Strontium additions in AZ31 magnesium alloy
Bibliographic record
Abstract
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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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".