Phase-separation kinetics of a multicomponent alloy

Abstract

Phase separation kinetics of multicomponent 350-grade maraging steel has been investigated on both recrystallized as well as cold-worked specimens by small-angle x-ray scattering, wide angle x-ray scattering, and transmission electron microscopy at two different temperatures, viz. 430 \ifmmode^\circ\else\textdegree\fi{}C and 510 \ifmmode^\circ\else\textdegree\fi{}C, for different aging times. Unlike previous observations, at both the temperatures, dynamical scaling behavior is observed at the early stages of phase separation accompanied by diffuse interface of the secondary phases. Porod exponents have been found to be greater than 4. At late stages, the precipitate-matrix interface becomes sharp --- the Porod exponent is close to 4 but clear deviation from the dynamical scaling behavior is evident. At $430\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ the phase separation is attributed to the formation of an ordered $\ensuremath{\omega}$ phase through a mechanism involving chemical ordering and the $\ensuremath{\omega}$-like lattice collapse in the bcc structure. Time (t) dependent population averaged precipitate radius follows ${t}^{1/5}$ power law indicating cluster diffusion mechanism of Binder-Stauffer type for the entire range, 30 min--72 h, of aging time. At $510\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$the phase separation is attributed to the formation of ${\mathrm{Ni}}_{3}(\mathrm{T}\mathrm{i},\mathrm{M}\mathrm{o})$ with ${\mathrm{DO}}_{24}$ structure through the process of nucleation and growth. Average precipitate radius follows ${t}^{1/3}$ Lifshitz-Slyozov power law for the entire range, 5 min--18 h, of aging time. The system, despite being multicomponent and complex, appears to follow two distinct time-temperature-transformation curves. As far as the effect of cold work on phase-separation behavior is concerned, it has been found that cold work facilitates the growth of the precipitates. Also, it narrows down the size distribution and enforces strong spatial correlation of the precipitates. Cold working the material is found to be detrimental to the dynamical scaling behavior.