Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment

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1 Materials Science and Engineering A (2004) Nanostructured surface layer on metallic materials induced by surface mechanical attrition treatment K. Lu a,,j.lu b a Institute of Metal Research, Chinese Academy of Sciences, Shenyang National Laboratory for Materials Science, 72 Wenhua Road, Shenyang , China b LASMIS, University of Technology of Troyes, Troyes, France Abstract In terms of the grain refinement mechanism induced by plastic straining, a novel surface mechanical attrition treatment (SMAT) was developed for synthesizing a nanostructured surface layer on metallic materials in order to upgrade the overall properties and performance. In this paper, the SMAT technique and the microstructure of the SMAT surface layer will be described. The grain refinement mechanism of the surface layer during the SMAT will be analyzed in terms of the microstructure observations in several typical materials. Obvious enhancements in mechanical properties and tribological properties of the nanostructured surface layer in different materials were observed. Further development and prospects will be addressed with respect to the SMAT as well as the performance and technological applications of the engineering materials with the nanostructured surface layer Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Surface; Mechanical attrition; Grain refinement; Properties 1. Introduction In most cases, material failures occur on surfaces such as fatigue fracture, fretting fatigue, wear and corrosion, etc. These failures are very sensitive to the structure and properties of the material surface. Optimization of the surface microstructure and properties is an effective approach to enhance the global behavior and service lifetime of materials. With the intensive and extensive investigations on nanostructured materials in the past decades, more and more experimental evidence showed that this new class of materials possess novel properties and performances that are fundamentally different from their conventional coarse-grained polycrystalline counterparts, such as high hardness and strength [1 3], enhanced physical properties [1 3], improved tribological properties [4], and superplasticity at low temperatures [5,6], etc. It is reasonable to expect to achieve surface modification by generation of a nanostructured surface layer so that the overall properties and behavior of the materials are significantly improved. Corresponding author. Tel.: ; fax: address: lu@imr.ac.cn (K. Lu). Conventionally, a nanostructured surface layer can be made on a bulk material by means of various coating and deposition technologies such as PVD, CVD, sputtering, electrodeposition, and plasma processing, etc. (as illustrated in Fig. 1a). The coated materials can be either nanometer-sized isolated particles or polycrystalline powders with nano-sized grains. The coated layer and the matrix can be different or made of the same kind of material. The predominant factors in this process are the bonding of the coated layer with the matrix and the bonding between particles while maintaining the nanostructure. An alternative approach to synthesis of a nanostructured surface layer is to transform the original coarse-grained surface layer of a bulk material into nano-sized grains while keeping the overall composition and/or phases unchanged. Such a process may be referred as surface selfnanocrystallization of bulk materials, as schematically shown in Fig. 1b. So far, three kinds of techniques have been developed for synthesizing bulk nanostructured materials [7]: (1) Starting with isolated nanometer-sized particles. The nanometer-sized particles can be generated by means of various techniques including PVC, CVD, electrochemical and hydrothermal methods, precipitation from /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.msea

