The effect of alloying elements on the microstructural development and mechanical properties of multiphase nickel-rich nickel-aluminum alloys

The research work presented in this study is devoted towards examining bulk multiphase nickel-rich alloys derived from the intermetallic compound β-NiAl. The microstructural development and structure - property relationship of nickel-rich NiAl alloys with different alloying element additions is considered. Additions of chromium, chromium plus titanium, copper as well as higher order additions to nickel-rich NiAl alloys are investigated. The general theme of this research is to produce bulk alloys with fixed compositions and investigate their microstructural stability with respect to high temperature treatments as well as their room temperature mechanical behavior.; Many high temperature industrial applications can benefit from these studies. Examples include diffusion coating of nickel-base superalloys, joining of NiAl and nickel bearing substrate and the future application of NiAl alloys in high temperature structural applications.; In this work, Cr is selected as an alloying element due to the ability of Cr to enhance the room temperature fracture toughness and ductility of NiAl. The possibility of producing a β − γ/γ ′ mixture in high aluminum content Ni-Al-Cr alloys is investigated. The effect of Cr content and high temperature heat treatment on the stability a β − γ/γ′ mixture is studied. The precipitation of various morphologies of the A2 type α-Cr phase and their effect on the stability of the produced microstructure is evaluated. The formation of an intradendritic L1₀ type martensite and the precipitation of γ′ from the martensitic regions as a result of high temperature aging treatments is studied.; The intent of alloying NiAl with Cr and Ti is to produce microstructures with the potential for combined improved low temperature ductility and high temperature creep resistance. The role of Ti levels in the stability of a β/β ′ two phase mixture is discussed. The mechanisms of β ′ decomposition as a result of high temperature aging treatment and their correlation with the formation of γ′ precipitates are investigated. Mechanical testing of Ni-Al-Cr-Ti alloys showed brittle fracture in all samples and the ultimate tensile strength increased considerably in alloys with a β/β′ intradendritic two phase mixture. The effects of intradendritic α-Cr precipitates and interdendritic eutectics involving α-Cr and β′ or β on the mechanical properties are also discussed.; Microstructural development in arc melted bulk alloys developed to resemble microstructural features within different regions of transient liquid phase (TLP) bonds between NiAl and nickel are also investigated. The original TLP bonds utilized a 50 μm thick copper foil as the low melting-temperature interlayer material and hence the analogue bulk alloys contained Ni, Al and Cu as major constituents. The reproduction of microstructural features of the actual bonds in analogue bulk alloys with compositions matching those of the bonds strongly supports the hypothesis that the Al : Cu ratio was the dominant factor in the microstructural evolution within the joints. Correlations between the failure modes in tensile tested analogue bulk alloys and the fracture mechanisms encountered during testing of TLP joints are drawn.; Investigations evolving complex bulk alloys derived from the microstructure of the diffusion zone of NiAl coatings on nickel-base superalloys have shown the formation of a highly stable β − γ/γ′ mixture. However, high temperature treatments resulted in the precipitation of the extremely brittle σ phase.