Effect of Ca/Nd on Thermal Stability and Mechanical Properties of AZ71 Magnesium Alloy
| Autor: | S. J. Huang, 黃仕傑 |
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| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 103 This study investigates thermal stability and mechanical properties of AZ71 magnesium alloy with addition of 0~5wt.%Ca and 1~2wt.%Nd. The ignition temperature of AZ71 is 574°C. By adding 0.5 and 2wt.%Ca to AZ71, the ignition temperature increases to 815°C and 1063°C. The formation of low temperature eutectic β-Mg17Al12 is reduced due to formation of Al2Ca. The ignition point cannot increase further when Ca addition is higher than 2 wt.%. The combustion occurs when magnesium evaporates through the loose magnesium oxide surface film to react with oxygen at high temperature. On the other hand, CaO forms dense MgO-CaO composite oxide film on the surface and protect the magnesium from ignition. The ignition temperatures of AZ71+0.5Ca+1Nd, AZ71+0.5Ca+2Nd are 825°C and 783°C, respectively. The results show that Nd cannot increase ignition temperature. The larger Nd atoms prevent them from diffusing to surface and form protective oxide films. The surface films are still mainly MgO-CaO composite oxide film. When over 1 wt.%of Nd is added, the AlxNdy precipitates are formed in aggregation, which can be harmful for dense oxide film formation. Both tensile strength and ductility decrease with Ca concentration. Nd is found to reduce the negative effects of Ca on tensile strength and ductility at high temperatures. Both solid solution and precipitation strengthening by forming AlxNdy compound give rise to increase of strength. When over 1 wt.% of Nd is added, aggregation and coasenimg of AlxNdy precipitates are formed and result the adverse effects on tensile strength and ductility. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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