Microstructural Investigations of Novel High Temperature Alloys Based on NiAl-(Cr,Mo)

Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae by adding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to 1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo)$_{ss}$ reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trend predicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diffraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientation relationship between NiAl matrix and (Cr,Mo)$_{ss}$ reinforcing phase was (100)$_{NiAl||}$ (100) $_{Cr,Mo}$ and ⟨100⟩ $_{NiAl||}$ ⟨100⟩ $_{Cr,Mo}$. ... mehrAn orientation relationship described by a rotation of almost 60° about ⟨111⟩ was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observed