Mechanisms of plastification in multiaxial loaded magnesium

Magnesium alloys are of particular importance for lightweight design structures due to their high strength- to-weight ratio and economical manufacture. The design of magnesium components relies upon our understanding of how the alloy behaves under various types and magnitudes of loading. In-depth study is required to correlate mechanical tests, such as tension, compression and shear, with deformation mechanisms at the microstructural scale. The hexagonal atomic structure of magnesium gives rise to complex deformation behaviour under stress. Magnesium sheets subjected to uniaxial compressive strain have been shown to deform through the formation and growth of bands of crystalline defects known as twins. These bands have a large influence on the mechanical properties, playing a significant role in strain-hardening and fracture initiation. Current theoretical models do not account for these strain distributions. Also, biaxial compressive deformation behaviour is not currently quantified due to the complications of performing such tests. This fundamental research project aims to characterise the relationship between microstructural deformation and macroscopic behaviour of the magnesium alloy AZ31B by a range of complex tests (including multiaxial compression, shear and nano-indentation) and state-of-the-art microstructural analysis. Particular focus will be placed upon the initiation, growth and distribution of twins, their interaction with other defects such as grain boundaries and precipitates, and their role in fracture initiation. The new JEOL JEM-F200 transmission electron microscope at the Paris Lodron University of Salzburg (PLUS) will be used to reveal the very earliest stages of deformation at the atomic scale. In situ uniaxial and biaxial compression tests in a scanning electron microscope will be carried out to investigate the deformation at the microstructural scale. Biaxial quasi-static and cyclic tests with cruciform specimens will be carried out at the University of Applied Sciences Landshut (UASL) to characterize and analyse the deformation behaviour and the fatigue behaviour at the macroscopic scale. Results from these tests will be used to develop an elasto-plastic constitutive model for the finite element method to calculate the macroscopic stress and strain fields and to develop a fatigue model for cyclic biaxial loading. Microstructural analysis will be carried out at the Department of Chemistry and Physics of Materials at PLUS by Dr. Whitmore, an experienced microstructural scientist and microscopist, while testing will mainly be made at the Competence Center for Lightweight Construction at UASL by Prof. Huber, an internationally recognized researcher in materials modelling and lightweight design and Prof. Saage, an experienced materials scientist.

This three-year fundamental research project, funded by the Austrian Science Fund (FWF, Project number: I-4782) in cooperation with the Deutsche Forschungsgemeinschaft (DFG, Project number: 438040004), was an international cooperation between Paris Lodron University of Salzburg (PLUS) and the University of Applied Science Landshut (UASL). The aim of the project was to investigate the mechanisms of microstructural and macroscopic deformation within the AZ31B alloy of magnesium when subjected to uniaxial compression, pure shear and biaxial compression and tension. Project leader Dr. Lawrence Whitmore at PLUS worked closely with colleagues Professor Dr. Otto Huber and Ph.D. student Anton Nischler at UASL to design and carry out mechanical tests using state-of-the-art in situ analytical methods. All metals deform plastically when the applied stress exceeds the elastic limit, but not all metals deform in the same way. In the case of magnesium, a lightweight metal used in the automotive and aerospace industries, deformation is complicated due to the hexagonal crystal structure. By deforming samples inside an electron microscope using a specially designed mechanical testing stage, it was possible to observe the deformation taking place at the scale of microns in real-time as the load was increased. Bands of elastic strain formed within and across multiple grains, and with increasing load individual grains underwent a transformation through deformation twinning to form bands of plastic deformation visible on the surface. These twins formed in areas where the compressive stress was greatest and were aligned normal to the compressive stress vector. New insights gained from the project contribute to the knowledge base of the scientific community and have also provided empirical data for the development of an elasto-plastic constitutive law for Finite Element Method (FEM) simulation being developed at UASL, which can be used to simulate the anisotropic and asymmetric yield behavior of textured magnesium alloys when subjected to arbitrary stress states. A new method of preparing magnesium surfaces was developed that did not lead to corrosion, and this was used to make the high-resolution studies. Two anti-buckling guides for uni- and biaxial compression tests were developed. A range of 3D printed tools was also developed for preparing high quality samples. These tools demonstrate a new 'sustainable' paradigm where researchers can download designs for tools and equipment from the cloud, modify them for individual experiments and print them locally using 3D plastic or metal printers. The project has yielded important new results that can contribute towards the construction of more energy efficient vehicles and lightweight components for space exploration, with 10 publications in peer-reviewed journals and proceedings, as well as new ideas and new tools that can enhance sustainability. More information about the project can be found here: https://www.plus.ac.at/chemie-und-physik-der-materialien/projects/dma