Papers by Author: Yu Dong Zhang

Abstract: The recrystallization behaviors of cold rolled aluminum alloys in electric field up to 400kV/mm and the phase transformation processes of proeutectoid steels under magnetic field up to 14 Tesla have been experimentally examined. It has been found that both the electric field and the magnetic field have influence on the evolution of texture and microstructure characteristics. During the recrystallization annealing under the electric field of the cold-rolled 3104 aluminum alloy sheets, the electric field postpones the recovery and recrystallization progress. First principle calculation was performed to study the electric structures of aluminum atoms and vacancies. It shows that vacancies that are helpful for recovery are electrically negative. As the sample worked as anode during electric field annealing, it was covered with positive surface charges that attract the electronegative vacancies in the vicinity of the free surface and annihilate them. In this way, the recovery and then the recrystallization are postponed. The magnetic field applied changes the precipitation sequence of transitional carbides during low temperature tempering that makes the relatively high-temperature monoclinic χ-Fe5C2 carbide precipitated without following the usual precipitation sequence, i.e. by skipping the precipitation of the usual orthorhombic η-Fe2C carbide. To reveal the working mechanism of this phenomenon, first principle calculations were performed to study the formation energies of the two iron-carbide systems and their electronic and magnetic structures and properties. Calculation results show that η-Fe2C has lower formation energy, which is proved by the formation sequence observed during the usual low temperature tempering process. However, χ-Fe5C2 has the higher magnetic moment, which enhances the stability under the magnetic field through magnetization. Therefore, under the magnetic field its precipitation tendency is increased.

Abstract: Heavy deformation plus micro alloying could be an effective way to obtain ultrafine grain structure of metals. In the present work, an Al-Cu-Mg alloy was microalloyed with Zr to obtain homogeneous precipitates and then heavily deformed by conventional forging at high temperature. The possible refining processing routes were studied and the superplasticity behaviors of the alloy was investigated. Results show that the micro alloyed alloy can be stably refined to 3-5μm under conventional processing routes. The Al-3Zr precipitates act both as additional sites to enhance recrystallization nucleation rate and pins to impede grain growth to increase the thermal stability of the fine grain structure. However, as the Al3Zr precipitates remains along grain boundaries, the superplastic capability of the material is not high. At 430°C with 1×10-4S-1 strain rate, the elongation obtained was 260%.

Abstract: The new phase equilibrium of Fe-C diagram under magnetic field has been theoretically calculated. Results show that the magnetic field mainly shifts the γ⁄α+γ equilibrium line and the eutectoid point to the high carbon and high temperature sides. Based on this result, an experimental setup has been launched to investigate the effect of magnetic field on austenite decomposition in medium carbon and high carbon steels. The thermodynamic and kinetic effects of the high magnetic field on proeutectoid transformation at different cooling rates have been studied. It was found that for medium carbon steels, the magnetic field increases the amount of proeutectoid ferrite and accelerates the diffusional decomposition of austenite at medium and relatively fast cooling rates (10°C/min and 46°C/min). But there is no special grain growth along the field direction. The results led to a proposal of a new rapid annealing under a high magnetic field. However, when cooling is slow (2°C/min), the magnetic field shows a strong tendency to promote the proeutectoid ferrite grains to grow along the field direction through the magnetic dipolar interaction, which leads to the formation of an elongated grain structure. Moreover, the magnetic field also exhibits influence on the austenite decomposition in hypereutectoid steel by changing the amount of secondary cementite and lamellar spacing of pearlite.

Abstract: A 12-Tesla magnetic field was applied during the transformation from austenite to ferrite and then pearlite in a medium plain carbon steel at two different cooling rates. Results show that when cooling is slow, the magnetic field shows an effect of promoting proeutectoid ferrite grains to grow along the field direction that results in an elongated grain microstructure. However, when cooling is fast, the magnetic field mainly shows an effect of reducing the amount of low angle misorientations and increasing the amount of CSL boundaries. In addition, the magnetic field exhibits a slight enhancement of the <001> texture component in the direction that is perpendicular to the field direction (TFD).

Abstract: High magnetic fields were applied to the austenite to proeutectoid transformation and tempering process in a 42CrMo steel. The thermodynamic and kinetic effects of the high magnetic field on the austenite decomposition show that it can obviously increase the amount of the product ferrite and accelerate the transformation by enhancing the Gibbs free energy difference between the parent and product phases. Moreover, the magnetic field can considerably lower the amount of low angle misorientations of ferrite in pearlite colonies and obviously increase the frequency of S3-29 coincidence boundaries, especially S3 boundaries, of the ferrite. But it has no obvious effect on crystallographic orientation distribution. When the field is applied to the high temperature tempering process, it can effectively prevent the directional growth of cementite along martensite plate boundaries and twin boundaries by increasing both the cementite/ferrite interfacial energy and the magnetostrictive strain energy. Finally, particle-like cementite is obtained. The magnetic field also obviously retards the formation and growth of the ‘distortion-free’ regions of the matrix.

Abstract: High magnetic field is applied with the field direction parallel to the rolling direction during annealing of a cold rolled IF steel sheet. Results of X-ray ODF analysis show that, magnetic field annealing retards the normal recrystallization texture evolution for the IF steel sheet. It is worth noting that an abnormal increase of orientation intensity at {100}<110> is found after magnetic annealing for 25min at 650°C. When the magnetic field strength is increased from 10 Tesla to 14 Tesla, the a-fiber is further strengthened, especially the {100}<110> component. Combined with EBSD analysis results, it is considered that the magnetic field does not change the mechanism of recrystallization texture evolution for the IF steel sheet in the present case.

Abstract: The Weiss model is extended by substituting the molecular field coefficient lwith a short-range-ordering coefficient g valid around and above Tc. The temperature variations of susceptibility of fcc Fe are calculated with a band model. Based on the above models, the phase equilibrium diagram of Fe-C binary system under a magnetic field is calculated.

Abstract: The recrystallization texture, grain boundary character distribution (GBCD) and their influence on the stress corrosion cracking and intergranular corrosion of 2024 aluminum alloy were investigated. Results showed that the texture of Specimen A1 is characterized by the retained coldrolling texture; while Specimen A3 has strong recrystallized cube texture and high frequencies of CSL grain boundaries (especially S7), which shows high stress corrosion and intergranular corrosion resistance.

Abstract: The cold-rolled 3104 aluminum alloy sheets were annealed without and with an electric field. Results show that the electric field can greatly postpone the recovery and recrystallization processes, enhance the Cube texture component. The effect of the electric field lies in that it decreases the concentration of the electronegative vacancies by attracting them to the electropositive sample surface, thus reducing the stored energy for recovery and recrystallization.