Consolidation behavior of nanocrystalline Al–5at.%Ti alloys synthesized by cryogenic milling

Abstract

Nanocrystalline powders of aluminum with titanium addition of 5 atomic percentage (Al–5at.%Ti) were prepared by cryogenic milling (CM) at −85°C. The mean particle and average grain sizes of powders prepared by cryogenic milling were 6 μm and 16 nm, respectively and those of powders produced by room temperature milling (RM) were 19 μm and 21 nm, respectively. Since dynamic recovery was suppressed and fracture was promoted during CM, the particle and grain sizes of Al–5at.%Ti powders were effectively reduced by CM. The powders synthesized by CM were consolidated to full density by vacuum hot pressing (VHP). No serious grain growth was detected because the consolidation of nanocrystalline powders was possible at low temperature for short time. In this study, the smallest grain size, 34 nm, was observed in the specimen VHPed at 390°C for 10 min with the pressure of 500 MPa. As a result, CM powder exhibits better sinterability than RM powder revealing CM powder reached the full density at 390°C while RM powder reached the full density at 450°C on the same consolidation conditions. During the consolidation of nanocrystalline Al–5at.%Ti powder by VHP, pure Al region was formed at a triple junction, which was previously pored region, of the powder particles. The length of the pure Al region was a few μm and the grain size in this region was 100 nm. It is considered that the pure Al region was formed by relatively small Al particles with energetically enhanced surface and existing between the large particles during consolidation.