Observation and Analysis of β Phase in Steel Using Non-aqueous Electrolyte-Potentiostatic Etching Method

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.