【作者】 顾萌；

【导师】 李建国；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2007， 硕士

【摘要】 Fe-Ga合金是近年来出现的新型磁致伸缩材料,主要应用于声纳、主动减振等领域。Fe-Ga合金具有价格低廉,温度适应性好,较好的延展性,且饱和磁化场很小的优势。但是Ga的熔点低,普通的制备方法会造成Ga元素的损失。因此开发制备Fe-Ga合金的新方法成为该领域的一个研究热点。本文采用熔融玻璃净化法及循环过热技术,研究Fe₈₃Ga₁₇合金深过冷行为,以及过冷度对合金组织性能及择优取向的影响,并采用超高温度梯度定向凝固技术制备具有择优取向的Fe₈₃Ga₁₇合金,和深过冷技术制备有所比较。采用熔融玻璃净化和循环过热相结合的方法,使得Fe-Ga合金获得287K的大过冷度。从小过冷到141K过冷度,平均晶粒尺寸随过冷度的增加而逐步减小,并且具有相对比较均匀的粒度分布。在过冷度为176K时,枝晶被拉长,且具有一定方向性。当过冷度达到216K时,定向枝晶被细化。当过冷度大于252K,出现了大的等轴晶,试样内部大等轴晶与小等轴晶共存。随着过冷度的增加,大等轴晶晶粒的面积分数逐步增加。当过冷度达到287K时,就只有大等轴晶晶粒存在。该合金的组织演化过程经历了两次细化和一次粗化,在过冷度小于252K时,形核率随着过冷度的增大而增大,这就导致Fe₈₃Ga₁₇合金晶粒尺度随过冷度增加而下降,并且相对比较均匀。随着过冷度增大,合金内部发生再结晶,导致大晶粒吞并小晶粒生长,最终发生完全粗化现象。在过冷度达到176K时,Fe-Ga合金熔体中生成了短程应变有序相,沿着[100]方向上的Ga原子团簇将会加强此方向上的磁致伸缩性能。研究了淬火和退火对Fe-Ga合金组织的影响。,因为有序DO3相降低材料的磁致伸缩性能,而淬火可以抑制DO3相的生成,并且淬火过程中部分b.c.c基相转变成了Modified-DO3相,该相结构中,沿[100]方向存在Ga原子对,很大程度上增强了磁致伸缩效应,因此,淬火可以明显提高合金的磁致伸缩性能。退火工艺的缓慢冷却过程有利于DO3相的形成,并消除了能增强磁致伸缩性能的Ga原子团簇,所以退火降低合金的磁致伸缩性能。本文采用超高温度梯度定向生长制备Fe₈₃Ga₁₇合金,实验温度梯度控制在800K/cm,生长速度为6μm/s和10μm/s。在生长速率为6μm/s的超高温梯度Fe₈₃Ga₁₇合金定向凝固试样出现两个区域,分别为定向生长过渡区和生长稳定区。在定向生长稳定区中,形成了平行于轴向方向的几个粗大定向排列的柱状晶,横向晶界几乎完全消失,其择优取向为[110]方向。生长速率为10μm/s的超高温梯度Fe₈₃Ga₁₇合金定向凝固试样的择优取向受到阻碍。采用晶体生长速率与固液界面形态关系解释了这一现象。为了获得高取向度晶体,在制备过程中要协调好温度梯度和生长速度,保证固液界面为上凸形。两种制备方法分别得到具有[100]和[110]择优取向的结构,其中深过冷技术为制备[100]择优取向大磁致伸缩材料提供了一条可行的途径。更多还原

【Abstract】 Giant magnetostrictive materials have been widely applied in spaceflight and military field as the element of acoustic sensors, actuators, and other magnetomechanical devices. Recently, Fe-Ga system which is virtue of low-filed magnetostriction and good mechanical properties such as good ductility and high strength, has attracted increasing attentions as magnetostrictive materials. In this study, by way of molten glass denucleating and superheating-cooling cycles, the undercooling behavior of the Fe₈₃Ga₁₇ alloys was studied. The Fe₈₃Ga₁₇ bars with preferred orientation were obtained using super-high temperature gradient directional solidification method.The Fe₈₃Ga₁₇ alloys were undercooled up to 287K through molten glass denucleating and superheating-cooling cycles. When the undercooling was less than 54K, the structures were composed of coarse dendrites. With increasing the undercooling up to 216K, the grains were refined. More particularly, the dendrites tended to possess a preferred orientation when the undercooling was in the range of 141-176K. When the undercooling exceeded 252K, abnormal large grains coexisting with fine equiaxed grains appeared. With a further increase of the undercooling up to 287K, only big equiaxed grains existed. The grain refinement with the increasing undercooling is due to the increased nucleation rate as the undercooling was less than 252K. When undercooling exceeded 252K, recrystallization occurred in the melt, giving rise to the formation of big grains. Local shot-range ordering of Ga atoms was formed in the Fe₈₃Ga₁₇ alloy when the undercooling exceeded 176K, and the presence of clusters of near-neighbor pairs of Ga atoms contributed to the large magnetostriction.The quenching process can improve the magnetostriction of Fe₈₃Ga₁₇ alloy by restricting the formation of DO3 phase. In the quenching process, small volume fraction of the b.c.c matrix turned to the Modified-DO3 phase which has pairs of Ga along [100]. On the other hand, in the annealing process, the development of long-rang DO3 structure was formed and the clusters of Ga atoms was disappeared, leading to a decrease in magnetostriction.Using super-high temperature gradient directional solidification method, the Fe₈₃Ga₁₇ alloy bars with [110] orientation were prepared. The experimental temperature gradient was 800K/cm. When the growth velocity was 6μm/s, several coarse columnar crystals along the longitudinal direction and even without transverse interfaces were formed in the growth stability area. The preferred orientation of directional solidified sample was interrupted as the growth velocity was 10μm/s. The phenomena were explained by the relationship between growth velocity and the solid-liquid interface shape. In order to prepare samples with preferred orientation, it is necessary to make the growth velocity consistent with the temperature gradient, and to make sure the solid-liquid interface convex.Both bulk undercooling and directional growth can obtain structures with preferred orientation, and most importantly, the undercooling experiment improves the texture of [100] orientation, and provides a possibility to prepare magnetostriction alloys with [100] preferred orientation.更多还原