PHASE TRANSFORMATIONS AND MICROSTRUCTURE DEVELOPMENT DURING HEAT TREATMENT OF Mg-Y-Nd(-Mn)-Zn ALLOYS
Abstract
The aim of this work was to investigate and compare microstructure changes in squeeze cast heat treated Mg-2Y-1Nd-1Zn (WEZ211) and Mg-3Y-2Nd-1Zn-1.5Mn (WEZM3211) alloys. The WEZM3211 alloy was isochronally heat treated in 30 oC/30 min steps from room temperature up to 510 oC, the WEZ211 alloy was first isochronally heat treated in steps of 20 oC/20 min up to 440 oC, quenched and isochronally heat treated again.
Phase transformations were investigated by several methods: electrical resistivity was measured after each heating step; transmission electron microscopy was performed in selected states; differential scanning calorimetry was realized in the temperature range 20 - 510 oC. Electrical resistivity curve of the WEZM3211alloy exhibits a slight decrease around annealing temperature 150 oC and the main drop in the temperature range 240 - 330 oC.
This drop can be ascribed mainly to two precipitation processes - formation of thin basal plates of '' phase and subsequently prismatic plates of equilibrium & phase. After annealing at higher temperatures oval particles containing Mn, Y were observed.
Electrical resistivity of the WEZ211 alloy shows two decreases connected with formation and development of thin basal plates of 'phase and ordered phase of 14H type.