Hochfeste und duktile Magnesiumwerkstoffe durch Zugabe von Seltenen Erden
Zusammenfassung der Projektergebnisse
The challenges of today's advanced world are enforcing a strong demand for reducing automobile weight cutting down greenhouse gas emissions. Magnesium alloys have a key role in lightweighting but their present use in automobile structures is rather limited to die cast components due to insufficient formability of magnesium alloy sheet material at ambient temperatures. The key to enhance the formability is to alter the texture observed in conventional magnesium sheet. Dilute addition of rare earth (RE) elements is found to generate much weaker and less common textures that largely contribute to the enhancement of ambient formability and the reduction of the mechanical anisotropy. Developing a deep understanding of what mechanisms are responsible for altering the textures in Mg-RE alloys is essential in leading alloy design toward a new class of wrought magnesium alloys with excellent combination of strength, strain hardening capability and tensile ductility. In this respect, the project had a clear objective of identifying the alloy systems and key processing conditions that control the RE texture modification effect and its underlying mechanisms. The working hypothesis was that recrystallization at certain deformation sites, such as twins and shear bands is key to understanding how rare earth orientations emerge from nucleation and come to dominate during recrystallization growth. Atom probe tomography investigations on the distribution of Gd atoms in the matrix showed strong solute segregation to grain boundaries causing a drag effect on boundary migration that appeared to impede dynamic recrystallization (DRX) during deformation, and thus promote the formation of shear bands and compression twins. This was considered very important for RE texture formation because retarding of DRX controlled the amount of deformation stored energy in the deformed microstructure, and hence the spectrum of orientations that can nucleate during static recrystallization. From the annealing experiments, it was found that the addition of rare earth is not always sufficient to impart texture changes during recrystallization, which was strongly dependent on the choice of the rare earth (high vs. low solubility) and the annealing temperature. For a highly deformed Mg-1Gd alloy sheet with significant shear banding, a powerful recrystallization texture transition was obtained upon 350°C / 60 min anneal. Equal processing of an alternative Mg-1Ce alloy on the other hand revealed little variation from the typical basal deformation texture. While the formation of a favorable RE texture in the former alloy was attributed to selective growth of off-basal orientations over basal-oriented grains, the lack of RE-induced texture modification in the other alloy was primarily attributed to the detrimental role of secondary Mg12Ce precipitates in pinning the grain boundaries, and thereby restricting any oriented growth-related evolution of non-basal orientations. Combinations of rare earths and non-rare earths (Zn and Zr) demonstrated a clear transition in behavior, triggering the RE effect in the Ce containing alloy upon 350°C / 60 min anneal. For the Gd alloy, which was already identified as a suitable material for texture tailoring through annealing treatments, the addition of non-REs seemed to lower the transition temperature, at which texture modification took part in the binary system. The effect of recrystallization texture transition on the room temperature ductility of the material was investigated by means of tension tests at a constant strain rate of 5 x 10^-4 s-1 along the rolling direction of the sheets. The mechanical behavior of the quaternary alloys with combinations of REs and non-REs demonstrated a tremendous enhancement in strength and ductility compared to conventional Mg alloy sheet materials. Polycrystal plasticity simulations provided a useful insight into the deformation mechanisms that could have contributed to the experimentally measured tensile textures, and the activated non-basal deformation mechanisms that further augmented the RE-related ductility enhancement. Part of the project was dedicated to develop a statistical microstructural evaluation method capable of estimating grain boundary mobility as a function of grain misorientation angle in metals during recrystallization. The proposed method was validated using cellular automata simulations and experimentally applied to high purity aluminum single crystals that were cold rolled and artificially scratched in order to nucleate random orientations during subsequent annealing. The results showed reasonable success in determining the grain boundary mobility distribution during recrystallization growth suggesting potential application of this method to hexagonal metals.
Projektbezogene Publikationen (Auswahl)
- Triggering rare earth texture modification in magnesium alloys by addition of zinc and zirconium. Acta Mater. 2014; 67, 116-133
Basu, T. Al-Samman
(Siehe online unter https://doi.org/10.1016/j.actamat.2013.12.015) - Twin recrystallization mechanisms in magnesium-rare earth alloys. Acta Mater. 2015; 96, 111-132
Basu, T. Al-Samman
(Siehe online unter https://doi.org/10.1016/j.actamat.2015.05.044) - Twinning effects in deformed and annealed magnesium–neodymium alloys. Mater. Sci. Eng. A 2015; 647, 91-104
C. Drouven, I. Basu, T. Al-Samman, S. Korte-Kerzel
(Siehe online unter https://doi.org/10.1016/j.msea.2015.08.090) - Determination of grain boundary mobility during recrystallization by statistical evaluation of electron backscatter diffraction measurements. Mater. Charact. 2016; 117, 99-112
Basu, M. Chen, M. Loeck, T. Al-Samman, D.A. Molodov
(Siehe online unter https://doi.org/10.1016/j.matchar.2016.04.024)
