Martensite Transformation from δ-ferrite during the Melt-quenching Process of δ⁄γ Duplex Stainless Steel

In order to reveal the mechanism of the ferrite→austenite phase transformation during rapid quenching, the microstructure of melt-quenched δ⁄γ duplex stainless steel ribbons has been investigated by means of transmission electron microscopy. Two types of plate-like transformation products are observed within δ-ferrite columner grains. One involves plenty of stacking faults on close-packed planes, and the diffraction pattern shows 18R stacking of close packed structure. The other comprises α-ferrite and γ phase, where the ferrite is surrounded by the thin γ films, and the orientation relationship between them is close to that of the Kurdjumov-Sachs. These plates are considered to be formed by the martensitic transformation from δ-ferrite to the close-packed structure or δ-ferrite to austenite and again from austenite to α-ferrite. The lattice invariant shear strain of the former transformation could be the stacking faults on close packed planes.