Solute Behavior in the Initial Nucleation of V- and Nb-Containing Carbide
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INTRODUCTION
VANADIUM and niobium play an important role in microalloyed steel manufacture, especially in improving mechanical properties by precipitation strengthening.[1–3] The transformation from the austenite to ferrite can bring about interphase precipitation of MX-type carbides with NaCl crystal structure (e.g., fcc),[4,5] and tempering solute-supersaturated dislocated martensite also leads to complex carbide precipitation. The mutual intersolution and growth of various types of carbides are affected by the intrinsic solubilities and diffusion of carbon-forming elements, which makes the precipitation extremely complex, particularly in steel containing multiadditions of Cr, Mo, V, etc.[6,7] During the nucleation stage of the alloy carbides, the dislocation and distinct interface are considered active initiators of the precipitation sequence, because it represents an initial energetically favorable stage to be nucleation sites.[8–10] The sophisticated understanding of the nucleation and associated diffusion behaviors of carbon and carbon-forming elements helps to obtain a reasonable explanation of the formation mechanisms of the alloy carbides, and further optimizes service performance at ambient or elevated temperatures by controlling the sizes, morphology, and density of the precipitates in the multicomponent steel. However, the characterization on structure and composition is restricted by the analytic resolution of conventional methods because of the small sizes of the fine precipitates. Binary MX-type carbides have been well characterized at nanoscale by high-resolution Q.D. LIU, Doctoral Student, W.Q. LIU and S.J. ZHAO, Professors, are with the Institute of Materials Science, Shanghai University, Shanghai 200072, P.R. China. Contact e-mail: shijin. [email protected] Manuscript submitted February 22, 2011. Article published online July 14, 2011 3952—VOLUME 42A, DECEMBER 2011
transmission electron microscopy (HRTEM), high angle annular dark field (HAADF), electron energy loss spectroscopy (EELS), and small-angle neutron scattering (SANS) and directly giving out the platelike crystal lattice morphology and quantitative composition distribution.[11–16] With regard to multicomponent carbides, the competitive affinity of alloy elements such as V, Nb, and Ti to C is strongly influenced by cohesive energy and electron structures at the coherent interfaces between ferrous matrix and MX-type carbides. The compositional and structural evolution of transient phases is also affected by the diffusion coefficient of the carbonforming elements from the aspect of kinetic behaviors. Consequently, the nucleation of multiple carbides is more complex than that of simple binary carbides. The 3D atom probe (3DAP), a helpful instrument with atomic spatial resolution and high compositional sensitivity, can complement the traditional inefficient characterization of the early stages of precipitation reactions, particularly when particles are small.[17] Recently, attention has been drawn to the characterization by 3DAP at the atomic scale of the cl
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