EXPERIMENTAL-STUDY ON DISSOLUTION OF ALPHA-PHASE IN GAMMA ALPHA/GAMMA-DIFFUSION COUPLES OF THE FE-CR-NI SYSTEM/

The dissolution kinetics of body-centered-cubic (bcc) alpha-ferrite in face-centered-cubic (fcc) gamma-austenite matrix was experimentally s tudied using four sets of gamma/alpha/gamma diffusion couples of the t ernary Fe-Cr-Ni system in order to understand the general characterist ics of the dissolution reaction of a solid-solution phase in multicomp onent systems. The gamma/alpha/gamma diffusion couples consisting of a n alpha-phase sheet with thicknesses of 0.07 to 0.1 9 mm and gamma-pha se plate's with a thickness of 2 mm were annealed at 1373 K for variou s times between 3.6 x 10(3) and 3.6 x 10(6)s. Although the alpha phase should completely dissolve in the gamma phase after sufficiently long annealing, the thickness of the alpha phase transiently increased wit h increasing annealing time at an early stage of the reaction. After r eaching a maximum value, the thickness of the alpha phase decreased wi th increasing annealing time. The composition of the moving alpha/gamm a interface changed with annealing time after overlap of diffusion zon es took place in the alpha phase. The change in the interface composit ion and the transient thickening of the alpha phase were qualitatively explained using approximate solutions of the diffusion equations for the semi-infinite alpha/gamma diffusion couples.