Microstructure and phase stability in a Nb–Mo–Cr–Al–Si alloy

The microstructures in as-cast and heat-treated samples of an Nb–27Mo–27Cr–9Al–9Si (in at.%) alloy have been investigated using X-ray diffraction and electron microscopy techniques. The as-cast alloy comprises a dendritic A2 solid solution surrounded by a eutectic mixture of A15 and C14 phases. After heat treatment at 1,000 C, there is extensive precipitation of A15 and C15 phases within the A2 dendrites, while the A15/C14 eutectic remains essentially unchanged. After heat treatment at 1,500 C, the precipitates within the A2 phase exhibit the A15 and C14 structures; these are coarser and more equiaxed than those formed at 1,000 C, and there is also extensive coarsening/spheroidization of the A15/C14 eutectic. Small particles of two unknown phases were also observed within the A2 dendrites in the heat-treated samples. The orientation relationships between the phases have been identified and these are used to deduce the way in which the microstructure develops.