טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShepelev Dmitry
SubjectThe Influence of Minor Additions on the Structure and
Mechanical Properties of Magnesium Alloys
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Emeritus Menachem Bamberger
Dr. Alexander Katsman
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

This research explores the possibilities to improve the mechanical properties of the alloys AZ91E and Mg-Ca-Zn, by minor addition of micro and sub-micro particles.

Ceramic particles of α-Al2O3 and SiC can serve as such additives to refine the microstructure of Mg-based alloys. However, direct introduction of ceramic particles into Mg matrix is limited by the poor wetting of those particles by liquid Mg and their massive agglomeration. Mg/α-Al2O3 and Mg/SiC master alloys were prepared using a method based on the insertion of the ceramic particles into a molten Mg bath, under a nitrogen atmosphere. These master alloys were used to refine AZ91E alloys by introducing various amounts of ceramic particles to manufacture AZ91E%Al2O3, AZ91E%SiC and AZ91E%SiC alloys. The alloy AZ91E%Al2O3 was grain refined to ~ 4 0?m and the alloys AZ91E? were grain refined to ~50?m as against 110?m in non-refined counterparts. The mechanical properties of the modified alloys are substantially better than those of a non-refined AZ91E alloy.

The microstructure and mechanical properties of Mg-Ca-Zn alloys with 1 wt.% Zr were investigated in as-cast and heat treated conditions. A substantial decrease in grain size (from 65 mm for the Mg-Ca-Zn base alloy to 22 mm) was observed. The alloy was exposed to solution treatment at 410°C for up to 96 hours followed by aging at 175°C for up to 96 hours. The nanoscale mechanical properties of grain boundary precipitate depleted zones (PDZ's) were analyzed using combined nanoindentation and atomic force microscopy (AFM). The width of PDZ's was examined after different solution treatments (ST) at 410˚C and different exposures to 175˚C.  An increase in ST duration from 10 h to 96 h at 410˚C resulted in expansion of PDZ's from ~0.75?m to ~3?m, while the following aging at 175˚C for up to 24h did not lead to detectable change in PDZ's. The analysis indicates that the lowest hardness was found in the region where Zn2Zr precipitates density was low, regardless of solute concentration.