On the formation of nanocrystalline aluminides during high pressure torsion of Al/Ni alternating foils and post-processing multilayer reaction

High pressure torsion of alternating foils of elemental Al and Ni is used for the production of highly deformed and highly reactive nanostructured Al/Ni multilayers. A continuous grain refinement of the microstructure from the center to the edge of the as-processed disc from approximately 25 µm down to 20 nm is observed. Using a combination of laboratory and synchrotron techniques the formation of a few Al ₃ Ni nanocrystallites inside Al layers is confirmed in the as-processed multilayered material at high shear strain. Thermodynamic calculations at room temperature of heats of mixing and enthalpy values of phase formation for Al ₃ Ni agree with the experimental observations. Post-processing annealing was performed using in situ TEM and calorimetry. The first phase that forms during the solid state multilayer combustion is confirmed to be Al ₃Ni followed by Al ₃Ni ₂, NiAl and Al ₂Ni ₃ . High-temperature chip-calorimetry revealed that the onset of reaction is characterized by an activation energy of 1.4 eV (135 kJ/mol-at) and a total heat of reaction of − 39.0 kJ/mol-at. The latter confirms the self-propagating high-temperature synthesis mechanism of reaction of the as-processed multilayered material.