Long distance chemical gradient induced by surface nanocrystallization

Natural structures are wonderful examples of what can be done with a fairly limited selection of resources. Natural structures have this feature due to their complex architecture, gradient chemical compositions and microstructure. Inspired by nature, many attempts have been made to manufacture man-made materials with similar gradients (e.g. surface nanocrystallization by severe surface plastic deformation (S²PD)). Gradient microstructure of alloys manufactured by the S²PD techniques has been very well studied. However, the distribution of chemical compositions within the alloy after the S²PD treatment has rarely been explored. The current work found that by using S²PD combined with one-year long natural aging treatments, the microstructure and chemical composition of the Al-Zn-Mg-Cu alloy (7034 Al alloy) exhibited a new gradient distribution measured from the treated surface to the pristine interior. Zn (Cu) content on the surface layer increased from 10.2 (1.3) to 57.7 (5.9) wt.%, along with an Mg surface layer depletion after treatments. The thickness of the observed chemical gradient layer was surprisingly as thick as 45 μm. Also observed were the similar phenomena in other alloys after the same treatments. The findings provide potential improvements in bioinspired materials manufacturing, rare resource saving and surface functionalization.