Papers by Author: Zhao Xia Chen

Abstract: This study reports the effect of aging duration on the super-elastic response of Fe-30%Ni-18%Co-10.5%Al-2%Nb-0.15%B (at.%) poly-crystals in compression. The aging temperature was 600°C and the aging durations were 20h, 45h, 60h and 72h, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were used in the work. The results show that with prolonging the aging duration, the super-elastic strain rises firstly and then descends. The super-elastic strain reaches the maximum 10.5% when the aging duration is 60h. The crisis stress for stress-induced martensite (σ_M) has no obvious changes, being about 250MPa when the aging duration is between 20h and 60h. But σ_M increases markedly when the aging duration prolongs to 72h. The hardness of the specimens changes in the same way as the superelastic strain, and reaches the maximum of 497HV₁₀ when the aging duration is 60h. During the aging process, two factors react. One is the decomposing and reducing in size of the undissolved phase (σ). The other is the formation of the precipitation phase (γ'). Nb can dissolve into the matrix phase (γ) adequately and promote the formation of γ'. The combination of the two factors improves the strength and superelasticity of the specimens till the over-aging arises corresponding to the 72h aging duration.

Abstract: The ultrasonic impact treatment on U70 raill steel was carried out under different technology by using HJ-II ultrasonic impact machine. The electric current was 1.5 A, the amplitude was 24 µm and the impact frequency was 10000 HZ during the treatment, the peening duration was 60 min. The influence of impact on microstructure was observed with the scanning electron microscope of 6360LA type and high resolution transmission electron microscope of JEM-2100 type, and the hardness of impacted surface was also researched before and after impact treatment. The wear resistance was tested by using M-2000 type wearing machine. The experimental results indicate that the surface strength of rail steel can be improved evidently by using ultrasonic impact treatment. The surface microstructure of the rail steel could be refined to nanoscale and the hardness and wear resistance of impact layer were significantly improved through the ultrasonic impact technology. Compared to the specimen without treatment, its surface hardness was increased 26.4%, and the wear resistance was increased 40.63%. The depth of plastic deformation zones of the specimen with 60 min treatment is nearly 100µm. In the plastic deformation zone, ferrite is refined to nanoscale grains so that the matrix is strengthened.

Abstract: The surface of U70 rails was treated by ultrasonic peening method. Keeping the electric current 1.5 A, the amplitude 24 µm and the frequency 10000 HZ during the treatment, the peening duration was 10 min, 20 min, 40 min and 60 min respectively. The wear property, wear appearance, and structural appearance of specimens have been studied by M-2000 wear tester and 6360LA scanning electron microscope. Results show that ultrasonic peening treatment can significantly improve the wear resistance of U70 rails, and the longer the peening duration, the stronger the wear resistance. The wear mass loss of the specimen with 60 min treatment is only 27 % of the one untreated. The reason is that ultrasonic peening can form plastic deformation zones. The depth of plastic deformation zones increases with the peening duration. The depth of plastic deformation zones of the specimen with 60 min treatment is nearly 100 µm. Longer peening duration means severer plastic deformation. In the plastic deformation zone, ferrite is refined to smaller grains so that the matrix is strengthened.