Fine structure of transition layer formed between NiAl melt and W substrate during self-propagating high-temperature synthesis

Ni-Al intermetallic was joined to a tungsten substrate by using the technique of SHS welding. A transition layer whith complex gradient structure about 400 μm thick was found to contain W based dendrites, pseudo-binary NiAl-W eutectic, and the precipitates of W containing phase below 50 nm in their size. In order to better understand the morphology of the transition layer, acid leaching with 4 % HCl – 3 % H2O2 aqueous solution was used to remove a NiAl layer from the burned sample, leaving intact the W based phases and W. The presence of the symmetry axes of the first, second, and third order in the dendrites is indicative of a low crystallization rate. In leached samples, the transition zone exhibited the presence of the branching bundles of W containing fibers around 10 μm long and 50 nm in diameter. On the surface of leached samples, there were the columnar dendrites oriented normally to the surface and the globules of the W based phase (80 – 86  at.  % W, 16 – 14  at.  % Ni and 0 – 4  at.  % Al). The above structure of the transition zone explains strong joining between the W substrate and SHS produced NiAl.