טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentHarosh Shlomi
SubjectPhase Formation and Precipitations Hardening in Mg-Sn-Zn-Al
Alloys
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Emeritus Menachem Bamberger
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

In recent years, research and development of magnesium alloys have been greatly promoted by the lightweight requirement in the automotive & aircraft industries. However, commercial applications of Mg alloys are limited because of poor creep resistance at temperatures above 1200C. Previous study on Mg-Sn-Zn alloys, revealed the precipitation of MgZn2 that forms to MgZn and formation of Mg2Sn particles. However, at 175ºC the addressed alloys were overaged after 96 hours and showed poor corrosion resistance. In order to improve the castability and corrosion resistance an addition of  2wt% Al was used.  The aim of this thesis is to investigate the structural stability of Mg-Sn-Zn-Al based alloys that exhibit improved structural stability at elevated temperature. A further improvement was expected by addition of 1wt% MM (50%Ce-25%La-20%Nd-5%Pr) or 1wt% Sr. At the as-cast condition, SEM micrographs indicate a very fine micro-structure (DAS below 12 μm) for all the investigated alloys. Alloying the Mg-Sn-Zn basic system with Al resulted in the formation of eutectic phase Mg32(Al,Zn)49 rather than MgZn which was formed in Mg-Sn-Zn alloys. Addition of 1% Ce-based MM resulted in the formation of a ternary phase Al2(Ce,La)Zn2 on the grain boundaries. The study focuses on the precipitation hardening, phase formation and structural stability during aging at elevated temperatures of solution treated samples. After solution treatment (450°C) and aging at 225°C, Vickers hardness measurements show that this alloy maintains an increase of 30% in hardness for periods of up to 32 days. all of the cases, overaging was not observed even after aging for 32 days at 225ºC. Furthermore, Very fine homogenously distributed precipitations (less than 500nm) were found. EDS, XRD, Auger and SAXS characterization methods were employed in order to identify the phase structure of the alloy and the origin of precipitates. The EDS at the HRTEM was utilized to analysis the chemical composition of submicron precipitates. There was no evidence for the presence of the deleterious γ-Al12Mg17 phase. Thermodynamic data was collected from DSC scans of as-cast samples and found compatible with theoretical calculations. The main conclusion of this research is that the Mg-Sn-Zn-Al based alloys are promising candidates for future commercial elevated temperature applications.