Peculiarities of a NiAl/Mo Transition Zone Formed during Self-Propagating High-Temperature Synthesis

Abstract—This work has demonstrated the possibility of joining (“welding”) a coating of intermetallic compound NiAl to an Mo substrate using the self-propagating high-temperature synthesis (SHS) method, without melting the Mo substrate. It has been revealed that an intermediate layer is formed between the Mo substrate and the welded NiAl coating. The main component of this layer is a cellular rod-like pseudobinary NiAl–Mo eutectic consisting of branched Mo threads 200 nm thick and of a NiAl matrix. It has been found that the microhardness of the Mo/NiAl transition layer is higher (2860 MPa) than that of the welded NiAl coating (2360 MPa) and of the Mo substrate (1830–1990 MPa). This indicates the occurrence of a strengthening of the NiAl intermetallic compound owing to the dissolution of Mo in it and to the formation of a pseudobinary NiAl–Mo eutectic and of Mo nanoprecipitates in the NiAl dendrites. Upon mechanical fracture of the sample, the Mo threads in the eutectic cells undergo significant plastic deformation. Using the selective chemical etching method in a mixture of HCl + H₂O₂, Mo-based structural components in the form of branched bundles of threads with a thickness of about 200 nm and a length of up to 300 μm have been separated.