【作者】 贺谦；

【导师】 刘林；

【作者基本信息】 西北工业大学 ， 材料物理化学， 2005， 硕士

【摘要】 包晶凝固是十分重要的相变过程,多年来,包晶凝固的实验研究还主要集中在少数几种属于非小晶面长大的包晶合金,如Fe-Ni、Ti-Al、Pb-Bi及Zn基等合金;而更多的包晶合金相图上反应生成的是具有固定成分的金属间化合物的包晶相,如磁性材料NdFeB中的Nd₂Fe₁₄B相、高温超导材料Y-Ba-Cu-O中的123相等,它们都属于小平面相,目前人们对此类包晶合金的研究才刚刚开始。因此,对NdFeB合金凝固组织进行研究对于丰富包晶凝固理论以及改善合金性能有着十分重要的意义。 本文采用LMC定向凝固法对亚包晶成分(Nd_11.18Fe_84.46B_4.36)、包晶成分(Nd_12.16Fe_83.01B_4.83)和过包晶成分(Nd_13.29Fe_81.99B_4.72),三种成分（at%）的NdFeB合金的凝固组织进行分析研究。 研究结果表明:亚包晶成分NdFeB合金的最终凝固组织由α-Fe枝晶相、包晶Nd₂Fe₁₄B相组成,这与平衡凝固下的组织构成相同;包晶和过包晶成分合会的最终凝固组织均由α-Fe枝晶相、包晶Nd₂Fe₁₄B相和富Nd相组成,这与平衡凝固下的组织构成有很大的区别。随着抽拉速率的提高α-Fe相的一次枝晶间距呈减小的趋势。在同一种抽拉速率下,α-Fe相的含量按照亚包晶、包晶和过包晶的顺序递减,Nd₂Fe₁₄B相的含量则呈现出递增的趋势;随着抽拉速度的提高,三种成分凝固组织中的α-Fe相的体积百分比都表现出了先减后增的趋势,T₁相的体积百分比都表现出了先增后减的趋势。 利用激光表面快速熔凝技术对(Nd_13.29Fe_81.99B₄.72)过包晶成分（at%）NdFeB合金的凝固组织进行了分析研究,得到以下结果: 在熔池底部均出现了铸态残留的α-Fe枝晶,其一次枝晶间距处于10⁰μm～10¹μm的数量级。由于冷却速率较快,T₁相作为初生相直接从液相析出,并以枝晶组织形貌生长,在熔池底部有较强的定向性,在熔池的中上部只存在局部定向,其一次枝晶间距也存在明显差异。在熔池的顶部,出现了细小的枝晶组织其枝晶间距为10⁰μm的数量级。更多还原

【Abstract】 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.更多还原