Investigation Of Effect Mechanism Of Si And Ni In As-cast Ferrite Ductile Iron By Electronic Theory

Production of as-cast ferrite ductile iron requires no heat treatments and,therefore,has advantages such as short production cycle and energy conservation.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 research their microscopic effects.Therefore,the present study uses the CASTEP,a program based 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 transformation 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 sparse 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 suppressed 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.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 suppresses the formation of cementite and the growth of pearlite.Because of its low stability,the Si-doped cementite releases Si atoms,which drives less C atoms to segregate at grain boundaries of the austenite and,as a result,blocks the growth of the cementite.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 because 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 increases 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 decreased,which,accordingly,increases both the tensile strength and the hardness but decreases 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-sensitive embedding energy of C and Si atoms segregated in a 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 segregate 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 presents a softening trend on the whole level.The environment-sensitive 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.Consequently,C atoms or some compounds will separate 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 raise the DBTT of the ferrite ductile iron while Ni atoms will lower it.An oscillometric impact tester and a TEM(Transmission Electron Microscope)are used to detect the ductility of the ferrite ductile iron under different temperature as well as to observe the dislocation at the fracture.The test results show that the ductility of the ferrite ductile iron decreases apparently with 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 purpose 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 sensitive to the compression deformation.