Effects of hot pressing temperature and annealing temperature on microstructure and compressive properties of a bulk nanocrystalline AZ61 magnesium alloy contain Ti

Involving rare-earth-free Mg alloys, developing microstructure of nanocrystalline matrix and dispersing stable nano-scale precipitates, being a great challenge, is the key to achieve ultra-high strength. In this work, bulk nanocrystalline magnesium alloy was prepared by mechanical milling and subsequent vacuum hot pressing. The effect of hot pressing temperature on microstructure, densification and mechanical properties was analyzed. After hot pressing at 723 K for 60 min, the powders got densified and the average grain size of magnesium matrix was similar to 76 nm. Nano-scale Ti3Al precipitates with a particle size of similar to 10 nm were detected by X-ray diffractometer and high-resolution electron microscope. After that, annealing treatment was carried out at 573 K for various times. Both strength and ductility got improved evidently. After annealing at 573 K for 80 h, the yield strength, compressive strength, fracture strain and hardness were 498 MPa, 553 MPa, 4.9% and 1.44 GPa, respectively. Based on analysis on strengthening mechanisms, the contribution rates owing to grain boundary strengthening effect, Orowan effect and load transfer effect were calculated to be 64%, 33% and 3%. It was certified that nanocrystalline magnesium phase together with nano-scale Ti3Al precipitates play a crucial role in realizing ultra-high strength. (C) 2021 Elsevier B.V. All rights reserved.