Characterization of dualphase/dispersion mixed microstructure in a Fe-Nb-C alloy

The alloy Fe-1.15%Nb-0.30%C forms during the transition from the two-phase high-temperature state (900, 950, 1000, 1050 degrees C) with gamma + carbides (NbC, Nb2C) into the state alpha + carbides + pearlite after phase transformation at A(c1) during cooling the dualphase/disspersion mixed microstructure. Because mean chord lengths <(D)over bar gamma> : (D) over bar((alpha + Perlit) )approximate to 1:1 were obtained, the grain growth processes were evaluated at room temperature (test upon the coercitive field strength H-C). In and near the former gamma-grain boundaries form NbC and Nb2C. This means, at first the dispersed microstructure exists. During the phase transformation gamma double right arrow alpha + pearlite the dualphase microstructure superposes. The mutual penetration of both the types of microstructure is clear to observe. The dispersion microstructure is evaluated by scanning electron microscopy (mean particle radius <(r)over bar p>, particle density n(p), specific phase boundary area S-V((P/matrix))), the dualphase microstructure by optical microscopy (volume weighted mean chord length (D) over bar((alpha + pearlite)), specific phase boundary area S-V((alpha + pearlite))). The strength properties include the influence of both microstructural types. They are evaluated by means of the Hall-Petch relationship. The dispersed carbides hinder the grain growth at each temperature. After their solution (and reprecipitation) an accelerated grain growth follows at T greater than or equal to 950 degrees C. The influence of the dispersion microstructure can be seen from the very flat slopes of the Hall-Petch straight lines, First findings were obtained relative to the affect of grain sizes and microstructural type on the Young's modulus.