The phase and morphological transformation during the remelting process was investigated by isothermal soaking and rapidly quenching of preformed AZ91D magnesium alloys in semisolid state. It was revealed that the morphological transformation of preformed alloys is crucial to obtain homogenously fine spheroidal grains and affect the final forming ability. The transformation is divided into two stages, local remelting of the whole experiment and partial remelting of the respective grains, which behave as liquid bands and liquid cells structures, respectively. In the partial melting, the lower melting point phase, β-Mg17Al12, diffused to the grains boundary and center of the grains and separated to Al2Mg and Mg. The Al2Mg and Mg phases with lower melting points melt into cells structures. The final microstructure of the remelting experiments is composed of cells structures, spheroidal grains and liquid phase.

Structural evolution of bulk undercooled Ni/11.56 at.% Pb monotectic alloy was investigated systematically by using molten glass denucleating combined with superheating cycling. Within the achieved undercooling range 10/286K, the solidification structures were classified into three categories. When the undercooling was less than 50K, the structures consisted of coarse dendrites and interdendritic lead phase. With the undercooling increasing into the range of 70/232K, the dendrite clusters were refined and fine lead particles separated out from the supersaturated primary dendrite arms because of solute trapping. When the undercooling exceeded 242K, the granular grains formed and fine lead particles homogeneously distributed in the whole sample. The phase selection of high temperature melts was analyzed by adopting steady state nucleation theory. The calculated results shown that undercooled Ni/11.56 at.% Pb monotectic alloy melts solidified in the form of a(Ni) dendrites essentially during the stage of rapid solidification and after recalescence, the interdendritic residual melts solidified in the equilibrium mode. Based on the observation of the solidification structures and the calculated results with BCT dendritic growth model, it was confirmed that the granulation mechanism of the granular grains was owing to the primary dendritic disintegration and subsequent recrystallization.

Microstructural evolution of bulk undercooled Ni-40wt%Pb hypermonotectic alloy was systematically investigated by using molten glass purification and superheating cycling. Within the achieved undercooling range, the microstructures were classified into three categories. When the undercooling was less than 50K, the microstructure consisted of coarse dendritic grain and interdendritic Pb lumps. Increasing undercooling to the range 100 to 198K, macrosegregation was serious. When the undercooling was up to 292K, refined granular grain with homogeneously distributed fine Pb particles on it formed. The granulation mechanism of granular grains was owing to dendritic disintegration and subsequent recrystallization.

Effects of the crystal growth velocity on the crystal orientation and the microstructures of the Tb Dy Fe compound have been 0.27 0.73 1.9 investigated in an optical image zone melting furnace. The results indicate that the crystal growth velocity is very important to prepare oriented crystals of Terfenol-D. Without any seed crystals, when the velocity is lower than 5mm/ h, mainly k111l oriented crystals can be obtained. When the velocity is changed between 5mm/h and 15mm/ h, Terfenol-D crystals with k112l orientation or k110l orientation can be obtained. When the velocity is higher than 15mm/h, k113l oriented crystals can be prepared. The microstructures are in agreement with the X-ray diffraction patterns. The shape of the grains is round or that of a polygon. During the preparation of the Terfenol-D oriented crystal in an optical image zone melting furnace, the length of the molten zone, shape of the molten zone and temperature of the molten zone as well as the crystal growth velocity are important to prepare precisely oriented crystals of Terfenol-D. In the optical image zone melting furnace, a low crystal growth velocity can effectively suppress twin formation in Tb Dy Fe compounds. For the 0.27 0.73 1.9 composition of Tb Dy Fe , k111l oriented twinless single crystals or polycrystals can be possibly prepared in the furnace by well 0.27 0.73 1.9 controlling the solidification conditions.