טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAmir Finkel
SubjectThe Effect of Exposure to Elevated Temperatures on the
Microstructure and Hardness of Die Cast Mg-Ca-Zn
Based Alloys
DepartmentDepartment of Materials Science and Engineering
Supervisors Full Professor Rabkin Eugen
Professor Emeritus Bamberger Menachem


Abstract

Three different alloys based on Mg-5wt%Ca-6wt%Zn were cast into steel mold and than exposed to 160ºC for 40 days. Microstructural analysis, thermal analysis, micro-hardness and hardness measurements conducted in samples at predefined periods of time enabled monitoring the thermal stability of the cast alloys. The as cast structure is composed mainly of a-Mg solid solution, and in the grain boundaries 2mm elliptical precipitates of CaMg2 and eutectic structure of Mg and Ca2Mg6Zn3 were found. The a-Mg grain size was 10mm and no change was observed during exposure to 160ºC. The structure of the alloy did not changed during the treatment, but the amount of the inter-granular phases slightly increased. Exposure to 160ºC resulted in a decrease in micro-hardness of a-Mg grains, but no change in the overall hardness of the samples was observed during this time. Microstructural analyses point out on diffusion of the solute elements from the a-Mg grains to grain boundaries resulting in a decrease in the hardness of the a-Mg, which is compensated by the increase in hardness related to inter-granular precipitates. The diffusion coefficient of Ca in Mg matrix was calculated and found to be 2.1·10-17 m2/s, which is higher then the diffusion coefficient of Zn in Mg matrix by two orders of magnitude. Addition of 2wt%Si reduced the thermal stability of the alloy and the overall hardness, but 1wt%Nd addition improved the properties of the alloys. The properties of the studied alloy are competitive with those of rapidly solidified Mg-Ca-Zn alloys and much better than those of AZ91 alloy. In addition, the structural stability at 160ºC is better than that of rapidly solidified Mg-Ca-Zn alloys and AZ91.