Regular Structure Formation of Hypermonotectic Al-In Alloys

Since the liquid / liquid separation occurs in hypermonotectic alloys and the liquid / liquid interface agitates mass transfer around solidifying front, it is rather difficult to achieve the aligned-rod structure. The high magnetic filed such as 10T achieved the aligned-rod structure in the Al-10at%In alloys. The in-situ observations of the monotectic solidification in the Al-10at%In alloys were performed using synchrotron radiation X-ray. Coarse and fine In rods coexisted during the unidirectional solidification without magnetic field. The local melt flow induced by the In-rich liquid / Al-rich liquid interface enhanced the mass transfer and consequently the coarse rods could continuously grow. The suppression of the local melt flow of which scale was several 10 m by the high static magnetic field resulted in the aligned-rod structure.

Fabrication of Porous Aluminium and Copper Media by Using Monotectic Solidification under a Magnetic Field

Unidirectional solidification of Al-In, Cu-Pb and Cu-Pb-Al monotectic alloys was performed under static magnetic fields up to 10T for formation of the regularly aligned-rod structure. mThe imposition of static magnetic fields exceeding 4T suppressed movement of the In liquid droplets at the Al solidifying front and enhanced the engulfment of the In droplets into the front. As a result, the eutectic-like structure was obtained in the Al-10 and 15at%In hypermonotectic alloys. The micro X-ray tomography indicated that the continuous In rods with diameters of 10-20 µm were aligned parallel to each other. In the case of the Cu-Pb and the Cu-Pb-Al alloys, the imposition of static magnetic fields also enhanced the formation of the aligned Pb rods. The electrochemical etching by using a 10% HNO3 solution successfully removed the minor phase, and the porous aluminum and copper with deep pores were fabricated.

On a Morphological Transition in Monotectic Solidification

Al-Bi and Al-In of exact monotectic composition were directionally solidified with solidification velocities ranging from 0.3 µm/sec to 5 µm/sec and temperature gradients from 2 K/mm to 6.5 K/mm. Three distinct microstuctures could be observed: fibers, strings of pearls or irregular arrays of second phase droplets. The transitions from fibers to strings of pearls is discussed theoretically and compared with the experimental results. The origin of this transition seems to be a Nichols and Mullins instability of the fibers during cooling from the monotectic to the eutectic temperature caused by surface perturbations driving volume diffusion of solute in the liquid fibers of the second phase.