2 K. Lu, J. Lu / Materials Science and Engineering A (2004) Fig. 1. Schematic illustration of three types of surface nanocrystallization processes: (a) surface coating or deposition; (b) surface self-nanocrystallization; (c) hybrid surface nanocrystallization. In terms of the underlying mechanism of the method (3), i.e., grain refinement induced by plastic deformation, we developed a new technique, namely surface mechanical attrition treatment (SMAT), to synthesize a nanostructured surface layer on bulk metallic materials [11]. Owing to the plastic deformation in the surface layer induced by the mechanical attrition, the coarse-grained structure in the surface layer is refined into the nanometer scale without change of the chemical compositions. This SMAT has been successfully applied in various kinds of materials including pure metals, steels, and alloys [12 16], on which a nanostructured surface layer up to 50 m thick has been obtained. And the obvious enhancement in the overall properties and performance of the materials is observed after the SMAT treatment. In this paper, we will report the processing of the SMAT, the microstructure and grain refinement mechanism of the surface layer induced by the SMAT, as well as properties of the nanostructured surface layer in different materials. solution, and so on. The most widely applied PVD method involves inert gas condensation. The second step is to consolidate these ultrafine particles into bulk materials in which the particles become nano-sized grains. (2) Starting with a noncrystalline structure. Nanocrystalline materials are obtained by nucleating numerous crystallites in the glass by annealing or mechanical activation [3]. These nuclei subsequently grow together (with a relatively low growth rate) and result in a nanostructured material. (3) Starting with a coarse-grained polycrystalline structure. By increasing the free energy of the polycrystals and generation of much more defects and interfaces (grain boundaries) in various nonequilibrium processes, such as ball-milling [8], severe plastic deformation [9], irradiation with high-energy particles, spark erosion, and sliding wear [10], etc., the polycrystalline structure will be transformed into nanocrystalline structures via different kinds of grain refinement mechanisms. 2. Surface mechanical attrition treatment The key point for realizing the surface self-nanocrystallization of a bulk material is to introduce a large amount of defects and/or interfaces into the surface layer so that its microstructure is transformed into nano-sized crystallites. Or in other words, a grain refinement process into the nano-scale is needed in the surface layer while the structure of the coarse-grained matrix remains unchanged. Surface mechanical attrition is an effective technique to realize the surface self-nanocrystallization on metallic materials. Fig. 2 illustrates the experimental set-up of the SMAT. Spherical steel balls with smooth surface (or of other materials such as glass and ceramics) are placed in a reflecting chamber that is vibrated by a vibration generator. Typical ball sizes are 1 10 mm in diameter and that can be different for different materials. The vibration frequency of the chamber is in the range from 50 Hz to 20 khz. When the balls are resonated, the sample surface to be treated is impacted by a Sample Vacuum v sample (a) Vibration generator (b) Fig. 2. (a and b) Schematic illustration of the surface mechanical attrition treatment set-up and the repeated multidirectional plastic deformation in the sample surface layer induced by impact of the flying balls.

3 40 K. Lu, J. Lu / Materials Science and Engineering A (2004) large number of flying balls over a short period of time. The velocity of the balls is about 1 20 m/s, depending upon the vibration frequency, the distance between the sample surface to the balls, and the ball size. The impact directions of the balls onto the sample surface are rather random due to the random flying directions of the balls inside the vibration chamber. Each impact will induce plastic deformation with a high strain rate in the surface layer of the sample, as schematically shown in Fig. 2b. As a consequence, the repeated multidirectional impacts at high strain rates onto the sample surface result in severe plastic deformation and grain refinement progressively down to the nanometer regime in the entire sample surface. The temperature rise at the sample surface induced by the repeated impacts was measured in a Fe sample, being about C, which varies with the intensity of impacts and the materials treated. Comparing the SMAT with other conventional surface treatment such as shot peening, one may find obvious differences in several aspects. Much larger balls (a few mm) are used in the SMAT than in the shot peening (0.2 1 mm in diameter). Spherical balls with smooth surface are necessary for obtaining a nanostructured surface layer in the SMAT. Balls with rough surface (as in shot peening) will wear and damage the nanostructured surface layer during the treatment. The velocity of the balls in the SMAT process is much lower (1 20 m/s) compared with the conventional shot peening (typically about 100 m/s). Conventional shot peening is a directional process in which the angle between the shot jet and the sample surface is normally fixed, close to 90 in many case. But in the SMAT, random directional impacts of the balls onto the sample are needed in order to facilitate the grain refinement process. Our experimental results on a number of materials including pure metals and alloys show that the nanostructured surface layer can be up to 50 m thick, in which grain sizes vary gradually from a few nanometers (in the top treated surface layer) to about 100 nm. Underneath is a refined structured layer (up to 100 m thick) consisting of submicrometer-sized crystallites or cells separated by either grain boundaries or subboundaries. In deeper layers are deformed coarse grains with various kinds of dislocation configurations such as dense dislocation walls, dislocation tangles, and dislocation cells. Fig. 3 shows a scheme for a cross-sectional view of the SMAT sample. The thickness of the nanostructured surface layer (as well as the refined structure layer) depends very much upon the material treated and the processing parameters (such as ball size, vibration frequency, temperature, etc.). In fact other mechanical treatment techniques are also possible to achieve surface nanostructure layers on metallic materials when large strains with a high strain rate are achieved, such as shot peening, hammer peening, laser shock treatment, surface rolling, and high-speed machining. However, the thickness of the nanostructured surface layer may be different for various treatment techniques due to the different strains and strain rates applied in the surface layer. With a nanostructured surface layer prepared, it is possible to modify the composition and/or phases of the nanostructured surface layer by exposure of the treated surface to different media that can be solid, liquid or gaseous. Solid solutions, compounds, or composites might be formed in the nanostructured surface layer so that specific properties can be obtained. This type of surface nanocrystallization process, i.e., hybrid surface nanocrystallization (shown in Fig. 1c) may provide an efficient way to enhance the surface properties. This process can be possibly performed by an in situ process combining the SMAT with the chemical reaction simultaneously. 3. Surface nanocrystallization mechanism Understanding of the formation process of nanocrystallites during the SMAT process is crucial for development of the SMAT. Owing to the gradient variation of the strain and strain rate from the treated top surface (both are very large) to the deep matrix (essentially zero, as schematically shown in Fig. 3), a gradient grain size distribution from a few nanometers (in the top surface layer) to several micrometers is developed in the SMAT sample, that provides a unique opportunity to examine the microstructure characteristics at different levels of strain and strain rate. Therefore, the underlying mechanism for deformation-induced grain refinement in the micrometer nanometer regime can be deduced. Analogous to the grain refinement mechanism during plastic deformation of bulk metals, formation of nanostructures from the coarse-grained polycrystals in the surface layer upon the SMAT involves various dislocation activities and development of grain boundaries. Plastic deformation behaviors and dislocation activities in metals depend strongly on the lattice structure and the stacking fault energy (SFE). For example, in those materials with high SFEs, dislocation walls and cells will be formed to accumulate strains and subboundaries are formed to subdivide coarse grains. While for those with low SFEs, plastic deformation mode may change from dislocation slip to mechanical twins (especially under high strain rate and/or low temperature) [17]. In the following, we take three typical examples with different lattice structures and SFEs to demonstrate the surface nanocrystallization process upon the SMAT treatment Fe [16] Fe is a typical bcc metal with a high SFE of about 200 mj/m 2. After a SMAT to a pure Fe bulk sample, obvious grain refinement was observed in the surface layer in which the top layer consists of ultrafine crystallites of about 10 nm in size, as indicated in Fig. 4. The grain/cell size increases gradually from 10 nm to several micrometers at about 60 m deep. Detailed cross-sectional TEM observations of the as-treated Fe sample revealed the grain refinement process, which involves the following elemental processes:

