|M.Sc Student||Gorny Anton|
|Subject||Precipitation Hardening in Mg-Sn-Zn-Y and Mg-Sn-Zn-Sb Cast|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Menachem Bamberger|
|Full Thesis text|
In this research, the precipitation process at Mg-Sn-Zn-Y and Mg-Sn-Zn-Sb alloys were studied at the as-cast condition in order to investigate the behavior of these alloys at elevated temperatures. In addition, the alloys were solution treated in order to allow better understanding of the precipitation mechanisms and test the alloys response to precipitation hardening.
Zn dissolves well in Mg and participates in solution hardening, same with Sn while Y and Sb additions lead to the formation of intermetallics with Mg which are stable at elevated temperatures.
The alloys were examined in the as-cast state, after solution treatment, followed by various aging periods and temperatures (150ºC-225ºC for 1-384hr.), TEM,SEM and XRD examination of the as-cast alloy reveals the coexistence of 3 phases, αMg matrix, the binary Mg2Sn intermetallic, Mg/MgZn eutectics and ternary MgSnY in case of Y containing alloy and Mg3Sb2 phase in the case of Sb containing alloy all located in the interdendritic regions.
MgSnY phase was found in as cast Y containing alloy. This phase was previously unmentioned in the literature. The phase remains unchanged during all thermal treatments which alloy undergo. TEM investigation of MgSnY phase suggesting primitive orthorhombic unit cell with lattice parameters a= 6.97 Å, b= 10.33 Å, and c= 13.27 Å.
Further examination of the solution treated specimens by SEM and TEM has shown that Sn and Zn phases were completely dissolved in the matrix during solution treatment and that MgSnY and Mg3Sb2 were not dissolved in the α-Mg matrix.
Hardness in Y and Sb containing alloys has 3 maximums. During aging first precipitate is MgZn2 needle shaped particles appeared from the beginning of aging process. They grew semicoherently to the αMg matrix. After 16 hours of aging second phase precipitates. It's Mg2Sn phase which precipitates on top of MgZn2 needles of certain size forming T-shaped particles.
TEM investigation of aged samples revealed formation of chains of precipitates on sub-grain boundaries which lie on MgSnY phases in case of Y containing alloy and on Mg3Sb2 phases in case of Sb containing alloy. These stable phases at elevated temperatures influence the mobility of sub-grain boundaries and improve structural stability during aging, subdivide effectively large grains into smaller sub-grains and present additional obstacle in the path of moving dislocations during plastic deformation. The presence of sub grains explains the third hardness peak.