Structure of composites consolidated from ball milled 7475 aluminum alloy and ZrO₂ powders

Nanocomposites were prepared from 7475 alloy powder ball milled for 40 h with additions of 2% Zr and 10 or 20 wt.% of ZrO₂, Y ₂O₃ stabilized powders. Two types of ZrO₂ powder additions of size near 30 nm and 0.3-0.5 Lm were used. Transmission electron microscopy studies confirmed the refinement of the aluminum solid solution grain size after milling, down to about 40 nm. The milled powders were consolidated using uniaxial hot pressing in vacuum at 380°C and at a pressure of 600 MPa. The hardness of consolidated samples was higher for the 20% ZrO₂ nanocrystalline ceramic powder addition than for 20% ZrO ₂ coarser powder, at 320 and 280 HV, respectively. Transmission electron microscopy studies allowed the determination of the grain size of aluminum solid solution to be near 100 nm after hot pressing and homogeneous distribution of ZrO₂ particles. The fractions of monoclinic ZrO ₂ were similar in the milled powder and in the hot pressed samples. ZrO₂ nanoparticles did not retard the grain growth, contrary to 2% of Zr which prevented grain growth during hot pressing. The compression tests showed 1 000 MPa of ultimate compression strength of samples with ZrO ₂ nanoparticles, slightly higher than those with ZrO₂ larger particle additions.