2相ステンレス鋼におけるα粒界から析出するμ目の形態と結晶学的特徴

抄録

The morphology and the crystallographic features of austenite (γ) precipitated at ferrite (α) grain boundaries have been investigated using Fe-25Cr-6Ni two-phase stainless steel. The starting structure of the alloy before aging in (α+γ) two-phase region was fully ferritic. The γ precipitation at high angle α grain boundaries (G.B.γ) was found to be classified into three main types according to the morphologic and the crystallographic feature; Type I consists of γ particles having the same variant of K-S orientation relationship (O.R.) and shows a film-like morphology by coalescence of these γ particles. Type II consists of γ particles of different K-S variants, showing an allotriomorph morphology by impingement of the γ. The γ of these two types are both holding K-S O.R. to only one of two adjacent α grains, not to the other. Type III consists of γ particles having the same orientation, the close-packed plane of γ being in parallel to those of both α grains, and shows a film-like morphology by coalescence of the γ.
The close-packed α planes which are in parallel relationship between G.B.γ and α grain in these three types were found to be nearly parallel to the plane of the grain boundary. In the case of Type I, only one K-S variant of γ was selected by taking most coherent orientation against unrelated α grain. The γ of Type II, however, was able to select 4 different K-S variants of the same close-packed parallel orientation relationship, for lack of coherent orientaion against unrelated α grain. For this reason the morphologic and the crystallographic feature of G.B.γ strongly depend on the orientation of α grain boundary plane. Also the morphologic and the crystallographic feature of spike-γ growing from G.B.γ depend on the α grain boundary orientation because of strong correlation among spike-γ, G.B.γ and α grain.