Papers by Author: Lei Wang

Abstract: The phase equilibria of new-type Co-based superalloys which include the system of Co-Al-W were studied by CALPHAD method. It was shown that γ+γ' two-phase regions were existed in the calculated phase diagrams. The compositions of new-type Co-based superalloys which can obtain γ+γ' microstructures were predicted due to the calculated phase diagram. The mole fractions of the constituent phases of Co-Al-W-Ni-Cr alloys were calculated. The temperature of γ' phase began to precipitate at about 1050°C.

Abstract: The dynamic strain aging (DSA) behavior of SA508-III steel was evaluated through tensile tests with different strain rates from 10^-4 to 10^-1s^-1 at 350°C. The OM, SEM and TEM were carried out to observe the microstructures and fracture morphologies of the steel. The results show that the serrated flows appear in the stress-strain curves when the strain rate is between 10^-3~10^-2s^-1, indicating that DSA occurs. Under the strain rate range, the tensile strength increases and the elongation and the reduction of area decrease. However, the fracture surface of the steel after tensile tests is still ductile. DSA in SA508-III steel at the strain rates from10^-3 to 10^-2s^-1 is mainly caused by the interaction between the internal solute atoms and dislocations, which leads to the dislocations multiplication and the formation of sub-grain boundaries and dislocation cell structure.

Abstract: Fatigue crack propagation behaviors coupled with the microstructure evolution of a hardworking Ni-based superalloy after long-time aging at 1023K were studied, to provide fundamental data for the fatigue life prediction of the superalloy. The results show that the microstructures remarkably change and the fatigue crack propagation resistance decreases with the aging time prolongs. It is found that the precipitation and the growth of topologically close packed (TCP) phases as well as the coarsening of γ' phase and carbides on grain boundaries can significantly affect the fatigue crack growth rate. On one hand, coarsened γ' phase and carbides at grain boundaries block dislocation movements near the crack tip, thus the fatigue crack propagation is hindered in near-threshold region and Paris region. On the other hand, the stress concentration accumulates continually with carbides precipitation increases, so that the grain boundaries become the main fatigue crack propagation rout. As well as, the effect of the TCP phases on the fatigue crack propagation behavior ascribes to the size and the distribution of TCP phases. Very small quantity of TCP phases contribute to pinning dislocation and enlarging fatigue crack propagation absorption energy, but high quantity of TCP phases with short rod shape changed to the needle, which gradually precipitate uniformly within the grain after 1000h besides on grain boundaries in the earlier aging, leads to much higher stress concentration degree. Those discussed above are the most important reasons why the fatigue crack growth rate increases after long-time aging.

Abstract: The microstructures and fatigue properties of the TNTZ added with Y or Y₂O₃ have been investigated. The results indicate that TNTZ added with Y or Y₂O₃ are all found to be composed of β phase and the small amount of Y₂O₃. The grain size of TNTZ added with Y or Y₂O₃ is smaller than that of TNTZ. The Young’s modulus of TNTZ added with Y or Y₂O₃ are maintained at a low level, and Young’s modulus of TNTZ added with Y is smaller than that of TNTZ added with Y₂O₃. The mechanical properties are both improved by adding Y or Y₂O₃, while the tensile strength of TNTZ added with Y₂O₃ is slightly higher than that added with Y. The high cycle fatigue limit of the alloys added with Y or Y₂O₃ are similar, while the low cycle fatigue strength of TNTZ added with Y is higher than that added with Y₂O₃. The improvement in fatigue properties ascribes to the microstructure refinement and the pining effect of Y₂O₃ particles. On the other hand, Y elements form Y₂O₃ with the Oxygen elements in the matrix, thus lead to the weakening of the Oxygen solution effect.

Abstract: The effects of strain rate on the tensile deformation behavior of quenching and partitioning (Q&P) steel applied to the auto industry were investigated. The results indicated that the strength of Q&P steel raised with increasing strain rate. The variation of elongation which presented the trend of declining (10^-4 s^-1~10¹ s^-1) followed by rising to the peak (8×10¹ s^-1) then falling again (10² s^-1~10³ s^-1), is mainly caused by the transformation from retained austenite to martensite, namely the TRIP effect.

