Microstructure evolution in AISI 304 stainless steel during near rapid directional solidification

The microstructure evolution of near rapidly directionally solidified AISI 304 stainless steel was investigated in the present paper. It is found that the microstructure consists of δ ferrite dendrites with developed sidebranches and interdendritic austenite (γ) under the temperature gradient (G) of 20 K mm -1 and growth rate (V) of 1·0 mm s -1 . Coupled growth microstructures of thin lamellar ferrite and austenite begin to form at a higher growth rate of 2·0 mm s -1 . The formation mechanism of the coupled microstructures is analysed based on the nucleation and constitutional undercooling criterion that the δ ferrite phase and austenite phase form alternately before the steady state growth of each phase is reached due to larger undercooling. With further increase of the growth rate up to 3·0 mm s -1 , the morphology of the δ ferrite transforms from lathy to cellular.