Transmission electron microscopy investigation of Mg‐Zn alloy processed by severe plastic deformation

Abstract

Magnesium alloys possess great potential owing to advantageous properties in various fields such as structural applications, electronic devices or hydrogen and thermal energy storage. Use of Mg alloys for biomedical implants also attracted enormous attention recently. Properties of Mg and its alloys can be significantly improved via design of alloys with precipitation hardening effect and grain refinement using severe plastic deformation (SPD) techniques. Mg‐Zn based alloys are among the most important Mg alloys and have been investigated for more than a hundred years. With a most recent development of various SPD techniques, there is a number of processing routes which may significantly improve the alloys properties. However, fundamental knowledge about an impact of SPD on microstructure of these alloys at nanoscale is insufficient or missing in many cases. In this work, deformation behaviour of an α‐Mg matrix and Mg‐Zn intermetallic particles in the Mg‐12wt.% Zn alloy subjected to equal‐channel angular pressing with applied back pressure (ECAP‐BP) is characterized using transmission electron microscopy (TEM) techniques at nanoscale. Magnesium with 99.9% purity and an appropriate amount of high‐grade Zinc were melted in a graphite crucible under an Ar atmosphere. The subsequent thermal treatment consisted of annealing at 320°C for 20 hours and quenching into warm water. The material was then processed by ECAP (4 passes via Bc route) with applied BP of ~400 MPa to prevent cracking during processing. The processing temperature was gradually decreasing from 200 °C to 185 °C, 170 °C and 155 °C for 1st, 2nd, 3rd and 4th ECAP‐BP pass, respectively, to obtain ultra‐fine grained structure. Following TEM techniques are employed for micro‐ and nano‐ structural analysis: Bright‐field (BF) imaging; selected area electron diffraction (SAED); high‐resolution transmission electron microscopy (HRTEM); high‐angle annular dark field ‐ scanning transmission electron microscopy (HAADF‐STEM) imaging; and electron energy‐loss spectroscopy (EELS). All techniques were performed using an FEI Tecnai TF20 X‐twin, which operated at 200 kV.