                      !"  # $   %# %%  Extensive research work was devoted to Mg-based alloys strengthened by precipitation hardening. In this framework, the Mg-Zn-Sn system was considered a promising candidate for a creep resistant Mg-alloy. Small additions of alloying elements forming high temperature phases (HTP) were used to improve the structural stability of the Mg-Zn-Sn alloy. Phase formation during solidification was analyzed using thermodynamic calculations. The influence of HTP-particles on stabilization of sub-grain boundary structure was found to be of great importance in improving structural stability of the alloys at elevated temperatures. Mechanisms of precipitation hardening were investigated using the modified Langer-Schwartz model calibrated for Mg-Zn-Sn alloys.  The Mg-Zn-Sn system was considered a promising candidate for a creep resistant Mg-alloy strengthened by precipitation hardening. Precipitation hardening is usually considered the result of interaction between dislocations and new phase particles precipitated during aging. In the case of Mg-Zn-Sn alloys, MgZn 2 and Mg 2 Sn intermetallic phases distributed within the Mg grains are responsible for strengthening the α-Mg-matrix. However, the coarsening process leading to overaging substantially diminishes the strengthening effect. It was found that small additions of alloying elements forming high temperature phases (HTP) improve the structural stability of the Mg-Zn-Sn alloy [1.2]. The present work was undertaken in order to investigate the microstructure evolution of the Mg-Zn-Sn-alloy with additions of Y and Sb which form stable high temperature phases in the Mg-Zn-Sn-system and to elucidate a mechanism of HTP-influence on the microstructure stability.      The phase formation during the solidification of the investigated alloys were evaluated with the aid of "ThermoCalc", a software package for calculating phase diagrams and phase formation during solidification. Thermodynamic analysis of the quaternary systems Mg-Zn-Sn-Y and Mg-Zn-Sn-Sb performed with the ThermoCalc program shows that the additions of Y and Sb to the molten Mg lead to the formation of high temperature phases during solidification. In the case of Y this is the ternary phase MgSnY, and in the case of Sb, this is the binary phase Mg 3 Sb 2 . The formation temperatures of MgSnY and Mg 3 Sb 2 are different: the MgSnY phase starts to form at 570 o C whereas the Mg 3 Sb 2 phase is formed below 525 o C. So MgSnY particles have more time to grow from the melt. This means the Y is more effective structure stabilizer than Sb since may lead to higher volume fraction of HTP-particles in the quenched alloys.          HTP-particles may serve as effective barriers for moving grain- and sub-grain boundaries, therefore restrain the growth and disappearance of sub-grains during solution treatment. As a result, a distinct sub-grain microstructure is preserved as opposed to its absence in the base Mg-Zn-Sn alloy. These processes Advanced Materials Research Vol. 95 (2010) pp 51-54 Online available since 2010/Jan/12 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.95.51 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 84.229.185.10-22/04/10,23:26:02)