Phase Separation Kinetics Of Fe-Cr Alloy:a Simulation With Phase-field Method

Fe-Cr alloy is an important duplex stainless steel, has been the key structure material of nuclear power plants owing to their excellent anti-radiation and strength at high temperatures. Fe-Cr alloys are susceptible to embrittlement when aged at temperatures in the range of 300～500℃, known as "475℃ embrittlement". This is attributed to the phase separation of the solid solution into an ultrafine mixture of Cr-riched a’and Fe-riched a phases, in which the Cr-riched α’phase leads to substantial deterioration of fracture toughness and thermal corrosion performance. Moreover, the phase separation mechanism of α’ phase in Fe-Cr alloys with different Cr contents and aging temperatures is different. Thus, the research of α’ phase separation mechanism, morphology evolution and the coarsening behavior of the α’ phase are essential for improving the performance of Fe-Cr alloy.Based on the Cahn-Hilliard phase-field equation, a Fe-Cr alloy phase field kinetics equation was established and solved with a semi-implicit fourier-spectral method by using the periodic boundary conditions. With Matlab7.0 programming, we investigated the effects of Cr content and aging temperature on the phase separation mechanism, as well as the separation, kinetics evolution and coarsening behavior of the α’ phase in Fe-Cr alloys.The α’ phase in Fe-42 at.% Cr alloy looks like ellipses and vermicular shapes without preferential alignment at the initial stage. As the aging progressing, the vermicular shape of α’ phase gradually changes to ellipses. The mutual effects of supercooling and diffusion during phase decomposition result in a larger decomposition rate at 725 K.The phase separation occurs by spinodal decomposition in Fe-Cr alloys with Cr composition 25,28 at.% aged at 610 K and 35 at.% aged at both 610 K and 700 K. While it occurs by nucleation and growth in Fe-25 at.% Cr alloy aged at 700 K. An transition characteristic of phase separation kinetics is presented from non-classical nucleation and growth to spinodal decomposition in Fe-28 at.% Cr alloy aged at 700 K.The kinetics evolution of decomposition, growth and coarsening stage can be devided by the synthetic variation of volume fraction, particle number density and radius of the α’ phase. With the increase of Cr content, the volume fraction and coarsening rate changes faster, the particle number of the α’ phase and the equilibrium volume fraction increase. The α’ phase separated via non-classical nucleation and growth has a large growth and coarsening rate and results in a larger average radius than that of spinodal decomposition.