Grain boundary segregation in magnesium alloys and its role in controlling the microstructure and mechanical properties
Final Report Abstract
The combination of Zn and RE as solute additions in wrought Mg alloys led to unique sheet texture development upon rolling and subsequent annealing characterized by pronounced basal pole peaks at ±40° TD. Since this texture was not observed in the Zn-free alloy subjected to similar processing, it is suggested that synergetic effects of multiple solute species are crucial in terms of the RD→TD texture transition taking place during recrystallization. With respect to the mechanical properties in tension, the addition of Zn led to a remarkable enhancement in the yield strength, strain hardening capability, and failure ductility, which can be attributed to precipitate and solute strengthening effects combined with a fine grain size and a favorable soft texture. Targeted variation of the relative Zn content in solid solution, i.e. modifying the RE/Zn atomic concentration ratio revealed texture variations favoring off-basal texture components. With increasing relative Zn fraction there was a clear preference for the TD-type texture. This indicates that the synergistic solute effects can be manipulated by controlling the RE:Zn solute ratio. APT of Mg-Gd-Zn alloys with a different relative Zn solute fraction showed that solute segregation is controlled by the Gd:Zn solute ratio in the matrix. The negative segregation energy, as well as the peak concentration at the grain boundary were found to increase with a higher relative Zn solute fraction. Surprisingly, the absolute amount of Gd concentration was of little importance. Identified solute clusters revealed a frequent solute composition of Gd:Zn equal to 0.33 in all ternary alloys despite different nominal Gd:Zn ratios of the original composition. Accordingly, the formation of stable clusters capable of grain boundary cosegregation is likely to require a lower RE:Zn ratio. This would explain the enhanced segregation in alloys with a large relative Zn solute fraction. With respect to the mechanical behavior, tensile tests in RD and TD directions revealed a planar anisotropy of the yield stress. The magnitude of this anisotropy depended on the Gd:Zn ratio, particularly in TD. EBSD-assisted slip trace analysis revealed enhanced activation of non-basal slip accompanied by a relative decrease of basal slip events. This was particularly the case for the Mg-Gd-Zn 1:2 alloy with the highest relative Zn fraction. A comparison of active slip systems under strain in RD and TD in this alloy showed similar activation of all common slip systems in magnesium. However, the fraction of grains exhibiting twinning was doubled for straining in TD, which was the reason for the observed RD/TD anisotropy of the yield stress. Considering the importance of the atomic solute ratio compared to the absolute solute concentrations shown in this work, the influence of clustering on solute-boundary interaction appears to be crucial for the development of a synergistic alloy design concept tailored towards desirable TD-type textures for enhanced formability. Level-set computer simulations of grain growth were tested by varying the interplay of boundary curvature, remaining dislocation density gradients after recrystallization and solute drag as a result of solute segregation at grain boundaries. The simulation outputs were validated by systematic quasi-in-situ EBSD mappings performed to track the evolution of local and global microstructural characteristics as a function of annealing time. The residual sum of squares was applied to assess the variance in the experimental and simulated data sets. The results showed that the observed favorable growth for some grains is controlled by several drivers of different importance at different stages of annealing. With increasing annealing time, stabilization of the microstructure is attributed to a reduction of dislocation density gradients between large and small grains, which at a certain point cannot overcome the drag force from segregated solutes at grain boundaries. The modeling approach has therefore shown a promising potential to incorporate various key aspects of grain growth with comprehensive complexity and reasonable accuracy.
Publications
- Grain boundary co-segregation in magnesium alloys with multiple substitutional elements. Acta Materialia 208 (2021), 116749
Pei, R. ; Zou, Y. ; (...); Al-Samman, T.
(See online at https://doi.org/10.1016/j.actamat.2021.116749) - Texture Selection Mechanisms during Recrystallization and Grain Growth of a Magnesium-Erbium-Zinc Alloy. Metals 11 (2021), 171
Mouhib, F.-Z.; S., Fengyang; (...); Al-Samman, T.
(See online at https://doi.org/10.3390/met11010171) - Synergistic effect of Y and Ca addition on the texture modification in AZ31B magnesium alloy. Acta Materialia 233 (2022), 117990
Pei, R. ; Zou, Y. ; (...); Al-Samman, T.
(See online at https://doi.org/10.1016/j.actamat.2022.117990) - Synergistic effects of solutes on active deformation modes, grain boundary segregation and texture evolution in Mg-Gd-Zn alloys. Materials Science and Engineering A 847 (2022), 143348
Mouhib, F.-Z. ; Pei, R. ; (...); Al-Samman, T.
(See online at https://doi.org/10.1016/j.msea.2022.143348)
