过冷Fe-Ni合金熔体凝固组织和凝固行为研究

【作者】 孙向涛；

【导师】 郝维新；

【作者基本信息】 太原科技大学 ， 材料加工工程， 2009， 硕士

【摘要】 本文采用熔融玻璃并结合循环过热的方法,使Fe-15at.%Ni、Fe-22.5at.%Ni合金分别获得了300K和320K的过冷度,系统研究了这些合金凝固组织随过冷度的变化规律以及深过冷熔体中亚稳相的析出行为。采用BCT模型分析了过冷熔体中晶体的生长方式以及凝固组织的演化规律,借助经典形核理论和瞬态形核理论详细讨论了过冷Fe-Ni合金的合金中亚稳相δ与稳定相γ竞争形核的热力学及动力学机制,提出了过冷Fe-Ni合金中形成亚稳相的临界条件。采用XRD和TEM技术分析了合金相结构,借助于VSM研究了合金的软磁性能。结果表明:1.Fe-Ni合金在所能达到的过冷度范围先后呈现出粗大树枝晶、第一类粒状晶、细小树枝晶和第二类粒状晶,发生了两次粒化现象,即由粗大树枝晶向第一粒状晶转变和细小树枝晶向第二类粒状晶转变的过程。2.通过对合金凝固组织演化规律分析表明:凝固组织第一类粒化机制为枝晶熔断机制,第二类粒化机制为枝晶碎断+再结晶机制,即由于固液界面前沿液相不能及时补充凝固收缩造成的体积差,使初生枝晶在收缩应力下发生碎断,由于晶体缺陷和变形能存在,组织发生再结晶从而形成具有平直晶界的等轴晶组织。3.通过对组织观察,发现过合金冷度超过某一临界过冷度时有亚稳相枝晶核的析出,并且亚稳枝晶核的数量随过冷度的增加而增多,亚稳相枝晶核多分布于晶粒内部少量沿晶界分布,亚稳相析出的临界过冷度Fe-15at.%Ni合金为110K,Fe-22.at.%Ni合金为185K。4.采用经典形核理论和瞬态形核理论分析了过冷Fe-Ni合金熔体中亚稳相δ和稳定相γ竞争形核行为,当熔体过冷度大于某一临界过冷度时亚稳相将做为初生相优先形核,理论分析计算与试验结果定性吻合。5.过冷Fe-Ni合金亚稳相和稳定相在冷却过程中相结构发生了变化,即凝固组织中γ相转变为马氏体,亚稳相δ相一部分转变为α铁素体,其余部分先转变为γ相,然后γ相转变为马氏体。6.对Fe-Ni合金的软磁性能测定分析表明,采用深过冷快速凝固技术有利于提高软磁合金的饱和磁感应强度Bs和降低剩余磁感应强度Br,这为制备高性能软磁材料提供了新的方法和途径。更多还原

【Abstract】 Fe-15at.%Ni and Fe-22.5at.%Ni alloy melts were bulk undercooled up to 300K and 320K respectively by combining the glass fluxing technique with superheating-cooling cycles. The solidification microstructure evolution with undercooling and solidification behaviors of metastable phase were investigated systematically. The growth pattern of crystal and evolution behavior of Solidification Structure were described based BCD model. Based on the classical nucleation theory and the time-dependent nucleation theory, the thermodynamics and the kinetics for the competitive nucleation of the metastable 8 and the stable phases y have been calculated, the critical condition that the metastable phase appeared in Fe-Ni alloy was brought forward. The Phases structure and soft magnetic Properties were studied by using XRD、TEM and VSM, respectively. The main research work and conclusions are Presented as follows:1. The solidification structures with coarser dendrites, the first granular grains, finer dendrites and the second granular grains were appeared orderly in Fe-Ni alloy. The granulations of solidification microstructure would take place twice at a large Undercooling range, where the first is that the coarse dendrite turned into the first granular grain under the smaller undercoolings and the second is that the finer dendrite turned into the second granular grain under the higher undercoolings.2. The analysis of microstructure evolution in Fe-Ni alloy showed that the first granulation mechanism is mainly controlled by dendrite re-melting within the smaller undercooling, and the second by dendrite breaking-up and recrystallizing within the larger undercooling. That is Primary dendrite broken under theshrinkage stress because the forefront of solid-liquid phase interface can not add the size difference by the Solidification shrinkage in time, solidification structure occurred recrystallization and form Equiaxed grains with Straight grain boundaries due to Crystal defects and deformation energy. 3. The metastable dendrite cores were appeared in melts when the melts temperature exceeded the critical undercooling by the observation of solidification microstructures, and the amount of metastable dendrite cores increase with the increasing of undercooling. Most of such dendrite cores are distributed in the grains, while some of them lie in the grain boundaries. The critical undercooling of metastable in the Fe-15at.%Ni alloy is 110K and the Fe-22.5at.%Ni alloy is 180K.4. The competitive nucleation behavior in undercooled melts was analyzed with classic nucleation theory and time-dependent theory, respectively, when the melts temperature is greater than the critical undercooling, the metastable phase will nucleate preferential and solidify as primary phase from the undercooled melts. The results of calculations based the theories are accordance with the experimental results qualitatively.5. The structures of metastable phase and stable phase changes in the cooling process of melt. The stable phaseγtransform to martensite, Part of the metastable phase 5 transform to ferrite, the rest transform toγ, firstly, and then transform to martensite.6. The measures of soft magnetic properties of Fe-Ni alloy showed that it is propitious to for high undercooling and rapid solidification to enhance saturation induction density B_s and reduce residual magnetic flux density B_r, which provided new ways and approachs for the preparation of high-performance soft magnetic materials.更多还原