Using first-principles calculations, we systematically study the potential energy surfaces and dissociation processes of the hydrogen molecule on the Mg（0001） surface. It is found that during the dissociative adsorption process with the minimum energy barrier, the hydrogen molecule first orients perpendicularly, and then rotates tobecome parallel to the surface. It is also found that the orientation of the hydrogen molecule in the transition state is neither perpendicular nor parallel to the surface. Most importantly we find that the rotation causes a reduction of the calculated dissociation energy barrier for the hydrogen molecule. The underlying electronic mechanism for the rotation of the hydrogen molecule is also discussed in the paper.
A nanostructured surface layer was fabricated on a AZ91D magnesium alloy by using a high-energy shot peening(HESP).HESP induced structure along the depth of the treated sample surface layer was characterized by means of X-ray diffractometer (XRD),transmission electron microscope(TEM) and high resolution transmission electron microscope(HRTEM).The experimental results show that a deformed layer of about 50 μm has formed after HESP treatment and the average grain size increases from about 40 nm in the surface layer to about 200 nm at the depth of 40 μm.The surface nanocrystallization can realize intercoordination of the dislocations slipping and dynamic recrystallization.The nanocrystalline grains have stacking faults and dislocation in their interiors.The microhardness of the top surface is about triplicate that of the coarse-grained matrix.
