INVESTIGATION OF THE DEFORMATION MECHANISMS OF MAGNESIUM ALLOYS AT ROOM AND ELEVATED TEMPERATURE

Abstract

Magnesium alloys are promising candidates for aerospace and automotive industries, mostly due to their high specific strength and stiffness, good heat dissipation, and superior damping performance compared to traditional structural materials. However, wrought Mg alloys show poor ductility and formability at low temperatures due to anisotropy resulting from their hexagonal closed packed structure. This dissertation aims to develop a quantitative understanding of the deformation mechanisms that give rise to poor formability and strain incompatibility of Mg alloys at the grainscale level under tensile loading. In order to reveal the accommodation of deformation mechanisms of Mg alloys, two alloys have been selected, AZ31 and WE43 Mg alloys. The AZ31 Mg alloy is used in low temperature applications due to its good mechanical properties but can deform superplastically at low stresses at elevated temperatures, whereas the WE43 Mg alloy retains its strength at elevated temperatures (up to 250 °C). To this end, the deformation mechanisms in Mg alloys at room and elevated temperature at a slow strain rate is quantified at the microstructural scale by utilising HRDIC tensile testing. This is complemented by electron backscattered diffraction to reveal the underlying grain orientations. In this thesis, firstly, a speckle formation method, the styrene-argon assisted gold remodelling, has been improved and applied to map strains at the grain scale for large scale plastic deformation of an AZ31 Mg alloy by HRDIC. To this end, in situ HRDIC tensile testing in an AZ31 Mg alloy is performed using a custom-built test rig integrated within a high resolution scanning electron microscope. Secondly, a novel approach is developed to conduct in situ HRDIC tensile testing at elevated temperatures. This method is specifically designed to avoid significant relaxation of the sample during the imaging process. As a result, the microstructure evolution and strain quantification of an AZ31 Mg alloy at 200 °C have been followed under tension to 50% strain with nearly 300 incremental steps at grain scale strain resolution (48 nm) under high resolution SEM. Finally, the deformation mechanisms of AZ31 and WE43 Mg alloys with different microstructural properties (i.e. alloying elements and precipitate content) are investigated at 200 ˚C and slow strain rate using in situ HRDIC tensile testing. The new speckle method allows for delineating the activation of intragranular twinning, slip, and quantifying in-plane tangential displacement along grain boundaries (GB) using a new algorithm. GB shear (GBS) is significant at room temperature at 2.3% strain in the AZ31 (as large as 140 nm for certain GBs). However, local deformation by slip within a mantle region extending 450 nm from the GBS leads to displacement along the boundary rather than sliding at the boundary itself. At elevated temperature tests, the AZ31 showed superplastic tendency with nearly constant stress despite its low strength, whereas the WE43 showed high strength and failed at 33% strain. The deformation of the AZ31 was mainly accommodated by GBS and GB migration, generating grain rotation, formation, and break up with only a few grains deformed by basal slip where the Schmid factor for the basal slip was high. In contrast, the deformation of the WE43 was accommodated mostly by non-basal intragranular slip, while basal slip was observed.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Timothy Burnett (Supervisor)