The purpose of this study is to perform a fundamental study on the design and the development of an advanced high temperature intermetallic alloy by investigating the relationships between the phase transformation and the microstructure of two phase $NiAl/Ni_3Al$ alloys. $NiAl/Ni_3Al$ two phase alloys. The γ ′-Ni_3Al (L1_2 structure)$ and β-NiAl (B2 structure) have excited a good deal of interest for their possibility of application. The γ ′-Ni_3Al$ alloy is attractive due to the anomalous temperature dependence of flow and enhancement of room temperature ductility by boron addition. However, the $Ni_3Al$ alloy has problems such as high density, poor oxidation resistance and low melting temperature. The beta-NiAl alloy has attractive properties such as low density, high melting temperature and good oxidation resistance.
The microstructural features of $NiAl+Ni_3Al$ two-phase (Ni-34Al)-X (X=Ti, Si, Nb) alloys are divided into three branched: Lamellar, Mesh1 and Mesh2. The most important factor of classification of microstructural features is austenite transition temperature($A_s$) of Ni-Al-X alloys. The $A_s$ temperature is sharply lowered with increasing the amount of ternary elements such as Ti, Si and Nb.
The formation of the microstructure of the Lamellar alloy is brought about by following successive phase transformation: β prime (L1_0)→Ni_5 Al_3 (Pt_5 Ga_3)→ β+γ'. β'-martensite is quickly transformed by a re-ordering reaction to the $Ni_5 Al_3$ phase at 450$^\circ$C after quenching from 1300$^\circ$C. It is found that two types of β+γ' structure depended on ternary elements of the Mesh1 alloy are formed. The $Ni_5 Al_3$ phase of Si added Mesh1 alloy slowly precipitates from β-NiAl. This result indicates that the formation of the $Ni_5Al_3$ phase from β-NiAl is ignored. However, the phase transformation of $β→Ni_5Al_3$ of Ti and Nb added Mesh1 alloy is quickly formed by re-ordering at 250$^\circ$C. This difference is the reason of division of two types of microstructural features. The formation of the microstructure of the Mesh2 alloy is brought about by only phase transformation : β→β +γ'. The martensite temperature of Mesh2 alloy is too low to transform to β' -martensite. The γ' phase precipitates from the β phase and Widmanstatten structure is formed as observed in other alloy systems.
