range was 10-6 - 10-3. Rods of various compositions were die cast and machined to single clamped damping bars. Their transition temperatures and amplitude dependent damping was determined in as cast and homogenized state. The damping in the investigated shape memory alloys was found to be generally much higher than in metals without martensitic transition. In as cast state some alloys exceeded the damping of a Sonoston type alloy measured in comparison for strains higher than 3 x 10-5. The influence of grain size on damping was investigated by additional sand casting and the use of Boron for grain refinement. It was found that only the material with the biggest grains had a noticeable higher damping over the whole measured strain range. Homogenization heat treatment can still extremely increase the damping of CuAlMn alloys. After homogenization this extremely high damping decreases slowly to medium values in the order of as cast alloys.