Mechanisms of plastification in multiaxially loaded textured magnesium

• Applicant: Professor Dr.-Ing. Otto Huber
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Lightweight Construction, Textile Technology
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 438040004

• Final Report Year: 2024

Final Report Abstract

Magnesium (Mg) alloys are becoming increasingly important for lightweight structures and the associated material and energy savings due to their high specific static and cyclic strength, high availability and economic production. However, the hexagonal lattice structure results in complex deformation behavior. Recent studies on textured Mg wrought alloys under static and cyclic uniaxial compression loading have shown that strong strain localization occurs due to the formation of macroscopic bands of twinned grains (BTGs), which have a significant influence on the mechanical behavior. In this project, a 3D phenomenological constitutive law for anisotropic and asymmetric plastic flow behavior has been developed that is capable of representing this strong macroscopic strain localization using the finite element method (FEM). Quasi-static experiments have been developed and performed with the objective of encompassing the biaxial tension-tension and compression-compression quadrants of the yield surface. For this purpose, cruciform specimens were used along with a novel anti-buckling device. Shear specimens were also used, to investigate the tension-compression quadrants of the yield surface. The relationships between microstructural deformation mechanisms and the macroscopic yielding behavior of textured magnesium, exemplified by twin-roll cast AZ31B, under uniaxial and multiaxial loading were investigated by mechanical tests and microstructural analysis. Uniaxial in-situ compression tests and shear tests were conducted in a scanning electron microscope (SEM). The microstructure was analyzed for the initiation, growth, and distribution of twins using transmission electron microscopy (TEM) by the University of Salzburg and electron backscatter diffraction (EBSD) by the University of Applied Sciences (UAS) Landshut. Based on these complementary investigations, the entire yield surface for AZ31B was established. A material routine was developed to map the elasto-plastic constitutive model for finite element method (FEM) simulations and implemented into the open source FEM program CalculiX to calculate stress and strain fields for monotonic multiaxial loadings. This enables, for the first time, the numerical simulation of 3D anisotropic and asymmetric elasto-plastic material behavior together with the strong strain localization of test specimens and components made of AZ31B with high accuracy. Accurate elasto-plastic strain fields are an essential basis for the application of the fatigue model "Concept of Highly Strained Volume," developed at the Lightweight Design Competence Center of the University of Applied Sciences Landshut (LLK), for the calculation of fatigue life under cyclic multiaxial loading.

Publications

• A Novel Hot-Bent Uniaxial Test Sample to Characterise the Cyclic Material Behaviour of AZ31B Wrought Magnesium Structural Components, LCF9 Ninth International Conference on Low Cycle Fatigue, Berlin: DVM, 2022
  Nischler, A. & Huber, O.
  
• Entwicklung eines Materialmodells zur Simulation des inhomogenen 3D-Dehnungsfelds von texturierten Magnesium-Knetlegierungen mittels der FEM, in: 11. Landshuter Leichtbau-Colloquium. Leichtbau – Werkstoffe, Konstruktionen, Fertigungstechnologien, Landshut: LC-Verlag, 2023. ISBN 978-3-9818439-7-2
  Nischler, A. & Huber, O.
  
• In Situ Uniaxial Compression of Textured Magnesium AZ31B. Metals, 14(1), 20.
  Whitmore, Lawrence; Nischler, Anton; Saage, Holger & Huber, Otto
  
• In-situ Uniaxial Compression of Magnesium AZ31B, in: 5th Conference & Exhibition on Light Materials, Trondheim, 2023
  Whitmore, L., Nischler, A. & Huber, O.
  
• Preparation of magnesium AZ31B for electron backscatter diffraction (EBSD) analysis, in: 11. Landshuter Leichtbau-Colloquium. Leichtbau – Werkstoffe, Konstruktionen, Fertigungstechnologien, Landshut: LC-Verlag, 2023. ISBN 978-3- 9818439-7-2
  Whitmore, L., Nischler, A. & Huber, O.
  
• In Situ Pure Shear Tests on Textured Magnesium AZ31B Sheets. Metals, 14(4), 404.
  Whitmore, Lawrence; Nischler, Anton; Saage, Holger & Huber, Otto
  
• Modelling the Elasto-Plastic Material Behavior of Textured Hexagonal Close-Packed Metals. Advanced Structured Materials, 501-530. Springer Nature Switzerland.
  Nischler, Anton; Denk, Josef; Mader, Florian; Whitmore, Lawrence; Saage, Holger; Prahl, Ulrich & Huber, Otto