Grain refining mechanism and casting structure of Mg-Zr alloy.

Role of Lanthanum in Grain Refinement and Tensile Properties of Cast Mg–La–Zr Alloys

“…710) In earlier, it was realized that undissolved zirconium particles in the melt play an important role in grain refining, causing copious heterogeneous nucleation and growth. 8) This view was further supported by more recent studies of Qian et al 11,12) The majority of active undissolved particles observed at the center of the cores of grains were in the range of 1 to 5 µm in size. 10,13) In the previous investigation, 9) a combined addition of zirconium and lanthanum was found to confer more marked grain refining than zirconium alone.…”

“…This addition provided a soluble zirconium amount of approximately 0.5% and a sufficient number of undissolved zirconium particles (insoluble Zr) that act as active heterogeneous nucleation sites, thus ensuring good grain refinement. 8) The melt was also stirred to ensure proper mixing and good grain refining effect prior to pouring at 700°C into an open cast-iron mold with trapezoidal-cavity (18 mm top © 13 mm bottom © 10 mm high © 100 mm long) with a sufficient amount of riser to avoid shrinkage porosities in the cast bars for tensile testing. The melt was also poured to a bottom pour cast-iron mold with a cylindrical cavity (20 mm dia.…”

“…Since the grains in the MgLaZr alloys were finer than those of the MgZr alloy and of greater number per unit area, it was previously thought 9) that due to lanthanum's lack of solubility in magnesium, there would be a buildup of solute lanthanum at the growth front, restricting the crystal growth. It was also thought that a combination of the greater growth restricting factor of zirconium 16) with the available potent zirconium particles present in the melt 8) would lead to an increase in the number of crystal formations as might have occurred in the MgLaZr alloys, resulting in a much greater grain refinement effect. However, the grain growth in the MgZr alloy might have taken place right after solidification, while it did not occur or was hindered in the MgLaZr alloys due to the presence of lanthanum.…”

“…In order to confirm if this difference in grain size was indeed due to the result of solid state grain growth or difference with the nucleation event during solidification, in the following discussion, particular attention is given to the forms of zirconium-rich cores. Since undissolved zirconium particles contribute significantly to the grain refining of magnesium by acting as nucleants 8) and also the subsequent formation of Zrrich core structures occur due to a zirconium solute built-up in the liquid adjacent to growing grains during solidification, Zr-rich cores and their morphologies can be a good indicator for nucleation events and the growth history of grains associated with solidification conditions.…”

Feasibility of Friction Stir Welding for Joining and Microstructure Refinement in a ZK60 Magnesium Alloy

“…It is generally believed that the Zr particles are effective grain refiners during solidification of magnesium alloys due to the low mismatch between magnesium and zirconium. 33) Our present study indicates that the particles containing Zr might have played an important role for the grain refinement during FSW of ZK60. Further studies will be necessary to discuss more in detail the role of particles containing Zr for the grain refinement caused by FSW process.…”