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Diffusion Study of Mg-based Alloys: An Experimental and Modeling Approach
Sazol Kumar Das and In-Ho Jung
Department of Mining and Materials Engineering, McGill University, Montreal, QC, Canada, H3A 0C5
Anisotropic impurity diffusion coefficients of Al, Zn, Y, and Gd wereinvestigated for the first time using diffusion couples with Mg singlecrystal.
Anisotropic impurity diffusion coefficients of D ┴ / D ║ = 1.0 ~ 1.3,which is similar to Mg self-diffusion.
Solute diffusion coefficients decreases with increasing temperatureand can be neglected at common materials processing temperature forMg-based alloys.
The Al grain boundary diffusion coefficient was determined withmisorientation by developing a new technique with CFE-SEM EDS andfound to be about two order of magnitude faster at 75° misorientationbetween adjacent Mg grains.
Phase boundaries of stoichiometric compounds and solubility limits ofsolid solutions in the Mg-Y and Mg-Gd systems were determinedbetween 703 and 803 K and the phase diagrams of these two systemswere re-optimized.
ABSTRACT
Diffusion of alloying elements is key to control high temperatureprocesses (annealing, rolling, extrusion, etc.).
However, there is no systematic diffusion study or database for Mg.
Mg has anisotropic hcp crystal structure (c/a ratio: 1.62).
Self-diffusion in Mg is anisotropic D ┴ / D ║ = 1.13-1.26 [1-3].
Diffusion of Ag, Cd, In, Sb also shows anisotropy in hcp Mg [4].
Al and Zn are the prime alloying elements in Mg.
RE (Y and Gd) are also known to improve the property of Mg alloys.
There is no experimental study for anisotropic diffusion of Al, Zn, Y, and Gd in hcp Mg.
PROBLEM STATEMENTS
Diffusion of Ag, Cd, In, Sb and Sn in hcp Mg [4].[1] P. G. Shewmon ,Trans. Metall. Soc. AIME 206, 918 (1956): Single crystal radiotracer sectioning.
[2] J. Combronde et al., Acta Metall. 19, 1393 (1971): Single crystal radiotracer sectioning.
[3] M. Mantina , PhD thesis. Penn State University, 199 (2008): First principles calculations.
[4] Combronde et al., Acta Metall. 20, 37 (1972): Single crystal radiotracer sectioning.
To determine the anisotropic impurity diffusion coefficients of Al, Zn,
Y, and Gd in hcp Mg using single crystal Mg diffusion couples.
To determine grain boundary diffusion coefficients of solutes with
adjacent grains misorientation angles using Mg polycrystalline
diffusion couples.
To develop a diffusivity (mobility) database for Mg alloys.
To develop ICME tools for the solidification and homogenizationprocesses of Mg alloy production.
OBJECTIVES
Experimental conditions:Mg/Al: Temperature: 638 – 693 K; Duration: 2 – 4 daysMg/Zn: Temperature: 553 – 593 K; Duration: 4 – 20 daysMg/Y: Temperature: 723 – 803 K; Duration: 2 – 5 daysMg/Gd: Temperature: 703 – 743 K; Duration: 2 – 7 days
Diffusion equation (Fick’s 2nd law):
Solution of diffusion equation:
APPROACH
Pulley
Drum
Lid
Upper
Winding
Melt
Baffle
Crystal
Lower
Winding
Sketch of vertical Bridgman method [5].
Schematic of special cutting jig.
Mg Single Crystal
Before
After
Gas flow meter
Thermocouple
Controller
Steel Tube
Al
Mg
Before
X
MgAfter
Atmosphere-controlled low-temperature furnace. Samples before and after diffusion annealing.
EBSD, XRD Special Cuting
PolishingClamping on a-
and c-axisDiffusion Annealing
Quenching Mounting
Modeling
x
CD
xt
C mm
i
m
)])(())([(
)(211112
1 j
i
j
iii
j
i
j
iii
j
i
j
i CCDDCCDDx
tCC
FESEM-EDS EPMA-WDS
Slow Cuting
RESULTS
RESULTS
SEM-BSE image of interface microstructures of Mg-X diffusion couples.
Matano PlaneMatano Plane
Solute (Al, Zn, Y, Gd) concentration profiles through diffusion couple interfaces (EPMA-WDS).
Mg24Y5 Mg2Y YMg
MgY
Mg5Gd Mg3Gd Mg2Gd GdMg
MgGd
Mg4Gd6
Mg17Al12 MgMg2Al3Al Mg2Zn11 MgZn2 Mg2Zn3 MgZn
Al profile along grain boundary in comparison with bulk profile in hcp Mg.
Anisotropic impurity diffusion coefficients of Al, Zn, Y and Gd in theHCP Mg matrix were experimentally determined for the first time.Diffusivities of Y and Gd are about one order of magnitude lower thanthose of Al, Zn, and Mg.
Grain boundary diffusion coefficients of Al are two order of magnitudehigher at around 75° misorientation between adjacent Mg grains.
Phase boundaries in the Mg-Y and Mg-Gd systems were determinedaccurately in the solid solutions and intermetallics, and phase diagramswere re-optimized.
SUMMARY
693 K for 2 days
773 K for 2 days
593 K for 4 days
743 K for 2 days
693 K for 2 days
773 K for 2 days
593 K for 4 days
743 K for 2 days
[5] Prof. Tae Kwon Ha’s Research laboratory, Gangneung-Wonju National University, South Korea.
500μm
GB2 G1 G2 GB1 GB3
67o 76o 36o
o o o
hcp Mg
intermetallic layer Al
EBSD map
SEM SE micrograph
New technique to determine grainboundary diffusion coefficients withadjacent grain misorientation anglesusing CFE-SEM EDS from diffusioncouple experiments.
Calculated phase diagrams of the Mg-Y and Mg-Gd systems with experimental data.
CALPHAD XLIIICentral South University, ChangshaJune 1-6, 2014Changsha, China
Polishing
Impurity diffusion coefficients of Al, Zn, Y, and Gd in comparison with Al grain boundary and Mg self-diffusion coefficients in hcp Mg.
Publications:
- S. K. Das and I. -H. Jung, “Effect of the Basal Plane Orientation on Al and Zn Diffusion in hcp Mg”, Materials Characterization, 2014 (accepted).
- S. K. Das, Y. B. Kang, T. K. Ha and I.-H. Jung, “Thermodynamic Modeling and Diffusion Kinetic Experiments of Binary Mg-Gd and Mg-Y Systems”, Acta Materialia, 71, 164 (2014).
- S. K. Das, N. Brodusch, R. Gauvin, I.-H. Jung, “Grain boundary diffusion of Al in Mg” Scripta Materialia, 80, 41 (2014).
- S. K. Das, Y. –M. Kim, T. K. Ha, R. Gauvin, and I. –H. Jung, "Anisotropic diffusion behavior of Al in Mg: Diffusion couple study using Mg single crystal" Metall. Mater. Trans. A. 44, 2539 (2013).
- S. K. Das, Y. –M. Kim, T. K. Ha and I. –H. Jung, “Investigation of anistropic diffusion behavior of Zn in hcp Mg and interdiffusion coefficient of intermediate phase in Mg-Zn system”, Calphad, 42,
51 (2013).
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