δ/γ2相ステンレス鋼におけるM₂₃C₆とσ相の析出

抄録

Precipitation of M₂₃C₆ and σ-phase during isothermal aging of δ/γ duplex stainless steels has been studied mainly by means of transmission electron microscopy coupled with energy dispersive X-rayanalysis. M₂₃C₆ particles form either at γ/γ or atγ/δ boundaries. The particles are crystalographically related to either one of the γ grains atγ/γ boundaries or the γ grain atγ/δ boundaries. The M₂₃C₆ particles nucleating atγ/δ interface boundaries grow into the untransformed δ-ferrite as an M₂₃C₆/newγ eutectoid structure. The habit plane is the{111} _γ//{111}_M23C6, which is most parallel to the edgewise growth direction. Since C atoms formingM₂₃C₆ are supplied from theγ behind the eutectoid structure, the reaction will finally stop after entire consumption of C atoms. Thenσ-phase particles nucleate on theδ/newγ interface boundary in Nenno orientation relationship with the new γ, and grow into theδ-ferrite as aσ/δ cellular structure with the (001)_σ //(111)_γ habit plane. In some cases, impingements between σ andγ grains nucleating separately occur, and result in a tangled eutectoid structure without any specific orientation relationship. Since the partition of alloying elements betweenγ andδ-ferrite phases accelerates precipitation of both M₂₃C₆ and σ-phase, the solution treatment in theδ-ferrite single phase region largely retards these reactions.