Characterization and properties of nanocrystalline surface layer in Mg alloy induced by surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline (NC) surface layer about 40 μm was synthesized on an AZ91D Mg alloy. The microstructural features of the surface layer produced by SMAT were systematically characterized by optical microscopy (OM) observations, X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations, and hardness measurement was also carried out in order to examine the hardness variation along the depth. Experimental results show that the microstructure is inhomogeneous along the depth. In the region from top surface to about 10 μm in depth, the grain size increases from about 50 nm to 100 nm. In the adjacent region of about 10–40 μm in depth, the grain size increases from about 100 nm to 400 nm. The grain refinement can be attributed to the activity of dislocation and occurrence of dynamic recrystallization. Moreover, emergence of stacking faults in the grain interior is a novel discovery. After the SMAT, the micro-hardness of the surface layer was enhanced significantly compared with that of the original sample.