Hot workability of nanocrystalline AZ91 magnesium alloy

This study examined the hot deformation behavior and workability characteristics of nanocrystalline AZ91 Mg alloy by performing hot compression tests with a Gleeble-3800 machine. To this end, a nanocrystalline alloy powder with a crystallite size of 25 nm was synthesized via mechanical milling of a pre-alloyed AZ91 Mg alloy powder for 14 h. The mechanically milled (MM) AZ91 powder was subsequently cold pressed at 600 MPa into cylindrical compacts measuring 10 mm in diameter and 12 mm in height. Then, the powder compacts with a relative green density of 91% were hot-compressed at temperatures ranging from 150 °C to 500 °C and at true strain rates ranging from 0.001 s−1 to 10 s−1.The true stress-true strain curves peaked at low strains, after which the flow stress increased moderately. Processing maps were developed for all of the hot compression tests at strains of 0.1, 0.5, and 0.8, which represented a safe deformation domain at deformation temperatures and strain rates in the ranges of 250-350 °C and 0.1-10 s−1. The crystallite size of the nanocrystalline AZ91 Mg alloy hot-compressed within the aforementioned domain was measured to be 140 nm, which is considered very fine for Mg alloys and resulted in a high hardness value of 133 HV for the hot-compressed alloy.