| 其他摘要 | Large amount of researches have been focused on nanostructured materials during the past decades because of their unique structures and properties, among them severe plastic deformation induced nanostructure is one of the interests. A nanostructured layer can be fabricated on the surface of metallic materials by surface severe plastic deformation, which incorporates the surface modification technique with nanomaterials.
In the present work, a new surface severe plastic deformation method, Supersonic Particles Bombarding (SSPB), was used to have the treatment on 40Cr steel that is widely applied in industry. The principle of producing supersonic particles was analyzed, the processing parameters were explored, both the surface and cross sectional microstructures of 40Cr steel treated by SSPB were characterized by means of X-Ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), etc., and the micro-hardness, the wear resistance and the thermal stability of 40Cr steel after SSPB treatment were also evaluated. The main conclusions were obtained as follows.
Analysis on principle of Supersonic Particles Bombarding technique and optimization of processing parameters
The kinetics of dual phase fluid formed by gas and particles was analyzed theoretically, and the particle velocities were calculated, which was compared with the measured velocities. It was indicated by both calculation and measurement that the particle velocities were supersonic. The optimized processing parameters were: the glass particles with size of 0.4-0.6mm, the gas pressure of 1.5 MPa and the treatment time of 5 min per area.
Microstructure characterization and deformation analysis after SSPB treatment
1. Equiaxed nanocrystallines with random crystallographic orientation were obtained on the top surface of SSPB treated 40Cr steel. The surface layer was composed of nanostructure area、deformed area and matrix, and the depth of nanostructure and deformed area was about 100μm.
2. Geometrically necessary boundaries (GNB) and incidental dislocation boundaries (IDB) were formed in the deformed ferrites at the distance of 50-60μm to the surface. The original coarse ferrites were divided into different sizes finer structures, and most of the GNB were high angle boundaries, while the IDB low angle boundaries.
3. The deformation morphology of pearlite at the distance of 50-60μm to surface was related with orientation of the cementite plates to the surface. When the plates were parallel with or perpendicular to the surface, they could be curved in the deformation, and there were lots of high angle misorientations in both deformation induced dislocation boundaries in the ferrites and the bilateral ferrites of cementites. When the plates were at the angle of 45° to the surface, they could not be deformed, and the deformation induced dislocation boundaries in the ferrites were low angle misorientations, while there was some high angle misorientations between the bilateral ferrites of cementites.
4. The slip plane of ferrites in the pearlite was at the angle of 45° to the surface. When the cementite was parallel with or perpendicular to the surface, the dislocations would interact with the cementite bar and the shear zone would form after the dislocations moving a short distance along the slip plane. When the cementite was at the angle of 45° to the surface, the slip plane of ferrites was parallel to the cementite, and the dislocations would interact with the cementite bar after the dislocations moving a long distance along the slip plane, which would result in difficulty for deformation of the cementite.
Mechanical properties and thermal stability of SSPB treated 40Cr steel
1. Micro-hardness at the top surface was twice as hard as the matrix of SSPB treated 40Cr steel. A compressive stress layer with depth of about 400μm was formed on the treated 40Cr steel, and the maximal compressive stress was 400MPa. The compressive stress decreased with increase of the distance to the surface, which was changed to the tensile stress at the distance of 400μm to the surface. Under dry sliding、LP lubricating and LP+ZDDP lubricating conditions, the wear rate of polished SSPB 40Cr steel was the lowest and that of only SSPB treated 40Cr steel was the highest.
2. Nanostructure kept stable when the SSPB treated sample was annealed at 600℃, and the difference of nanostructure and matrix microstructure could be observed. Even if annealed at 750℃, the difference in structure still could not be distinguished. The recrystallization of the deformed sample occurred at 800℃. The micro-hardness of the top surface decreased slightly when annealed at 400℃, decreased remarkably at 500℃, and reached the half at 700℃. |
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