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The microstructures of five austenitic Fe-Al-Mn-C-(Si) The
microstructure of an Fe-8.5Al-4.5Ni-1.6C alloy, prepared by
conventional casting, has been investigated by using optical
microscopy (OM), transmission electron microscopy (TEM) and
energy- dispersive X-ray spectrometer (EDS). In the as-quenched
condition, the microstructure of the present alloy was
austenite phase containing (Fe,Ni)3AlC carbides (k'-carbide).
The k' -carbides having an L'12-type structure were formed
within the austenite matrix during quenching by spinodal
decomposition. This feature is quite different from that
observed by other workers in various Fe-Al-Mn-C alloys. When
the present alloy was aged at 550 C for short times, fine k'-
carbides existing in the as-quenched alloy had grown along
<100> directions, and no precipitates could be detected on the
grain boundary. However, prolonged the aging time at 550 C,
coarse (Fe,Ni)3AlC carbides ( k-carbide ) started to appear on
the grain boundaries. Subsequently, these coarse k-carbides
grew into the adjacent grains through a y ?+ k-carbide
transition. With further aging at 550 C, a spherical B2-type
particles were started to form within the ferrite region. The
orientation relationship between k-carbide and (?+ B2) phases
was determined to be (111)k-carbide // (110)?B2, and [110]k-
carbide// [001](?B2). Consequently, the phase decomposition of
y ( ?+ k-carbide ) ( ?+ B2 + k- carbide ) would continue to
occur during aging , and finally the stable microstructure of
the present alloy aged at 550 C should be a mixture of
Widmanstdtten-like k-carbide, ferrite phase and spherical B2-
type precipitates dispersing within the ferrite phase. It is
noted here that the B2 phase has never been observed by other
workers in various Fe-Al-C, Fe-Al-Mn-C, Fe-Al-Ni-C and Fe-Al-(
Mn,Ni)-C alloys.
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