Effect of temperature on solid-state formation of bulk nanograined intermetallic Al₃Ni during high-pressure torsion

A bulk form of nanograined intermetallic Al₃Ni was produced by severe plastic deformation using high-pressure torsion (HPT). Powder mixtures of 75 mol% Al and 25 mol% Ni were processed by HPT at a selected temperature in the range of room temperature (RT) to 573 K under a pressure of 6 GPa. X-ray diffraction analysis revealed that the Al₃Ni intermetallic formed after processing for 50 revolutions at RT but, as the processing temperature increased, less revolutions (i.e. lower imposed strain) were required for the formation of Al₃Ni. Observations by transmission electron microscopy showed that the microstructure consists of ultrafine grains having a size of 300-2000 nm after 3 and 10 revolutions. Once the Al₃Ni formed after a higher number of revolutions, equiaxed nanograins with a size of ~30 nm prevailed with a significant increase in hardness. The increase in hardness was more significant when processed at higher temperatures because of increasing the fraction of Al₃Ni. It was shown that the solid-state formation of Al₃Ni occurred due to enhanced diffusion (i.e. decreased activation energy for diffusion) through the presence of high density of lattice defects.