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 σ_y^d, of duplex steels, linear models for σ_y^d (Voigt-model) cannot serve for an accurate prediction of σ_y^d. A non-linear rule of mixture, however, is a more sophisticated approach to calculate σ_y^d. Micromechanical models combined with finite element computations are efficient tools to accurately predict the influence of the topology of the microstructure on σ_y^d 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 σ_y^d 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 σ_y^d. 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.