Understanding the deformation differences between AZ31 and ZEK100 wrought magnesium alloys through Digital Image Correlation study

Abstract

Rare earth contained magnesium alloy ZEK100 (Elektron 717) with a weakened split basal texture has greater formability than commercial magnesium alloy AZ31 with a strong basal texture when deformed at room temperature (RT). Although the uniaxial strain to failure of AZ31 (23% with tension along the rolling direction) is relatively similar to that of ZEK100 (27%), the biaxial strains to failure are markedly different, with a Von-Mises equivalent failure strain of around 4.2% for AZ31 while for ZEK100 this is more than double at approximately 11.0%. Several reasons have been postulated to explain the difference between AZ31 and ZEK100, including texture, slip activity, and twinning behaviour. To understand this, High-Resolution Digital Image Correlation (HR-DIC) work was conducted to investigate the different local strain behaviours in AZ31 and ZEK100 at the microstructural scale. HR-DIC work in this study was in combination with ex-situ electron-backscattered diffraction scanning (EBSD) to measure the active slip systems. Results show the plastic deformation of AZ31 is mainly attributed to easy basal slip and twinning. Strain distribution in AZ31 is very heterogeneous - Localized strains were found at grain boundaries (GBs) where the strain intensity can become almost 40 times larger than the applied macroscopic strain. Sharp planar slip bands were observed in AZ31, indicating the occurrence of concentrated plastic deformation there. The basal slip was also found to be dominant in ZEK100, however this was not so strongly confined to sharp bands. Diffuse non-basal slip was also extensively observed. Twins were much less commonly observed in ZEK100 than in AZ31. As a result, ZEK100, which has a weakened split basal texture, exhibits less heterogeneous strain distribution and lower maximum strain intensity at the grain level. It is proposed that the more diffuse slip permits a relatively 'uniform' plastic deformation compared to AZ31 and the reduced strain localization enables a higher macroscopic strain to be tolerated before failure. Although the local strains found at GBs of ZEK100 could still reach up to 30 times, this value is lower than in AZ31. Furthermore, there were significantly fewer of these high local strains in ZEK100 compared to AZ31. Such a result implies ZEK100 can survive a larger macro plastic deformation than AZ31 because in the latter alloy the development of a network of high regions of local strain produces early fracture. Crystal plasticity modelling was used to help interpret the observations. Both Visco-Plastic Self-Consistent (VPSC) and Crystal Plasticity Finite-Element Methods (CPFEM) were used. Results predicted by VPSC and CPFEM modelling work are consistent with HR-DIC measurements. VPSC simulation can predict the overall slip and twinning activities, while CPFEM can predict the strain localization at the grain level. Modelling allows the effect of texture and single crystal parameters to be isolated. These simulations indicate that the texture difference is of secondary importance to the difference in critical resolved shear stress for each system in explaining the different deformation behaviours between AZ31 and ZEK100.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Joseph Robson (Supervisor) & Joao Quinta Da Fonseca (Supervisor)