Analysis of interdiffusion of Dy, Nd, and Pr in Mg
- PDF / 313,179 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 54 Downloads / 174 Views
The diffusion characteristics of Mg–rare-earth diffusion couples were studied. Cylinders of pure Mg and rare earth (Dy, Nd, and Pr) were abutted and annealed at 500 °C for 100 h or 300 h. Point-by-point composition profiles were collected starting in pure Mg, across the diffusion zone, and ending in the pure rare earth, using energy dispersive x-ray spectroscopy with a scanning electron microscope. The intermetallic phases that resulted due to diffusion were identified and compared to existing phase diagrams, for which the data is limited. For each diffusion couple, a plot of concentration versus distance perpendicular to the original plane of contact was obtained and analyzed using the Boltzman–Matano method. The interdiffusion coefficients for each set of phases were then calculated. The results show that diffusion through the intermetallic phases is much slower than is expected in a solid solution.
I. INTRODUCTION
Rare earths are routinely added by industry to improve mechanical properties in cast Mg alloys. The first Mg– rare-earth alloys developed used mischmetal (50% Ce, 30% La, 15% Nd, and 5% Pr) to reduce microporosity in wrought alloys such as Mg–1.25Zn–0.17MM.1 Rare earth (RE) additions are especially effective in improving the creep resistance of magnesium-based alloys.2 The rare earths also refine the grain size and improve the strength, ductility, toughness, weldability, machinability, and corrosion resistance.3 Recently developed alloys containing one or two rare earths have been found to possess improved properties over the early mischmetal alloys. Didymium (80% Nd–20% Pr mixture) is the most effective, followed by cerium-free mischmetal, mischmetal cerium, and lanthanum, in the order of decreasing effectiveness. Mg–Al–Zn–Nd alloys have good corrosion resistance in an aqueous saline solution. A Mg–Y–Nd–Zr alloy was shown to have good corrosion resistance, good castability, and stability to 300 °C.4 Despite the widespread use of rare earths in Mg alloys, little is known about the diffusion rates of rare earth in Mg or vice versa. The only known diffusion study thus far that the authors could find is that of Lal and Levy, who determined the diffusion coefficients of Ce and La in Mg.5 In their study, pure Mg and Mg–rare-earth alloys were abutted to form diffusion couples. Their results are summarized in Table I. In a somewhat related study, liquid Mg was used to extract Nd from solid FeNdB magnet scrap. In this investigation it was shown that the diffusion rate of the Nd through the scrap was extremely rapid.6 Calculations J. Mater. Res., Vol. 16, No. 11, Nov 2001
http://journals.cambridge.org
Downloaded: 30 Mar 2015
based on the composition profiles obtained gave a value of 4.61 ×10−8 cm2/s at 700 °C for the diffusion coefficient of Nd in liquid Mg. This study focused on the interdiffusion between Mg and the rare earths Dy, Nd, and Pr. The concentration profiles that developed in diffusion couples during isothermal anneals were examined and the phase layers identified. The Boltzman–Matano method w
Data Loading...
