摘要
铸态铁素体球墨铸铁不需要采用任何热处理工艺，因此具有生产周期短、节约能源等优点。合金元素是决定铸态球墨铸铁基体组织的重要因素，通过合金化的深入研究可以在铸态得到所要求的铁素体基体组织。近年来，围绕Si、Ni影响铸态球墨铸铁组织及性能的实验研究较多，但尚缺乏对其微观作用机理的研究。因此，本课题利用基于密度泛函理论的CASTEP程序，从电子层面研究了Si、Ni对铸态球墨铸铁基体组织、固溶强化及韧脆转变温度等方面的影响。在球墨铸铁复合材料中，碳纳米管能够细化基体组织的晶粒，提高材料的强韧性。但制备过程中碳纳米管与铁基间的润湿问题难以解决，因此为改善增强体与铸铁基体间的结合性构造出Si改性碳纳米管结构，为进一步提高球墨铸铁的力学性能提供了可能。
建立Si、Ni掺杂奥氏体及C在奥氏体、铁素体中扩散的原子模型，通过计算能量及电子结构分析Si、Ni影响奥氏体共析转变的电子机理。能带结构分析表明，Si掺杂奥氏体的能带在费米能级附近较为稀松且Si-Fe原子间的布居数值较大；而Ni掺杂奥氏体的能带向费米能级集中且Ni-Fe原子间的布居数较低。在C原子扩散方面，Si促进了C在掺杂铁素体、奥氏体中的扩散，而Ni抑制了C在掺杂体系中的扩散。因此，C的扩散能力是Si、Ni影响奥氏体共析转变的主要因素。
银行客户经理建立Si、Ni掺杂渗碳体的原子模型，对体系的能带结构及C原子的配位图进行分析。结果表明Si、Ni掺杂渗碳体的能带结构向费米能级集中，Fe-C原子间共价键作用减弱，Fe-Fe原子层间的金属键作用消失，
导致渗碳体形核不容易，抑制珠光体的形成。由于含Si渗碳体的稳定性很低，渗碳体向外排出的Si原子降低了C原子向奥氏体晶界偏聚的驱动力，这使得渗碳体的生长受阻。
通过计算含Si、Ni铁素体的弹性常数分析铁素体固溶强化的电子机理。研究结果表明，掺杂固溶体在[100]和[111]方向的抗变形能力减弱，而在[011]方向上的抗变形能力增强，这是因为含Si、Ni铁素体中原子间得失电子作用方向性明显，表现为体系的各向异性增大。态密度图表明，含Si铁素体的价电子在低能区形成较多，稳定性优于含Ni铁素体。Ni固溶于铁素体后p轨道的电子数增加明显，固溶体表现出较高的硬度。含Si、Ni铁素体的杨氏模量和剪切模量均有所增大，B/G值和泊松比相应减小，表现为材料的抗拉强度和硬度增大，韧性减小起到固溶强化作用。Si、Ni掺杂铁素体晶界后原子间共价键强度减弱，降低晶界的稳定性为脆性掺杂。
韧脆转变温度以下在铁素体球墨铸铁基体中，位错运动受到的阻力作用增大，难以发生塑性变形导致材料的脆化倾向增大，因此从位错运动角度分析Si、Ni对铁素体韧脆转变温度的影响。C、Si原子偏聚在完整晶体内的环境敏感镶嵌能高于偏聚在位错芯区的能量，故位错芯区能够形成能量低谷俘获C、Si原子，而Ni原子在位错芯区的偏聚能力较弱。Si在位错体系中原子的d轨道和p轨道的电子数增多，s轨道的电子数减少，这增加了位错体系的脆性；Ni在位错体系中Ni、Fe原子间无电荷转移情况，d轨道的失电子情况明显，整体呈现固溶软化态。C原子在含Si位错芯区的环境敏感镶嵌能大于C、Si单独在位错芯区的能量，故随温度降低将会有C单质或化合物析出恶化材料的韧性。由于Si、Ni对铁素体
位错表现出固溶强化和固溶软化的特性，将分别提高和降低铁素体的韧脆转变温度。
利用示波冲击试验机和透射电镜测定铁素体球墨铸铁在不同温度下的冲击韧性并观察断口处的位错。实验结果表明随着Si含量的增加铁素体球墨铸铁的冲击韧性明显降低，断口处的位错发生明显的缠结和钉扎现象，从而验证了从位错滑移角度解释Si降低铁素体球墨铸铁低温冲击韧性的电子机理。对铁素体球墨铸铁试样进行常温显微硬度测量及Si元素的X射线能谱分析，结果表明Si含量越高铁素体球墨铸铁的显微硬度越大，且Si不易在铁素体晶界处聚集。实验现象与计算得到的Si固溶强化铁素体的电子机理相符合。
为提高碳纳米管与铁基间的润湿性，建立Si改性碳纳米管吸附Fe原子模型。Si掺杂碳纳米管的Mulliken电荷分布图表明，电荷由Si原子转移到邻近的C原子，围绕Si 原子形成了一个活性中心，提高改性碳纳米管对Fe原子的吸附能力。布居数计算结果表明，Fe-Si原子间布居数和电荷密度较大成键特征显著，这有利于增强体与基体间结合性能的提高。对Si改性碳纳米管施加变形作用，Fe原子的吸附能对压缩变形更加敏感。
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关键词：球墨铸铁，Si、Ni掺杂，Si改性碳纳米管，第一性原理
Abstract
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Production of as-cast ferrite ductile iron requires no heat treatments and, therefore, has advantages such as short production cycle and energy conrvation. Alloying element is an important factor as it can determine the matrix of as-cast ductile iron. A deep study of alloying helps us to make qualified ferritic matrix in the as-cast condition. In recent years, a great number of experimental studies