Fumio Kurosawa1, Isamu Taguchi1 and Ryutaro Matsumoto2
1Fundamental Research Laboratories, Nippon Steel Corporation, Kawasaki
The β -phase (NiAl) in stainless steel (25%Ni-19%Cr-5%Al) has been studied using the non-aqueous electrolyte-potentiostatic etching method (the SPEED method).
The results obtained were summarized as follows:
(1) The β -phase was found to be stable when the steel matrix was electrolyzed at +1200∼+1500 mV vs SCE in 3% nitric acid-2% perchloric acid-methyl alcohol electrolyte.
(2) Three-dimensional observation of microstructures of the dual phase (γ -δ -ferrite) and the β phase in δ -ferrite of the stainless steel was successfully conducted by the SPEED method.
(3) The β -phase was found in δ -ferrite of the stainless steel. The β -phase was dice-, sphere-and needle-shaped. From the X-ray and electron diffraction analysis of the isolated residues, the β phase was identified as NiAl (bcc, a0=0.288 nm).
(4) The shape of the β phase of the stainless steel at 1423 K changed from dice-like into diamond-like with aging time. The growth mechanism was also discussed.
(5) The growth of the newly found diamond-shaped β phase was oriented to the eight directions of < 111 >. The crystallographic morphology of the β phase was studied by the SPEED three-dimensional observation method.
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