Abstract: The phase equilibrium and the precipitation behavior of the γ' phase in the Co-Al-W-Ni-Cr superalloys were studied by thermodynamic method. The phase equilibrium of the Co-Al-W-15Ni-5Cr alloy at 900 was calculated, and it was found that there were a γ' single-phase region and a γ+γ' two-phase region. The effects of Al/W ratio on the precipitation of the γ' phase were studied, and it was clear that higher mole fraction of the γ' phase can be obtained with the Al/W ratio of 2.5. The stability of the γ' phase can be improved by adding of Ni, as well as the servicing temperature of the Co-Al-W based superalloys. The results obtained in this work can be used for the developing of the new-type Co-based superalloys.

Abstract: The pulse current was performed on the IN718 alloy during tensile test at room temperature and elevated temperature in the present study. The effect of pulse current on the deformation behavior of the alloy and the mechanisms were investigated. The results show that the deformation resistance decreases and the elongation increases significantly of IN718 alloy during tensile test at room temperature and 1073 K with the increase of pulse current. And the effect of pulse current on the strength and plasticity of the alloy increases as pulse current energy increases. The electroplastic effect of pulse current can promote the dislocation motion, and thus decrease the deformation resistance of the alloy during the tensile deformation. The pulse current reduces the starting temperature of recrystallization, promotes the recrystalllization nucleation, and thus makes it possible that the dynamic recrystallization would occur at lower temperature than that in the conventional temperature field. This is the main reason for the decrease of deformation resistance and the increase of plastic deformation ability of IN718 alloy under pulse current.

Abstract: The microstructures and mechanical properties of the Ni-22Cr-14W-2Mo superalloys with various La contents have been investigated. The results indicate that the grain diameters of the alloys with different La additions are similar to that of the alloy without La. It has also been found that the Vickers Hardness of the alloys increases with the increasing of the La content form 0.000% to 0.087%. The tensile properties of the alloys can be improved by La additions, whereas the σ_0.2 proof strength and the elongations of the alloys first increase and then decrease with the increasing of the La content form 0.000% to 0.087%. The alloy with 0.026% La shows the best tensile properties. This result ascribes to the different types of the La-riched phases and the different sizes of them. The La-Ni intermetallic phases with large size precipitating in the alloys have deleterious effects on the improvement of the mechanical properties, if the La content is too high.

Abstract: To reduce fuel consumption and greenhouse gas emission, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications, which would create potential safety and reliability issues under dynamic loading, the mechanical behavior of DP steel considering the strain rate must be examined. In the present study, the dynamic tensile behavior of DP600 steel sheets was investigated using a high-speed tensile testing machine at various strain rates. And the quasi-static tensile testing was also conducted on the steel to understand the effect of the strain rate on the tensile property. The fracture mechanisms of the steel were also analyzed. The results show that the mechanical properties of DP600 steel are noticeably influenced by the strain rates. As the strain rate increases, the strength of the steel increases and the obvious yield phenomenon can be observed when the strain rate is above 0.01 s^-1. The fracture elongation of DP600 steels decreases with increasing strain rate from 0.001 to 1 s^-1, then increases up to the strain rate of 100 s^-1 and reaches the lowest value at the strain rate of 1000 s^-1. DP600 steel sheet exhibit typical ductile fracture characteristics with dimples morphology of the facture surface when tensile deformed at various strain rates.

Abstract: Effect of electric field treatment with different intensities on vacancy of Inconel 718 aged at 1073 K was investigated. The positron annihilation lifetime (PAL) and corresponding concentration of vacancies were measured by positron annihilation technique. It was found that atomic vibration at lattice site can be promoted by an external electric field performed on Inconel 718 superalloy during aged at elevated temperature. Some atoms owing the higher vacancy formation energy or larger radius will jump out from the normal lattice sites and form defects, which is leading to the increase of PAL of monovacancy. Under continuous effect on electric field, monovacancies will transform to three-dimensional vacancies of lower energy and vacancy clusters. Moreover, the vacancy concentration of Inconel 718 superalloy can be improved evidently by electric field treatment, and the average PAL of vacancies can be up to 6.3% when the electric field intensity is up to 8 kV/cm, which is very useful for controlling the microstructures evolution and properties of the superalloy, such as solute redistribution and precipitations.