表面纳米化是否生成纳米晶

Whether Nanograins Formed in Surface Mechanical Attrition Treatment

喷丸或滚压和摩擦磨损的表面具有相同本质,都是冷加工导致组织细化。20世纪七八十年代从摩擦学角度对表面变形组织进行了系统研究,所得结论已成为材料学教材中的重要内容。21世纪初纳米风带动表面纳米化(SMAT)研究,忽视已有成果,认为变形细化的组织就是纳米晶。早期研究已经明确,变形后组织细化的尺度对性能影响不大,关键因素是其错角,SMAT却仍然强调纳米化尺度越小越好;摩擦学研究认为要获得纳米晶则表层转动要达到一定值,但此种转动将破坏连续性导致表层材料剥落。当SMAT认识到错角是决定性能的关键,并通过超强滚压处理获得大错角纳米晶时,材料表面已布满裂纹。重点讨论了表面形变结构的本质和特征,指出用动态再结晶机理解释纳米晶在位错胞壁生长的观点是错误的,并从胞壁和晶界对位错阻力不同的角度说明胞和晶在Hall-Petch关系中的差异。

Shot peening or surface rolling holds the same nature of wear in terms of microstructure refining.In the 1970 s and 1980 s,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 21 st 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 HallPetch relation is described by the different resistant stress of cells and grains to dislocations.