| 其他摘要 | Recently, much attention has been focused on the surface modification technologies of magnesium alloys. Among them the surface mechanical attrition treatment (SMAT) technique, owing to its ability to fabricate a nanostructural surface layer with excellent properties in many metals, has been receive intensive investigations. However, up to now, the clear scenery of microstructure evolution and surface properties of the nanostructural surface layer in magnesium alloys induced by SMAT, especially the formation mechanism of nanocrystallits during plastic deformation, wear behaviors and surface alloying behaviors in magnesium alloys, is still lacking. Therefore, understanding of these problems becomes more and more crucial.
In this work, microstructural characteristics at different depths from nanostructural surface layer in AZ91D magnesium alloy samples induced by SMAT were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM), respectively. Hardness and microstructure evolution of the surface layer were measured and the grain refinement mechanism was summarized. Wear and surface alloying behavior of the SMAT AZ91D magnesium alloy samples were analyzed too. The main results are followed:
Ⅰ.The microstructure evolution and grain refinement mechanism of the SMAT AZ91D magnesium alloy:
1. Equiaxed nanocrystallines with random crystallographic orientations were obtained in the surface layer of AZ91D magnesium alloy sample processed by means of SMAT, where average grain sizes is 32 nm. The maximum of hardness is up to 1.8 GPa in the SMAT AZ91D magnesium alloy sample due to the grain refinement effect in surface. With an increasing depth from the top surface, sizes of grains or cells increase. Owing to the gradient variation of microstructures, the SMAT surface layer can be subdivided into three sections along depth from the top surface, i.e., the nanocrystalline structure regime (0-100 m), submicro-crystalline structure regime (100-600 m) and deformed twining regime (600-1500 m).
2. The following elemental processes are involved in the grain refinement process in SMAT AZ91D magnesium alloy:
a) At the initial stage,deformation twinning dominates the plastic deformation and divides the coarse grains into finer twin matrix lammellae.
b) With an increasing strain and strain rate, large numbers of dislocation arrays form and divide the twin matrix lammellae into subgrains with small misorientations;
c) Due to the high strain and strain rate in the treated surface, surface nanocrystalline layer is formed via dynamic recrystallization of the deformed structure.
Ⅱ. Surface alloying behavior of SMAT AZ91D magnesium alloy:
A 100 m to 150 m thick Al-enriched alloyed layer was formed on the surface of an AZ91D alloy through diffusion alloying at temperature as low as 380C after SMAT. Large fraction of lamellar microstructure observed within the surface alloyed layer was confirmed to be Mg17Al12 precipitates in Mg solid solution matrix by using TEM, of which hardness is about 1.5 times that of matrix. The alloyed layer shows an increased wear resistance than matrix due to the strengthening effect of Mg17Al12 phase. It illustrates SMAT generated a large number of grain boundaries in the nanocrystalline layer, which enhanced appreciably atomic diffusion. In the solid diffusion alloying investigation, the SMAT samples show a similar alloying behavior with the coarse-grained one.
Ⅲ. Friction and wear behaviors of SMAT AZ91D magnesium alloy:
1. The nanocrystalline layer fabricated by means of SMAT in the Mg alloy shows a slightly lower friction coefficient and improved tribological properties compared with those of the coarse-grained sample. Analyzing to the worn surface and debris, we concluded that both the samples show a similar abrasion mechanism -- the mixture mechanism of cutting, plowing in debris abrasion and oxidation. The enhancement in wear resistance of the SMAT Mg alloy is associated with the increased hardness and strength induced by grain refinement.
2. With the elevation of sliding speed, the wear volume rates of the coarse-grained and the SMAT samples decrease and reach into the same level at high sliding speeds. TEM observations of the subsurface structure evolution revealed that at a velocity of 0.028 m/s, the SMAT layer remains almost unchanged during sliding while the grain size of the subsurface of the CG sample is reduced into nanometer scale due to the plastic deformation and the increment of local temperature developed during sliding. |
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