First Reliable Diffusion Coefficients for Mg-Y and Additional Reliable Diffusion Coefficients for Mg-Sn and Mg-Zn
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our quest to experimentally measure the diffusion coefficients of key alloying elements in Mg alloys in order to establish reliable diffusion (mobility) databases for computational design and process optimization of Mg alloys. A novel liquidsolid diffusion couple (LSDC) was recently developed to effectively collect diffusion profiles above the eutectic temperatures[1]; it enables determination of diffusion coefficients for even very challenging cases such as very low eutectic temperatures and extremely low solute solubilities. The forward-simulation analysis (FSA)[2–4] enables reliable extraction of both interdiffusion and impurity diffusion coefficients from the LSDC profiles as well as regular solid-solid diffusion couple profiles. The combined LSDC and FSA method allowed us to obtain the first-ever set of impurity diffusion coefficients of Ca in Mg as well as reliable high-temperature diffusion data of Al in Mg for a more reliable impurity diffusion coefficient of Al in Mg.[1] New diffusion coefficient data for three additional key alloying elements Sn, Y, and Zn in Mg alloys are reported here.
WEI ZHONG and JI-CHENG ZHAO are with the Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210. Contact email: [email protected] Manuscript submitted July 16, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The geometry of the LSDCs (Figure 1) and the procedure to make them are exactly the same as those reported for Ca and Al.[1] A ~ 5-mm diameter hole with a screw thread at the upper neck and a flat bottom was made to ~ 18 mm depth in a pure Mg (99.95 wt pct) block of ~ 20 9 20 9 25 mm. Pure Mg, Zn (99.95 wt pct), Sn (99.95 wt pct), Fe (99.9 wt pct), and a Mg-25 wt pct Y master alloy were prepared to required pellets (~ 5 mm in diameter and 3–5 mm in height) using mechanical machining or electrical discharge machining (EDM) followed by grinding to a fine surface finishing. A steel screw was used to tighten the assembly after the prepared pellets were put in sequence into the hole of Mg blocks inside an argon protected glove box. The assembled LSDC was then sealed in quartz tube with 1/5 atm pressure of backfilled argon. The samples were then subjected to designed heat treatments. The Mg-M (M refers to Zn, Sn, or Mg-25Y) region was melted at the annealing temperatures and diffusion took place between melt and the surrounding pure Mg to form a Mg solid solution adjacent to the liquid pool formed in the center as schematically shown in Figure 1(c). The LSDC samples then were quenched and sectioned through center line, followed by metallographic preparation for microscopy characterization. The electron-probe microanalysis (EPMA) was employed to collect the diffusion profiles using a CAMECA SX100 electron microprobe with operation condition of 15 kV accelerating voltage, 30 nA beam current, and 40 deg take-off angle at various step sizes. Three Mg-Zn LSDCs were assembled using pure Mg and pure Zn, and were annealed at 450 C for 8 hours, 500 C for 6 hours, 550 C for 6 hours, respectively. The solubi
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