Microstructure and mechanical behavior of NS/UFG aluminum prepared by cryomilling and spark plasma sintering

• Nanostructured/ultrafine grained bulk Al is produced via cryomilling and SPS.
• Its yield strength and compressive strength are 380 MPa and 420 MPa, respectively.
• A semi-quantitative insight into the strengthening mechanisms is provided.
• The changes in chemistry of the Al powder that occurred during SPS are studied.

We report on a study of the microstructure and mechanical behavior of a bulk nanostructured (NS)/ultra-fine grained (UFG) aluminum fabricated by cryomilling and high-pressure spark plasma sintering (SPS). The compressive yield stress of the consolidated material is determined to be 380 MPa, which is significantly higher than that of commercial strain hardened aluminum (124 MPa). Microstructural studies reveal that the nanometric grains are embedded inside a matrix of ultra-fine grains in the bulk material. The corresponding average grain size is approximately 125 nm and with a distribution of grains that are smaller than 500 nm. Moreover, chemical analysis of the powder particles and the consolidated samples indicates that carbon and oxygen levels remain unchanged, and that there is a slight decrease in the nitrogen level and a significant reduction in the hydrogen level after spark plasma sintering. Semi-quantitative analysis suggests that the mechanisms that contribute to the strength of the consolidated material include: grain boundary strengthening, second phase strengthening and dislocation strengthening.