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

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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 Al3Ni 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 Al3Ni 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 Al3Ni followed by Al3Ni2, NiAl and Al2Ni3. 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.