Whether Nanograins Formed in Surface Mechanical Attrition Treatment

Shot peening or surface rolling holds the same nature of wear in terms of microstructure refining. In the 1970s and 1980s, systematic studies of surface deformed microstructure were carried out from the perspective of tribology, and the conclusions obtained have become important achievement addressed in the textbook of materials science. However, following the fashion of nano tide at the beginning of the 21st century, the surface mechanical attrition treatment (SMAT) group neglected the contributions of the former generation and claimed that the deform-refined structure was nanograins. The early year study shows that the size of the deformed structure has little influence on the performance and the key factor is misorientation angles. But the SMAT group insists the finer nanosize the better. The tribological study indicates that to meet the characteristics of nanograin, a critical surface rotation to a certain value is required; but this kind of rotation will destroy the continuity and cause the surface material to peel off. As the SMAT group recognized the misorientation angle dominating the mechanical behavior and manipulated excessive rolling to realize the high angle nanograins, the surface was covered with cracks. The nature and characteristics of the deformed structure are clarified. It is pointed out that the dynamic recrystallization of nanograins initiated at the cell walls is specious, and the difference between cells and grains associated with Hall-Petch relation is described by the different resistant stress of cells and grains to dislocations.