Abstract
The aging behavior of a ferritic Fe-2.5 at%Ni-7.5 at%Al alloy was investigated by means of electrical resistivity and specific heat measurements, electron microscopy and Mössbauer spectroscopy.A different tendency in the resistivity change was observed by isothermal aging above and below 600°C. Electron microscopic examination and Mössbauer studies suggest that isothermal aging process of this alloy at a comparatively low temperature such as 550°C is a complex process consisting of two stages; the precipitation of an ordered aluminum rich zone (Al-ORZ) at the earlier stages, and the precipitation of a stable NiAl (CsCl-type) phase at the later stage. On the other hand, above 600°C, each isothermal aging run could be explained as a simple precipitation process of a stable NiAl phase. The Al-ORZ may be the (Fe, Ni)Al phase with the CsCl-type structure which was proposed by Hornbogen et al. as the α″-phase.Reversion of the Al-ORZ was recognized by reaging at a higher temperature; the zone can be regarded as a metastable phase which can be distinguished from the stable NiAl phase. It seems likely, however, that this Al-ORZ act as nucleus for the precipitation of the stable NiAl phase.
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