Phasenbildung, Phasenübergang und mechanische Eigenschaften des Funktionsmaterials Eisen-Palladium

Abstract

The alloy Iron-Palladium is a smart material whose properties can be strongly influenced in several ways. If the Palladium content is about 30%, the material undergoes a martensitic transformation. The transformation temperature strongly depends on the composition of the alloy. It was a major goal of this work to identify the factors that influence the stability of the austenitic fcc phase and the martensitic fct phase against formation of precipitates or a body centered phase and improve the phase transition. For this purpose, thin foils were prepared by the splat quenching technique. By systematic variation of composition, post annealing and by the addition of small amounts of Copper, the correlation of microstructure, Iron content, valence electron concentration, phase stability and transformation temperature was identified. In a relaxed state, that is, after a sufficiently long and hot annealing treatment, samples with a valence electron concentration of 8.59 or more have a fcc or fct structure. Below this concentration, the fct phase is only observed if it is stabilized by a defect rich micro structure. The temperature of the fcc-fct transformation increases linearly with the Iron content of the samples. By adding a suitable amount of Copper to the material, it is possible to increase the valence electron concentration at a given Iron content, and thus enhance the transformation temperature. In binary samples, a strong influence of the microstructure on the phase transformation was found. Especially the healing out of defects and lattice distortions and the coarsening of the columnar microstructure during annealing at temperatures of 800°C or above lead to a narrower transition range. Thus the material is completely martensitic at higher temperatures. For the characterization of the mechanical properties during the phase transition, a vibrating reed experiment was designed to meet the special requirements of measuring thin foils. The damping of the foils increases strongly after the phase transition. The elastic modulus of samples with a Palladium content of 29% was found to be 100GPa. Due to the tendency of these samples to transform to the body centered phase, the lattice softening during phase transition is less pronounced than in samples with a Palladium content of 30% that have an elastic modulus of 72GPa. A comparison of these results with stress-strain-measurements on a commercial ext ensometer helps to explain the special mechanical properties of thin foils. Austenitic samples are very soft and an elastic modulus of 25GPa was measured. Upon straining, a finely twinned stress induced martensite is observed. In the martensitic phase, an elastic modulus of 13GPa was measured, which is even softer, and a reorientation of martensite variants was observed. The elastic moduli that were calculated from the stress-strain-curves are considerably smaller than those calculated from the vibrating reed measurement. The difference between these values can for example be explained by the different annealing treatments of the samples or the different time scales and strain amplitudes of the two experiments. The interesting mechanical properties of the alloy Iron-Palladium, like the extremely low elastic modulus and the stress induced martensitic transformation, are made possible by the extremely flat potential energy landscape with small differences between the energies of the different crystal structures. However, these small differences between the energies of the different structures also lead to instability of the different phases towards each other. With the help of the insights into the interplay of valence electron concentration, Iron content, micro structure, phase stability and transformation temperature that were gained in this work, it is now possible to stabilize the martensitic phase at higher temperatures.