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.