Papers by Author: Wen Zhu Shao

Abstract: CP-Nb-Cr/Cu-Cd electrical contact material, which contains 1.7wt.% of Cd, 0.5wt.% of CP (man-made diamond particles), 2.0 wt.% of Nb and 0.7 wt.% of Cr, was fabricated by powder metallurgy process. The contact material was installed in different ac contactors covering current range from 40 to 630A. While tested under AC-4 electrical load, the electrical contact material was worn out gradually by repeated electrical arc, and microstructure of the arc-affected surface layer changed dramatically. Mechanism of arc erosion process and contact resistance fluctuation was discussed by corresponding microstructure observation and chemical composition analysis of the surface layer. CP-Nb-Cr/Cu-Cd electrical contact material is capable to be used in 63~250A ac contactors under AC-4 working conditions.

Abstract: This paper presents the high temperature deformation behavior of 7075 aluminum alloy after T6 heat treatment by using electron tensile test machine with a temperature range of 230 - 440 °C, strain rate of 0.01 s-1, deformation of 50 % and 100 %, respectively. The morphology of the fractured surfaces, dislocation and change of sub-grains before and after the tensile test were investigated by using scanning electron microscope (SEM) and electron back scattering diffraction (EBSD) technique. The results show that the fracture mechanism of 7075 aluminum alloy was ductile rupture. The thermal deformation of the 7075 aluminum alloy showed the steady-state flow characteristics, and the flow stress decreased with the increase of deformation temperature. Orientation angle offset of the grain boundary decreased with the increasing of the deformation at 440 °C, indicating that the continuous dynamic recrystallization occurred inside the 7075 aluminum alloy.

Abstract: Combined extrusion experiment (including direct and indirect extrusion) at 440 for large amount of deformation was carried out with the solution treated AA7050 aluminum alloy. Qualitative description and quantitative characterization were conducted employing electron backscattered diffraction (EBSD) technique on the microstructure of typical regions with different filler contents. These characteristic regions were filled at the following stages: ahead of filling (AF), beginning of filling (BF), mid stage of filling (MF) and the end of filling (EF). EBSD results showed that recrystallization fraction during direct extrusion were 8.3%, 13.5%, 9.3% and 11.2%, for AF, BF, MF and EF, respectively. Recrystallization fraction during indirect extrusion were 15.5%, 9.1% 5.2% and 9.9%，for AF, BF, MF and EF, respectively. It shows that the mode and the amount of deformation played an important role in DRX. DRX grains were formed continuously during direct extrusion, while during indirect extrusion, fewer DRX grains generated, and only originally generated DRX grains grew larger gradually.

Abstract: Hot compression tests at different temperatures and strain rates were performed on superalloy 718 annealed at 1100 °C for 30 min as well as 950 °C for 30 min in order to investigate the effect of the original microstructure on the hot deformation behavior. The results show that the δ phase precipitated on the grain boundaries of superalloy 718 prior to hot deformation leads to a decrease of the flow stress, which can be attributed to the drop of the matrix strength. The activation energy (Q) for hot deformation of the fine-grained material with δ phase on the grain boundaries is determined to be 381 kJ/mol, which is much lower than that of the coarse-grained material without δ phase (472 kJ/mol). It is suggested that the initial δ phase precipitated on the grain boundaries of the annealed material affect the thermal activation process of hot deformation for superalloy 718, resulting in the decrease of activation energy.

Abstract: A ceramic thermocouple (CTC) was assembled using SiC as the cathode and MoSi2 as the anode for the temperature test of the aluminum electrolysis. The NiFe2O4 was sprayed on the surface of SiC cathode for increasing the corrosion resistance ability in the cryolite. The corrosion behavior of NiFe2O4 in the cryolite was investigated. The results of the corrosion tests show that the NiFe2O4 has a very low corrosion rate in the cryolite, and a corrosion layer was formed during the corrosion test. The mechanisms of the NiFe2O4 corrosion are reactive corrosion and particles flaking. The thermo emf of SiC-MoSi2 CTC was tested from 25 °C to 1300 °C in air. The results show that thermo emf has a linear relationship with the temperature dependence, and reached 350 mV at 1300 °C. The CTC which consists of SiC-MoSi2 prepared in the same batch has a preferable interchangeability. By holding temperature at 1300 °C, the deviation of thermo emf is 0.5 %, which is better than the criterion of industry thermocouple, 0.75 %. The CTC response time was also analyzed.

Abstract: The nucleation and development of dynamic recrystallization (DRX) in hot deformed superalloy Inconel 718 during uniaxial compression were investigated by optical microscopy and electron back-scattered diffraction (EBSD) technique. The results showed that the discontinuous dynamic recrystallization was the predominant DRX mechanism in this alloy. The variations of partial crystallographic orientations led to the individual nucleation inside the deformed grains, which implied the occurrence of local continuous dynamic recrystallization. The progressive subgrain rotation can be confirmed neither near the prior high angle grain boundaries nor within the original grains. It was found that, as the strain increased, the initial twin boundaries were gradually transformed to ordinary mobile high angle boundaries. Meanwhile, the new twin boundaries were formed inside the recrystallized grain necklaces. It was suggested that the characteristics of the twin boundaries evolution with increasing strain were associated with the transformation of initial twin boundaries as well as the generation of new ones, which resulted in the development of DRX.

Abstract: Magnetic stability of a Fe-Cr-Co permanent magnet material at high temperatures has been investigated. Changes of microstructures and hyperfine field distributions after high temperature holding were analyzed by transimission electron microscopy (TEM) and Mössbauer spectroscopy. The results of experiments indicate that the changes of magnetic properties of the Fe-Cr-Co alloy tend to decrease, and then gradually keeps a certain level with increasing the holding time at high temperature. TEM observations show that microstructures of the alloy do not change distinctly after high temperature holding. Hyperfine interaction study through Mössbauer spectroscopy finds that the composition difference between α1 and α2 phases increases, and the composition of the two phases fluctuates. This study attributes the change of magnetic properties to element redistribution of Cr atom rather than the change of microstructure.

Abstract: Dependence of microstructure upon transfer stability of the powder-metallurgy copperdiamond electrical contact material with Cr and Nb addition during type-test process is investigated by optical microscope and SEM observation. During making and breaking process, micro-cracks occurred along grain boundaries under electrical and mechanical forces. Addition of cadmium into the composite increases oxidizable capability of this material, and also leads to oxide accumulation along grain boundaries. These factors reduce the reliability of electrical contacts in practice. Arc erosion quantities during commutation operation processes relates with grain size of matrix and particle size of the second metallic phase. The optimal grain size is 20~50µm and 10~20 µm for niobium particles in these tests.