The purpose of this study is to perform a fundamental study on the design and the development of an advanced high temparature intermetallics alloy by investigating the relationships between the microsturucture and the mechanical properties of two phase NiAl/$Ni_3Al$ alloys. NiAl/$Ni_3Al$ two phase alloys have been studied to get advantages of both NiAl and $Ni_3Al$ intermetallic compound, but they lack room temperature ductility and high temperature strength and so these have been their main drawback to their practical applications. Fortunately it was known that Fe addition improved their poor room temperature ductility through previous research, but Fe addition makes them weaker in high temperature. So the problems of their poor strength in high temperature are remained to be solved.
Carbon were added in the alloy for carbides precipitation with tatanium that has been known as 'carbide forming element', and the additions of 0.2wt%C and 3at% Ti could make carbides most adequately. Carbon doped alloys like NAF 25-10-C or NAF29-10-C have more refined microstructure than other alloys that have no carbides. That would be caused by carbides which had existed in as-cast microstructure showing equiaxed dendrites. The carides would supress the grain growth and the corsening of second phases. After heat treatment of (1250℃/14hr/W.Q. + 800℃/24hr/F.C.), NAF29-10 alloy showed the lamellar type microsturcture but NAF29-10-C alloy which had carbides showed much more refined 'mesh' type microstructure. This fine structure is due to the difference in phase transformation process from martensite to (β +γ). NAF29-10 alloy shows the process of M→β→$Ni_5Al_3$→(β+γ) but NAF29-10-C alloy shows the process of M→$Ni_5Al_3$→(β+γ).
When the alloys were quenched into water, stress due to the volume change in martensitic transformation let the crack be in their grain boundary. But in case of carbon doped alloy, these cracks could not be recovered. Although the reducing of heat- treatment time was attempted to suppress the quenching cracks in the alloys not having carbides, the cracks still made the specimen fractured in a severe intergranular fracture mode at the early stage of elastic deformation.
The β-rich alloy like NAF29-10-C showed high strength in room temperature but became soft very rapidly with increasing temperature similar to the case of βsingle phase. On the other hand the γ'-rich alloys such as NAF25-10 and NAF25-10-C showed high strength in high temperature and the 'positive temperature dependence of yield strength' similar to the case of γ' single phase.
