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摘要
In this study, the carbide precipitation at 925 degrees C in austenite (gamma) of a 0.04C-1.5Mn-0.10Ti-0.39 W (wt%) low-carbon steel was investigated by stress relaxation (SR) high-resolution transmission electron microscopy and atom probe tomography. First-principles calculations were employed to reveal the precipitation mechanism. Results indicate that a high dispersion of W- and Fe-rich MC-type ultrafine carbides (< 10 nm) forms during the very early stage prior to the onset of precipitation determined by SR. These ultrafine carbides possess a B1-crystal structure with a lattice parameter of 3.696 angstrom, which is quite close to that of gamma (3.56 angstrom). It can significantly decrease the misfit of carbide/gamma interface with a cube-on-cube relationship, thus assisting the carbide nucleation. As the time prolongs, a few spherical or polygonal Ti-rich (Ti, W)C particles (18-60 nm) are formed at the expense of the ultrafine carbides by nucleation and growth on them. These (Ti, W)C particles are identified with a "core-shell" structure (Ti-rich core and Ti, W-rich shell), which leads to a better-coarsening resistance compared with pure TiC in Ti steel. Calculation results show that the composition and structure of carbides at certain stage are closely related to a combined effect of W, Fe, and Ti atoms together with interstitial vacancies.
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单位哈尔滨工程大学
