Author: Ahmad

Experimental Investigation and Thermodynamic Modeling of the Mg-Mn-Nd Phase Diagram
Ahmad Mostafa1, Dmytro Kevorkov1, Mamoun Medraj1
1 Concordia University, Montreal, Canada

Motivation and Objective

Magnesium and its alloys have an increasingly important role in structural automotive and aerospace industry where weight saving is important. Magnesium alloys containing Mn and Nd exhibit very good mechanical properties at elevated temperatures. To date, the phase relations in the Mg-Mn-Nd phase diagram have not been studied thoroughly. The main purpose of this work is to determine the phase relationships in the Mg-Mn-Nd ternary phase diagram and its constituent binary systems by coupling the experimental investigation with thermodynamic modeling. A tentative isothermal section was constructed at 450°C.

Mg die-cast alloy for gear box developed by VW.

Results

The system was investigated experimentally at 450°C using diffusion couples and key alloys. High purity elements were used for sample preparation.
The Mg-Mn-Nd system was optimized using CALPHAD approach in the FactSage™ software package, taking into account all the experimental results. The Mg-Nd, Mn-Nd, and Mg-Mn binary models were used to optimize the Mg-Mn-Nd system.

Mg62Mn6Nd32

Mg71Mn3Nd26

Mn

SEM micrograph for Sample #1 in the as-cast condition showing the phase relations upon solidification.

The optimized Mg-Nd phase diagram. The results of the diffusion couple were found consistent with the intermetallic compounds in the thermodynamic model.

Mg71Mn3Nd26

Mn

5

Mg62Mn6Nd32

3

SEM micrograph for Sample #1 annealed at 450°C for 5 days showing decomposition of Mg2Nd phase

2

1

DSC Peaks analysis for sample #2

MgNd

Mg3Nd

Nd

Mg

Tentative isothermal section of the Mg-Mn-Nd system at 450°C

Mg41Nd5

SEM micrograph for a diffusion couple, designed between Mg and Nd end-members, annealed at 450°C showing the stoichiometric compounds of the binary phase diagram.

DSC results of samples #2, #3 and #5 compared with the calculated phase transformation temperatures

XRD pattern of sample #1 showing the detected phases

Conclusions

A combination of thermodynamic modeling and experimental studies was used to construct the phase relationships of Mg-Mn-Nd phase diagram. The ternary Mg-Mn-Nd system was modeled based on the available models of the binary systems. Key samples and diffusion couples were annealed at 450°C and studied using DSC, XRD and SEM/EPMA.
This work will provide valuable information for the Mg-Mn-Nd ternary system which can be used for alloy design.

References

M. Drits, Z. Sviderskaya, L. Rokhlin, Zh. Neorgan. Khim., 7 (1963) 2771-2777.
B. Mordike, K. Kainer, Magnesium Alloys and Their Applications, Willey-VCH, Weinheim, 2000.