【作者】 陈豫增；

【导师】 杨根仓；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2005， 硕士

【摘要】 本文以Fe-7.5 at.%Ni为研究对象,采用熔融玻璃净化并结合循环过热方法,系统研究了该合金凝固组织随过冷度的演化规律及深过冷熔体中亚稳相的析出行为,采用TEM和SEM手段观察了凝固组织中亚稳相枝晶核的形貌和分布。结合经典形核理论和瞬态形核理论,系统分析了过冷熔体中亚稳相和稳定相的竞争形核。采用BCT模型描述了过冷熔体中亚稳相和稳定相的生长行为,并探讨了过冷熔体中的竞争生长现象和亚稳相的演化机制,借助于TEM和XRD的分析手段探讨了合金的相演化规律,得出的主要结论如下: (1) 低过冷度下的组织细化是由初生枝晶在严重过热条件下的熔断引起:中等过冷度下的组织细化可归因为,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,导致初生枝晶在收缩应力的作用下发生破断,在随后的慢速凝固过程中由于晶体缺陷和变形能的存在,使组织发生再结晶从而形成具有平直晶界的细小等轴晶组织:大过冷度下凝固组织的粗化归因于γ相在较低的过冷度下形核和凝固后冷却过程中的晶粒合并。 (2) 通过对凝固组织的观察确定了亚稳相δ析出的临界过冷度△T=120K;发现亚稳枝晶核的数量随过冷度的增大而增多,亚稳相枝晶核多分布于晶粒内部少量沿晶界分布,晶粒内部的枝晶核呈椭圆状,晶界处的枝晶核呈长条状或三角状;枝晶核内部位错很少,且不存在亚结构,枝晶核与基体相存在着明显的相界面。 (3) 采用经典形核理论和瞬念形核理论对Fe-7.5at.%Ni合金过冷熔体中亚稳相和稳定相的竞争形核的计算与实验结果定性吻合,即当熔体过冷度大于某一临界过冷度时亚稳相将作为初生相从过冷熔体中析出,通过分析发现瞬态形核理论能更好的描述深过冷熔体中的竞争形核问题。 (4) 亚稳相的生长主要受到界面动力学过冷和热过冷的控制:在低过冷度和中等过冷度范围内过冷熔体内存在亚稳相和稳定相的竞争生长,稳定相会从竞争生长中胜出,最终成为生长相;大过冷度的条件下,熔体的凝固过程中亚稳相为初生相,稳定相作为次生相生长。 (5) 亚稳相枝品破断机制为枝晶过热重熔机制,大过冷下亚稳相的生长和演化过西北T业大学工学硕卜学位论文程为,形核,生长,过热重熔,熟化,最后被稳定相的生长包裹。(6)过冷Fe一7.sat.%Ni合金的相演化规律为,凝固组织中的Y相转变为马氏体,亚稳相6相的枝晶核在凝固结束后先转变为Y相,后由丫相转变为马氏体。关键词:Fe一Ni合金,过冷熔体,竞争形核,亚稳相,组织演化,相变更多还原

【Abstract】 Fe-7.5 at.% Ni alloys were undercooled with molten glass fluxing and cycling superheating methods. The solidification microstructure evolution with undercooling and solidification behaviors of metastable phase were investigated, systematicly. The dendrite core of metastable phase and its distributing in the solidification microstructures were observed by TEM and SEM techniques. Competitive nucleation in undercooled melts was analyzed with classic nucleation theory (CNT) and time-dependent theory (TDT), respectively. The growth of metastable phase and stable phase were described based on BCT model. The competitive growth between the metastable phase and stable phase were discussed and the evolution of metastable phase was elucidated. The phase transformation principle has been studied with TEM and XRD methods. Main conclusions are as follows:(1) The grains refining mechanism in low undercooling range is the dendrite remelting by the serious superheating. The grains refining in the medium undercooling range is due to the recrystallization induced by the high contraction stress, and in high undercooling range, the metastable phase δ solidifies firstly, thenY phase solidifies at a low undercooling; which makes grain size of Y phase increase, subsequently, the high interface energy leads to the mergence of the grains after solidification, so that the coarse grains form.(2) The critical undercooling (ΔT = 120K ) of metastable phase formation as primary phase was defined by the observation of solidification microstructures. The amount of metastable dendrite cores increase with the increasing of undercooling. Most of such dendrite cores are distributed in the grains with an elliptical morphology; some of them lie in the grain boundaries with triangular or lathy morphologies. There are few dislocations and substructures in the dendrite cores, and there are clear phase boundaries between such dendrite cores and the matrix.(2) The results of calculations based on CNT and TDT are accordance with the experimental results qualitatively, namely, the metastable phase will solidify asprimary phase from the undercooled melts when the undercooling of melts exceeds a critical value. By comparing the results of calculation of those two nucleation theories, we found that the TDT can describe the competitive nucleation in the undercooled Fe-7.5at.%Ni alloy melts better.(4) The growth of metastable phase is mainly controlled by interfacial kinetic undercooling and thermal undercooling. There exists the competitive growth in the low and medium undercooled melts, and the stable phase is growing phase, whereas, in the high undercooled melts, the metastable phase grows as the primary phase, and the stable phase grows as the secondary phase.(5) The fragmentation mechanism of metastable dendrite is the dendrite remelting due to the serious superheating in the melts, and the course evolution of metastable phase is nucleation, growth, dendrite remelting, ripening, and then enwrapped by the growth of stable phase;(6) The solid state phase transformation of the undercooled Fe-7.5at.%Ni alloy are that stable phase Y transform to martensite, metastable phase δ transform to Y , firstly, and then transform to martensite.更多还原