have been made to investigate the effects of Si and Ni on both structure and performance of as-cast ductile iron, but none were made to rearch their microscopic effects. Therefore, the prent study us the CASTEP, a program bad on DFT (density functional theory), to investigate, on the electronic level, the effects of Si and Ni on the matrix, the solution strengthening and the DBTT (ductile-brittle transition temperature) of as-cast ductile iron. For a ductile iron composite, CNTs (carbon nanotubes) can refine the matrix grains and thus strengthen the obdurability of the material. However, wettability of CNTs and the iron substrate is somewhat a difficulty during the preparation. Therefore, Si-modified CNTs is constructed to improve the bonding between the reinforcement and the substrate, which makes it possible to make further improvement of mechanical properties of the ductile iron.
Atomic models of Si- and Ni-doped austenite are built, in which, C atoms are diffusing in austenite and ferrite. The effects of Si and Ni on the electronic mechanism of austenite’s eutectoid transformati
on are analyzed through both energy calculation and electronic structure. As shown in the analysis of the energy band structure, the energy band of the Si-doped austenite near the Fermi level is spar and the population of Si-Fe atoms becomes larger, while the energy band of the Ni-doped austenite gets intensive towards the Fermi level and the population of Ni-Fe atoms becomes smaller. For the diffusion of C atoms, Si has promoted the diffusion both in the ferrite and the austenite, but Ni has suppresd the diffusion. Hence, the diffusion of C atoms to is a major factor whereby Si and Ni exert their effects on the eutectoid transformation of the austenite.
巧克力松饼Atomic models of Si- and Ni-doped cementite are built. The structure of energy band and the coordination of C atoms in the system are analyzed. The results show that: the energy bands of the Si- and Ni-doped cementite are intensive near the Fermi level; the role of Fe-C
covalent bonds becomes weak; and the function of Fe-Fe metallic bonds disappear, which suppress the formation of cementite and the growth of pearlite. Becau of its low stability, the Si-doped cementite releas Si atoms, which drives less C atoms to gregate at grain boundaries of the austenite and, as a result, blocks the growth of the cementite.
