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-p hase high-temperature state (900, 950, 1000, 1050 degrees C) with gamm a + carbides (NbC, Nb2C) into the state alpha + carbides + pearlite af ter phase transformation at A(c1) during cooling the dualphase/dissper sion mixed microstructure. Because mean chord lengths <(D)over bar gam ma> : (D) over bar((alpha + Perlit) )approximate to 1:1 were obtained, the grain growth processes were evaluated at room temperature (test u pon the coercitive field strength H-C). In and near the former gamma-g rain boundaries form NbC and Nb2C. This means, at first the dispersed microstructure exists. During the phase transformation gamma double ri ght arrow alpha + pearlite the dualphase microstructure superposes. Th e mutual penetration of both the types of microstructure is clear to o bserve. The dispersion microstructure is evaluated by scanning electro n microscopy (mean particle radius <(r)over bar p>, particle density n (p), specific phase boundary area S-V((P/matrix))), the dualphase micr ostructure by optical microscopy (volume weighted mean chord length (D ) over bar((alpha + pearlite)), specific phase boundary area S-V((alph a + 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 t emperature. 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 f lat slopes of the Hall-Petch straight lines, First findings were obtai ned relative to the affect of grain sizes and microstructural type on the Young's modulus.