4604 N.R.Tao et al.Acta Materialia 50 (2002)4603-4616 pioneering study of preparing bulk nc metals by approach to create localized plastic deformation, means of gas condensation and consolidation by resulting in grain refinement progressively down to Gleiter et al.,in the early 1980s,several processing the nanometer region in the surface layer of met- techniques have been developed to produce bulk nc allic materials [12-14].The SMA process,which materials,e.g.consolidation of ultrafine powders has been successfully applied in many material prepared by various kinds of techniques [7],crys- systems [15],has some unique advantages com- tallization of amorphous precursors [8],ball-mill- pared with the coating and deposition methods for ing and consolidation [9],severe plastic defor- SNC.For example,as there is no change in chemi- mation of bulk metals [10],and electrodeposition cal compositions of the nc surface layer and in the [11].However,due to the limitation of each of matrix,as well as a gradient variation in the grain these techniques,preparation of 'ideal'bulk nc dimension from nano-sized (in the top layer)to samples (free of contamination and porosity,bulk coarse-grains (matrix),bonding of the nc surface in size,uniform and small (of a few nanometers)in layer with matrix will not be a problem.In grain size)is still a challenge to material scientists. addition,many existing mechanical processing Meanwhile much effort has been concentrated on techniques are applicable for synthesizing nanos- improvement of these techniques for synthesizing tructures in the surface by modifying the pro- bulk nc materials in recent years,the techniques cessing parameters,such as shot peening,hammer for producing nanostructured surface layers are peening,surface rolling,laser shock processing,etc emerging quickly. [12].Therefore,this SNC approach has a great Most failures of materials occur on surfaces, potential in industrial applications.For further including fatigue fracture,fretting fatigue,wear development of the SMA technique,a clear under- and corrosion etc.,which are very sensitive to the standing of the underlying mechanism for forma- structure and properties of the material surface. tion of nanostructures during the treatment is Optimization of the surface structure and proper- necessary.The objective of this work is to reveal ties may effectively enhance the global behavior of the intrinsic mechanism for grain refinement dur- a material.As a result,the surface modification of ing the SMA treatment in a pure iron sample. engineering materials is found to process more and Plastic deformation induced grain refinement more industrial applications.With increasing evi- has been known for many years [10,16].Polycrys- dences of novel properties in nc materials,it is tals with submicro-sized grains were usually fabri- reasonable to propose to achieve surface modifi- cated via severe plastic deformation (SPD)of vari- cation by the generation of a nanostructured sur- ous metals and alloys by using equal-channel face layer so that the overall properties and angular pressing (ECAP)[10],high pressure tor- behavior of the material are significantly improved. sion [17]and cold rolling [18].By examining the This kind of surface modification,referred as sur- microstructure evolution in plastically deformed face nanocrystallization (SNC),will greatly metals,possible grain refinement mechanisms were enhance the surface properties without changing proposed,involving dislocation activities,forma- the chemical composition [12].It is also a flexible tion of subgrain boundaries and grain boundaries approach that makes it possible to meet specific [18,19].The refinement process of coarse grains structure/property requirements on surface of upon plastic deformation,in principle,depends on samples. many intrinsic and extrinsic factors,such as struc- Nanostructured surface layers can be produced ture and stacking fault energy of the material,the by means of various existing coating and depo- intensity of strains and strain rates,deformation sition techniques such as PVD,CVD and plasma temperature,and so on.Formation of nano-sized processing.Alternatively,SNC can be realized by grains was realized in intensive mechanical means of grain refinement into the nanometer attrition processes (such as ball-milling)in metals regime in the surface layer of a bulk material.Our and alloys [20-22],in which much larger strains previous investigations demonstrated that surface and strain rates were applied relative to the other mechanical attrition (SMA)is an effective SPD processes.In the milling process,fracture and4604 N.R. Tao et al. / Acta Materialia 50 (2002) 4603–4616 pioneering study of preparing bulk nc metals by means of gas condensation and consolidation by Gleiter et al., in the early 1980s, several processing techniques have been developed to produce bulk nc materials, e.g. consolidation of ultrafine powders prepared by various kinds of techniques [7], crys￾tallization of amorphous precursors [8], ball-mill￾ing and consolidation [9], severe plastic defor￾mation of bulk metals [10], and electrodeposition [11]. However, due to the limitation of each of these techniques, preparation of ‘ideal’ bulk nc samples (free of contamination and porosity, bulk in size, uniform and small (of a few nanometers) in grain size) is still a challenge to material scientists. Meanwhile much effort has been concentrated on improvement of these techniques for synthesizing bulk nc materials in recent years, the techniques for producing nanostructured surface layers are emerging quickly. Most failures of materials occur on surfaces, including fatigue fracture, fretting fatigue, wear and corrosion etc., which are very sensitive to the structure and properties of the material surface. Optimization of the surface structure and proper￾ties may effectively enhance the global behavior of a material. As a result, the surface modification of engineering materials is found to process more and more industrial applications. With increasing evi￾dences of novel properties in nc materials, it is reasonable to propose to achieve surface modifi- cation by the generation of a nanostructured sur￾face layer so that the overall properties and behavior of the material are significantly improved. This kind of surface modification, referred as sur￾face nanocrystallization (SNC), will greatly enhance the surface properties without changing the chemical composition [12]. It is also a flexible approach that makes it possible to meet specific structure/property requirements on surface of samples. Nanostructured surface layers can be produced by means of various existing coating and depo￾sition techniques such as PVD, CVD and plasma processing. Alternatively, SNC can be realized by means of grain refinement into the nanometer regime in the surface layer of a bulk material. Our previous investigations demonstrated that surface mechanical attrition (SMA) is an effective approach to create localized plastic deformation, resulting in grain refinement progressively down to the nanometer region in the surface layer of met￾allic materials [12–14]. The SMA process, which has been successfully applied in many material systems [15], has some unique advantages com￾pared with the coating and deposition methods for SNC. For example, as there is no change in chemi￾cal compositions of the nc surface layer and in the matrix, as well as a gradient variation in the grain dimension from nano-sized (in the top layer) to coarse-grains (matrix), bonding of the nc surface layer with matrix will not be a problem. In addition, many existing mechanical processing techniques are applicable for synthesizing nanos￾tructures in the surface by modifying the pro￾cessing parameters, such as shot peening, hammer peening, surface rolling, laser shock processing, etc [12]. Therefore, this SNC approach has a great potential in industrial applications. For further development of the SMA technique, a clear under￾standing of the underlying mechanism for forma￾tion of nanostructures during the treatment is necessary. The objective of this work is to reveal the intrinsic mechanism for grain refinement dur￾ing the SMA treatment in a pure iron sample. Plastic deformation induced grain refinement has been known for many years [10,16]. Polycrys￾tals with submicro-sized grains were usually fabri￾cated via severe plastic deformation (SPD) of vari￾ous metals and alloys by using equal-channel angular pressing (ECAP) [10], high pressure tor￾sion [17] and cold rolling [18]. By examining the microstructure evolution in plastically deformed metals, possible grain refinement mechanisms were proposed, involving dislocation activities, forma￾tion of subgrain boundaries and grain boundaries [18,19]. The refinement process of coarse grains upon plastic deformation, in principle, depends on many intrinsic and extrinsic factors, such as struc￾ture and stacking fault energy of the material, the intensity of strains and strain rates, deformation temperature, and so on. Formation of nano-sized grains was realized in intensive mechanical attrition processes (such as ball-milling) in metals and alloys [20–22], in which much larger strains and strain rates were applied relative to the other SPD processes. In the milling process, fracture and