Phase-field investigation on the microstructural evolution of eutectic transformation and four-phase reaction in Mo-Si-Ti system

By using phase-field method, we investigate the morphological evolution of three-phase eutectic transition and four-phase reaction in Mo-Si-Ti system through 2-D and 3-D simulations. For the eutectic transition, we focus on the two-phase growth of lamellar pair from an isothermally undercooled melt: L→Ti(Mo)5Si3+β(Mo,Si,Ti), and obtain a microstructure selection map for (mi) stable, (mii) unstable, and (miii) oscillatory growth (metastable mode), in terms of the Mo-composition and lamellar spacings. The underlying reason for these three different morphologies is clarified by analyzing the growth rate of the solidification front. In addition, we scrutinize the influence of interfacial energy on the solidification morphology and observe three different types of growth mode: (gi) curving, (gii) stable, and (giii) unstable growth. For the four-phase reaction, L+Mo(Ti)3Si→Ti(Mo)5Si3+β(Mo,Si,Ti), we observe the remelting of Mo(Ti)3Si phase and the formation of a lamellar pair consisting of Ti(Mo)5Si3 and β(Mo,Si,Ti) on the surface of the Mo(Ti)3Si phase after an interface of the lamellae pair phases is formed. A certain orientation angle with respect to the solidification direction is obtained for the lamellar pair growth during the four-phase reaction. In both eutectic phase transformation and four phase reaction, a comparison between the 2D and 3D simulations reveals the influence of the third dimension on the development of the lamellar pair.