Comparison between the microstructural evolutions of two modes of SHS of NiAl: key to a common reaction mechanism

Microstructures arrested at different stages of completion in case of both the modes of self-propagating high-temperature synthesis (SHS) of NiAl were studied in detail to compare the process evolution. In case of the thermal explosion mode where combustion cannot be arrested directly once the ignition point is reached, it was quenched after different extents of post-ignition combustion by varying the heating rate and using coarse Ni powder as in situ heat sink. This indirect method successfully simulates the wave-quenching operation, which is applicable only to the plane wave propagation (PWP) mode. The sequence of events accompanying the combustion was reconstructed on the basis of the incompletely converted microstructures in both the cases. It was shown for the first time that one unified reaction mechanism, very close to the one proposed by Aleksandrov and Korchagin [Comb. Expl. Shock Waves 23 (1988) 557], was operative in both the modes of SHS. This is irrespective of the fact that the pre-combustion diffusive reactions do not occur in the PWP mode.