安全教育班会记录The electronic mechanisms of ferrite’s solution strengthening are analyzed by calculating the elastic
constants of the Si- and Ni-doped ferrites. The results suggest that the anti-deformation ability of the solid solution to becomes weaker along the [100] and the [111] directions but stronger along the [011] direction. This is becau atoms in the Si- and Ni-doped ferrites are receiving and losing electrons directionally, which is reflected in the increasing anisotropy of system. As shown in the DOS (density of states) map, the Si-doped ferrite contains more valence electrons in the low-energy zone and is more stable than the Ni-doped ferrite. In the solid solution of the Ni-ferrite, the number of the electrons on the p-orbit increas apparently and, therefore, the solid solution shows a higher hardness. Both the Young’s modulus and the shear modulus of Si- and Ni-doped ferrites becomes larger, while the values of B/G and Poisson's ratio are decread, which, accordingly, increas both the tensile strength and the hardness but decreas the ductility of the material. The strength of atomic covalent bonds in the Si- and Ni-doped ferrites becomes weaker and the stability of the grain boundaries becomes lower. So, this is a kind of brittle dope.
In the matrix of the ferrite ductile iron, the dislocation motion is increasingly blocked under the DBTT. So, the plastic deformation is difficult to occur, and the material tends to become more brittle. For this reason, the effects of Si and Ni on ferrite’s DBTT are analyzed from the perspective of the dislocation motion. The environment-nsitive embedding energy of C and Si atoms gregated in a
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complete grain is higher than that in a dislocation core. As a result, a low energy zone will be formed in the dislocation core to capture C and Si atoms. But the ability of Ni atoms to gregate in the dislocation core is relatively weak. As for Si atoms in the dislocation system, there are more electrons on both the d-orbit and the p-orbit, but less electrons on the s-orbit, which makes the dislocation system more brittle. As for Ni atoms in the dislocation system, there is no electron transfer between Ni and Fe atoms and more electrons are lost from the d-orbit. So , the solid solution prents a softening trend on the whole level. The environment-nsitive embedding energy of C atoms in the dislocation core that contains Si atoms is higher than that of C and Si atoms alone in the dislocation core.
Conquently, C atoms or some compounds will parate out with decreasing the temperature, which deteriorates the material’s ductility. In the dislocation system, Si atoms have strengthened the solid solution while Ni atoms have softened the solid solution. Therefore, Si atoms will rai the DBTT of the ferrite ductile iron while Ni atoms will lower it.
An oscillometric impact tester and a TEM (Transmission Electron Microscope) are ud to detect the ductility of the ferrite ductile iron under different temperature as well as to obrve the dislocation at the fracture. The test results show that the ductility of the ferrite ductile iron decreas apparently wit
h increasing the content of Si atoms and there are entanglement and pinning phenomena in the dislocation at the fracture, which, from the perspective of dislocation slip, proves the electronic mechanism that Si atoms can bring down the ductility of the ferrite ductile iron under low temperatures. In addition, the microhardness of a specimen of the ferrite ductile iron is tested and the x-ray energy spectra of Si atoms is analyzed. The results show that the microhardness of the ferrite ductile iron becomes higher with increasing the Si content and Si atoms are not easy to aggregate at ferrite grain boundaries. For the electronic mechanism of Si atoms to strengthen the solid solution of the ferrite, the test results match well with the calculation results.
For the purpo of the wettability of both the CNTs and the Fe substrate, a model of the Si-modified CNTs adsorbing Fe atoms is built. As shown in the map of the Mulliken charges distributed in the Si-modified CNTs, the charges transfer from the Si atom to adjacent C atoms, forming an active center around the Si atom, which improves the ability of the modified CNTs to adsorb Fe atoms. The result of the population calculation suggests that: both the population of Fe-Si atoms and the electric density become larger, showing a great bonding ability. This will make the reinforcement and the substrate bonded more tightly. Applying a force to deform the Si-modified CNTs, it shows that energy to adsorb Fe atoms is more nsitive to the compression deformation.
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Key words：Ductile iron, Si- and Ni-doping, Si-modified CNTs, The first principle
