| Peritectic solidification is one of very important phase transformations. Recently, many aspects of peritectic solidification have been investigated mainly on alloys which grow in non-small plane style, such as Fe-Ni, Ti-Al, Pb-Bi and Zn-based alloys. But phases formed more frequently in a peritectic reaction are intermetallic ones, which have faceted plane, such as Nd2Fe14B phase of NdFeB alloys and 123 phase of Y-Ba-Cu-0 alloys. A study of solidification structure for NdFeB alloys is helpful to understand peritectic solidification theory and to improve the property of materials.In this thesis, the solidification microstructures of hypoperitectic composition ( Nd11.18Fe84.46B4.36 ) stoichiometic ( Nd12.16Fe83.01B4.83 ) and hyperperitectic composition(Nd13.29Fe81.99B4.72)NdFeB alloys (at%) are researched at the withdraw velocity ranging from 5μm/s to 500μm/s under directional solidification.The results indicate that the final microstructures in the composition of hypoperitectic alloys are primary α-Fe phase, Nd2Fe14B phase. For stoichiometic and hyperperitectic composition alloys, they are a-Fe phase, Nd2Fe14B phase and Nd-rich phase, respectively. With increasing withdraw velocity, primary dendrite arm spacing of primary a-Fe phase decreases, the volume fraction of primary a-Fe phase decreases and then increases, Nd2Fe14B phase increases and then decreases.The solidification microstructure of hyperperitectic composition (Nd13.29Feg1.99B4.72)NdFeB alloys(at%)are also researched at the scan rates ranging from 1500mm/min to 6500mm/min by laser surface remelting.The results show that there are some remaining a-Fe dendrites at the bottom of molten pool, their primary dendrite arm spacings range from 10~0μm to 10~1μm. Nd2Fe14B phase grow in dendrite as primary phase because of high cooling rate, they grow directionaily at the bottom of molten pool, with increasing distance from bottom, the directional growing are partial. There are fine dendrites at the top of the molten pool, their primary dendrite arm spacing range is 10~0μm. |
