The creep behaviour of Magnesium-Manganese based alloys
Notice bibliographique
Résumé
The concern for weight-reduction in vehicles has increased substantially in recent decades due to fuel consumption regulations. Extending the use of magnesium (Mg), the lightest structural metal, in automotive powertrain applications has been one of the routes to reduce vehicle weights. In this doctoral work, the creep behaviour of Mg-Mn based systems with cerium (Ce) and/or strontium (Sr) additions is studied in order to shed light into the new design principles for the development of creep-resistant Mg alloys. Transmission electron microscopy (TEM) is used to characterize the microstructures (the orientation, coherency, crystal structure and the morphology of the phases) as well as the features of creep deformation (twins, dislocations, sub-grains). In the Mg-Mn binary system, the dynamic precipitation of α-Mn was observed upon thermal exposure and during creep. Two different orientation relationships (OR) were determined between α-Mn (rods) precipitated upon heat treatment and the α-Mg matrix. The creep mechanisms were determined to be dislocational processes based on the activation energy (Qc) values (Qc (pure Mg) = 105 and 168kJ/mole; Qc (Mg-1.4Mn) = 127 and 154kJ/mole) obtained via long-term creep tests at different temperatures. The creep strengthening effect of Mn was due to the dynamic precipitation of α-Mn on dislocations (acting as heterogeneous nucleation sites) during creep. In the Mg-Ce-Mn ternary system, the intradendritic precipitation of Mg12Ce occurred heterogeneously where α-Mn precipitates acted as nucleants. The nucleation and growth of the α-Mn and Mg12Ce precipitates were mutually affected by the presence of the other. The dislocational processes were found to be rate-controlling, hence, the refinement in the Mg12Ce precipitate size via α-Mn precipitates enhanced the creep resistance by providing effective dislocation pinning. In the Mg-Sr-Mn ternary system, a new orientation relationship (OR) was determined between the Mg17Sr2 and α-Mn phases, which resulted in differing morphologies of the dynamically precipitated α-Mn depending on the region of precipitation (interdendritic and intradendritic regions). Creep deformation was again found to be dependent on dislocational processes: pipe diffusion at low temperatures and either dislocation climb or activated cross-slip at the higher temperatures (Qc (JM51) = 99 and 234kJ/mole; Qc (JM52) = 93 and 135kJ/mole). The dynamic precipitation of α-Mn on the basal dislocations of the intradendritic regions enhanced creep resistance via pinning by coherency strain fields. Mg-Sr-Mn-Ce alloys exhibiting high creep resistance were developed. The design principles were based on the results obtained from the creep behaviour studies of the pure Mg, Mg-Mn, Mg-Ce-Mn, Mg-Sr-Mn systems, as well as thermodynamic calculations by FactSage. The microstructure of the alloys consisted of the Mg17Sr2 intermetallic phase at the interdendritic regions which strengthened the grain boundaries, and dissolved Mn and Ce which resulted in the dynamic co-precipitation of Mg12Ce and α-Mn (Sr) in intradendritic regions during creep. The creep strain of the quaternary alloys was around to be four times lower than the strain of the ternary alloys.
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| Catégorie | Codex | Gemma |
|---|---|---|
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| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
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| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
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