Surface nanocrystallization and its properties of a rare earth magnesium alloy induced by HVOF–SMB

The nanocrystalline microstructure in the surface of a rare earth magnesium alloy induced by a new process named HVOF–SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding) has been characterized by means of X-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM). The effects of HVOF–SMB on mechanical properties and corrosion resistance of the magnesium alloy have been investigated by microhardness distribution and polarization curves, and the cross-sectional observation of the treated Mg alloy sample has been characterized by optical microscopy (OM). Results showed that HVOF–SMB can induce surface nanocrystallization with a grain size level less than 20 nm at topmost surface layer, and the microhardness at the top surface layer increases up to about 170 HV0.025 which is twice more than that of the substrate. The grain size increases as well as the microhardness decreases gradually with the depth varying from surface to substrate. The potentiodynamic polarization curves indicate that the HVOF–SMB treated surfaces show lower corrosion resistance in comparison with the untreated magnesium alloy in 3.5% NaCl solution. The underlying mechanism of the electrochemical properties on the nanocrystalline Mg-based alloy’s surface induced by severe plastic deformation in corrosion resistance is discussed.

Research highlights
► The surface self-nanocrystallization of a rare earth magnesium alloy induced by HVOF-SMB.
► The grain size increases as well as the microhardness decreases gradually with the depth varying from surface to substrate.
► Annealing treatment at high temperature enables growth of nanocrystalline.
► No obvious change in microhardness and no significant change in nano-grains after low-temperature annealing at 373 K.