The Aging Behavior of an Fe-Cr-Ni Duplex Alloy of Ferrite and Austenite

Abstract

The aging behavior of Fe-30Cr-8Ni alloy having two phases of α and γ with a microduplex structure or coarser structure was studied by means of optical microscopy, hardness measurements and the tensile test.
(1) On aging at 475°C, the strength was increased by spinodal decomposition of α-phase. In the coarser structure, α became deformed by mechanical twinning with aging. In the microduplex structure, α was deformed by slip and the elongation did not decrease so much in spite of aging for a long time, because α was difficult to be deformed by mechanical twinning owing to a fine microstructure and the strain hardening rate of the alloy was increased by preferential deformation of γ.
(2) On aging at 650 or 750°C, the transformation of grain boundary reaction type: α→γ^*+σ (γ^*: new γ-phase), occurred. This transformation was progressed by the lamellar growth in the coarser structure but in the microduplex structure by the growth of a domain consisting of γ^* and σ instead of lamella. There was a marked fluctuation in hardness at an early stage of hardening in the microduplex structure. The alloy was embrittled with this type of transformation and γ had no effect on the enhancement of ductility.