Analyse der Kinetik der Ausscheidungsprozesse in Fe-Si-Cu auf atomarem Niveau mittels Atomsondentomographie
Final Report Abstract
In this project, the suitability of nanometre-sized Cu-rich precipitates to produce a mechanically strengthened electrical steel with retained good soft magnetic properties for applications in efficient electric motors was investigated. The mechanical and magnetic properties of the material were studied. Extensive atom probe tomography investigations were performed to shed light on the initial stages of Cu precipitation. The main findings can be summarised as follows: • The hardness and the 0.2% proof strength of Cu-bearing alloys increase markedly upon annealing, while the total magnetic loss is only slightly influenced by the precipitation. • Loss separation analysis reveals that even though the total loss slightly decreases upon annealing, the excess loss component increases slightly due to precipitation. Hence, precipitation might increase losses at very high frequencies. • The equilibrium solubility of Cu in Fe is decreased by addition of Si. APT results agree reasonably well with thermodynamic database values extrapolated to low temperatures. • During annealing, Cu-rich precipitates form. They are Si-depleted, but contain a large amount of Fe, which is not expected from equilibrium solubility. • The precipitates are approximately spherical in shape. No other precipitation phases, including any Si-rich phases, emerge. • The newly-developed cluster finding approach is well suited for exact and reproducible classification of atoms as precipitate- or matrix-atoms. • The validity of this approach as well as the very limited influence of the local magnification effect on the results of cluster analysis has been proven by field evaporation simulations. • The nucleation, growth and coarsening stages overlap strongly during Cu precipitation. Even after overageing, there is a significant amount of Cu left in the matrix. • Simple simulations assuming entirely diffusion-controlled growth of pure-Cu precipitates strongly underestimate the growth rate of precipitates in the present alloy. • Cold working of the material prior to annealing increases the growth rate of precipitates, but not their nucleation rate. Additionally, the growth becomes anisotropic, probably due to accelerated diffusion along dislocation lines.
