Plasticity and dynamic recrystallization of magnesium and magnesium alloy crystals
Final Report Abstract
Within the scope of this project, the physical mechanisms of single crystal plasticity were investigated, placing special emphasis on deformation twinning, with the aim of advancing the current knowledge with respect to the deformation behavior of magnesium and its alloys at various temperatures. Specifically-oriented single crystals of different orientations were subjected to channel-die plane strain compression at room and elevated temperatures in order to access specific mechanisms of plasticity and recrystallization. The microstructure and texture evolution were characterized with respect to each orientation, deformation conditions and chemical composition (pure Mg vs. Mg-Gd). Pure Mg crystals of ‘hard’ orientations that were compressed along the c-axis displayed limited room temperature ductility, although pyramidal 〈푐 + 푎〉 slip was activated, and fractured along crystallographic {112‾4} planes as a result of highly localized shear. The addition of Gd did not reveal any marked improvement in the slip activity. The failure in c-axis compression at ambient temperature is therefore fostered by a tendency for shear localization, coupled with a high hardening rate of 〈c+a〉 slip, and is of a ductile nature. At elevated temperature, c-axis compression is aided by contraction twinning, dynamic recrystallization and a reduced tendency for shear localization. In the case of c-axis extension, large room temperature ductility was achieved by preventing the formation of a basal texture component due to multiple extension/contraction twinning and the occurrence of dynamic recrystallization inside contraction twin bands as a result of shear localization. The presence of high angle {101‾2} twin boundaries to serve as nucleation sites was found to be a prerequisite for the formation of {101‾1} contraction twins at ambient temperature. Dynamic recrystallization inside these bands was of a continuous type (recovery driven) and occurred readily at room temperature, yielding fine, and equiaxed grains. The recrystallized grains were rotated by 30° around the c-axis of their parent twin host as a result of fragmentation of the band due to prismatic slip. It was shown that the high number of 30° grain boundaries, known to appear frequently in polycrystalline material, is not a result of selective grain growth, but must be attributed to the process of fragmentation and further continuous recrystallization. Dynamic recrystallization was not found to operate in {101‾2} extension twins, owing to a lack of shear localization in this twin type and the ease of growth of {101‾2} extension twins. Thus, extension twins contribute to texture weakening by multiple twinning while contraction twins undergo dynamic recrystallization. Promoting both twin types is therefore essential in order to achieve texture randomization. While mechanical tests on Mg-Gd crystals did not display any marked improvement in ductility, the addition of Gd had a profound impact on the recrystallization behavior, entailing a significant grain refinement and texture weakening at elevated temperatures compared to pure Mg, which is highly beneficial in terms of ductility. At ambient temperature dynamic recrystallization was retarded, resulting in texture weakening through static recrystallization upon subsequent annealing. In conclusion, this project produced new important insights on the single crystal plasticity of magnesium which help to advance the current knowledge of the deformation behavior of polycrystalline Mg and its alloys. Moreover, the current study provides a scientific basis for further experimental investigations on bi-, tri- and oligo-crystals to systematically access the role of grain boundaries in the plasticity of magnesium and ultimately allow optimal materials design suitable for a variety of industrial applications.
Publications
- Mechanisms of exceptional ductility of magnesium single crystal during deformation at room temperature: Multiple twinning and dynamic recrystallization. Acta Materialia 76 (2014), 314-330
Molodov, K. D.; Al-Samman, T.; Molodov, D. A.; Gottstein, G.
(See online at https://doi.org/10.1016/j.actamat.2014.04.066) - On the diversity of the plastic response of magnesium in plane strain compression. Materials Science and Engineering A 651 (2016), 63-68
Molodov, K. D.; Al-Samman, T.; Molodov, D. A.
(See online at https://doi.org/10.1016/j.msea.2015.10.108) - On the role of anomalous twinning in the plasticity of magnesium. Acta Materialia 103 (2016), 711-723
Molodov, K. D.; Al-Samman, T.; Molodov, D. A.; Gottstein, G.
(See online at https://doi.org/10.1016/j.actamat.2015.10.043) - On the twinning shear of {101‾2} twins in magnesium – experimental determination and formal description. Acta Materialia 134 (2017), 267-273
Molodov, K. D.; Al-Samman, T.; Molodov, D. A.; Korte-Kerzel, S.
(See online at https://doi.org/10.1016/j.actamat.2017.05.041) - Profuse slip transmission across twin boundaries in magnesium. Acta Materialia 124 (2017), 397-409
Molodov, K. D.; Al-Samman, T.; Molodov, D. A.
(See online at https://doi.org/10.1016/j.actamat.2016.11.022) - Characteristic dislocation substructure in {101‾2} twins in hexagonal metals. Scripta Materialia 143 (2018), 81-85
Wang, F.; Hazeli, K.; Molodov, K. D.; Barrett, C.D.; Al-Samman, T.; Molodov, D. A.; Kontsos, A.; Ramesh, K.T.; El Kadiri, H.; Agnew, S.R.
(See online at https://doi.org/10.1016/j.scriptamat.2017.09.015)
