Abstract
The effect of σ-phase morphology on the tensile properties of δ-ferrite-austenite duplex stainless steels has been investigated. The ductility of specimens deformed at temperatures below 673 K was largely reduced by the precipitation of the σ-phase, but exhibited quite a large temperature dependence, similar to that of the brittle-ductile transition in ferritic steels. A lamellar structure consisting of layers of austenite and a σ-phase-austenite cellular structure was formed by aging a specimen solution treated in the δ-ferrite-austenite duplex phase region. When this specimen was deformed at lower temperatures, transverse microcracks originating from the cleavage fracture of σ-phase particles were linked by large longitudinal cracks, and final fracture occurred with the shearing of austenite layers. On deformation at higher temperatures, the coalescence of micro voids formed by the decohesion of σ-phase/austenite interfaces led to final fracture, with a large reduction in area. The transition temperature was lowered significantly by increasing either the austenite volume fraction before aging, or the fine dispersion of σ-phase particles within the parent austenite. Such microstructures were obtained by increasing the Ni content or by solutionizing the δ-ferrite single-phase region before aging.
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