Phase decomposition and microstructure of Fe-Ni-Al-C-(Ti,B) alloys were investigated by means of transmission electron microscopy(TEM) and X-ray diffraction. Also, room temperature and high temperature mechanical properties were investigated.
Fe-Ni-Al-C alloys system show high strength which is attributed from the finely dispersed $Ll_2$ type ordered precipitates, but unfortunately at high temperature, the brittle fracture which is initiated from the grain boundary occurs. An examinatin of the local sulphur content near the intergranular crack via AES(Auger Electron Spectroscopy) shows that the cause of high temperature brittle intergranular fracture is due to the stress-driven segregation of sulphur to the grain boundaries.
A series of new alloys additional Ti and B respectively have been investigated. In the Fe-Ni-Al-C-Ti alloy series, TiC phase particles are formed during melting, and in the Fe-Ni-Al-C-B alloys, $(Fe,Ni)_2$B phase particles were precipitated on 823K aging. Also, in the Ti and B added Fe-Ni-Al-C-(Ti,B) alloys, $Ll_2$ ordered spherical particles precipitated on aging at 823K for 100hours are identified. The average size of particles have been observed about 100A via the centered dark-field imaging.
From the tensile test results at various temperatures it has been found that Ti addition is benificial to the high-temperature strength, and an fractograph investigation by SEM(Scanning Electron Microscope) reveals B addition benificial for grain-boundary cohesion.
In a series of TEM images of tensile tested specimens, Well-developed planar slip bands have been observed, but the formation of planar slip band is disturbed as tensile test temperature is increased.
In conclusion, Fe-34.36at%Ni-7.17at%Al-3.72at%Ti-0.9at%C alloy has the best strength among the alloys studied and it's maximum yield stress is 731Mpa at 681K.
