Yield Strength of Nitrogen Alloyed Duplex Steels: Hall-Petch Analysis and Micromechanical Predictions

For the prediction of the yield strength of nitrogen alloyed ferritic-austenitic duplex steels, accurate knowledge on the single phases’ yield strength and their geometrical arrangement within the duplex microstructure is required. Since the matrix-inclusion character of the phases markedly influences the yield strength σyd of duplex steels, linear models for σyd (Voigt-model) cannot serve for an accurate prediction of σyd. A non-linear rule of mixture, however, is a more sophisticated approach to calculate σyd. Micromechanical models combined with finite element computations are efficient tools to accurately predict the influence of the topology of the microstructure on σyd only if the yield strengths of the single phases are distinctly different, i.e. the yield strength ratio of ferrite to austenite ψ is larger than 2. Experiments on duplex steels show, however, a marked influence of the phase arrangement on σyd even for ψ ∼ 1. To explain this behavior a modified non-linear rule of mixture is proposed, which incorporates the in-situ yield strengths of the phases as upper and lower bounds for σyd. The discrepancy between the experiments (and their analysis) and the predictions from the micromechanical approach convincingly demonstrates the necessity to include crystallographic details such as dislocation-interface boundary interactions and misorientation and crystal structures of adjacent grains in improved micromechanical models for the yield strength of duplex steel microstructures.