Strontium additions in AZ31 magnesium alloy
Notice bibliographique
Résumé
Magnesium is a low density metal that is an attractive alternative to steel and aluminum for weight reduction in automotive design. Automotive applications of Mg have steadily grown since the early 1990s. Limited deformation mechanisms at moderate temperatures and resulting poor formability have been the major limitations to the development of Mg wrought alloys. Specialized alloys with improved deformation behaviour exist for aerospace applications, however these are based on expensive alloy systems containing silver and rare-earth (RE) elements such as yttrium and neodymium. Effective use in high volume applications such as automotive components requires a more cost effective solution.Conventional low cost extrusion alloys based on the Mg-Al-Zn and Mg-Zn-Zr systems offer a solution however, they suffer from major problems of extrudability and poor formability. Strontium (Sr) has been introduced to replace RE additions in Mg alloys with an alkali-earth element. Sr has been used as an effective alloying element for grain and second phase refinement as it precipitates as intermetallic phases with high thermal stability. A fw studies have been conducted on the behaviour of these Sr-containing thermally stable precipitates during high temperature deformation. The primary objectives of this research were to study the microstructure, mechanical properties and texture evolution of AZ31 alloy containing various levels of Sr during hot deformation. In the first part of this study, the precipitation and solidification of AZ31-Sr castings was studied by thermal analysis and numerical modeling as a background to understand the formation of secondary phases and microstructural development. This was achieved by casting AZ31 alloys containing various levels of Sr, recording the cooling curves and comparing the findings with computer simulated generated using FactSage (thermodynamic calculation software). In the second part of this study the recrystallization and texture evolution during hot bar extrusion of AZ31-Sr alloys in three sections is characterized. In the first section (3.1) it was shown that Particle-Stimulated Nucleation (PSN) occurred in the deformed regions around the precipitates in certain hot working conditions at 350oC. Increasing the amount of Al-Sr stringers in AZ31 increased the potential sites for PSN. In the following two chapters, the yield asymmetry of AZ31-Sr extruded bars (section 5.1) and the effect of annealing before hot extrusion (section 6.1) were examined. The tensile and compressive strengths of extruded AZ31 alloys containing different levels of Sr depended more on the extrusion temperature than on the Sr concentration and also the yield asymmetry of the AZ31+xSr alloys increased with increasing extrusion temperature. However, the increase was less when Sr was present in the alloy. Section 6 reports results obtained from AZ31+0.8Sr subjected to annealing prior to extrusion at four different times. An increase in the duration of the pre-deformation anneal switched the plane facing the extrusion direction from first order prismatic (100) to second order prismatic planes (110). In the final of part of this thesis, the microstructures and textures of hot extruded AZ31-Sr tubes were examined. Section 7.1 reports on seamed tubes that were extruded using a porthole die. The complex strain path inside the die could be divided into several simple curvatures which occur during ECAP (Equal Channel Angular Pressing). In section 7.2 the expansion behaviour of seamless AZ31-Sr tubes was studied by compressing the tubes with pure-copper cylinder-inserts. The texture of the extruded tube is comprised of two components: hoop direction (HD) and radial direction (RD), which behave differently during expansion. The HD component undergoes extension twinning while the contraction and double twinning occurs in RD grains.
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Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| 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 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».