4 K. Lu, J. Lu / Materials Science and Engineering A (2004) Fig. 3. Schematic illustration of microstructure characteristics and distributions of strain and strain rate along depth in the surface layer subjected to the SMAT. Based on the velocity of the balls and the measured depth of the pit caused by an individual impact, the strain rate at the sample surface was estimated to be as much as s 1. (1) Development of dense dislocation walls (DDWs) and dislocation tangles (DTs): In order to accommodate plastic strains, dislocation activities are motivated in the original coarse grains, forming DDWs along (1 1 0) slip planes and random DTs in some grains (depending on the grain orientations). Development of these dislocation configurations results in subdivision of original grains by forming individual dislocation cells primarily separated by DDWs and DTs. The repeated multi-directional impact onto the sample surface may lead to a change of slip systems with the strain path even inside the same grain. The dislocations not only interact with other dislocations in the current active slip systems, but also interact with inactive disloca- tions generated in previous deformation. Therefore, the grains can be subdivided efficiently by the DDWs and DTs in the treatment. (2) Transformation of DDWs and DTs into subboundaries with small misorientations: Formation of subgrain boundaries subdividing the original grains is resulted from development of DDWs and DTs by accumulating of more and more dislocations with increasing strains. At a certain strain level, for minimizing the total system energy, dislocation annihilation and rearrangement occur in DDWs and DTs, which will transform into subboundaries (with larger misorientations across relative to the DDWs) separating individual cells. The subgrain boundaries are actually formed by recombination Grain/cell size (nm) Mean microstrain (%) Distance from surface (µm) 0.00 Fig. 4. Variations of the grain/cell size with the depth from the SMAT surface of the Fe sample determined by means of XRD analysis ( ), TEM (( ) equiaxed, (H) short-axis, (N) long-axis) and SEM ( ) observations. The mean microstrain ( ) was also determined at different depths by means of XRD analysis.

5 42 K. Lu, J. Lu / Materials Science and Engineering A (2004) of a high density of dislocations and usually have small misorientations of a few degrees, i.e., small-angle grain boundaries. (3) Evolution of subboundaries to highly-misoriented grain boundaries: With further increasing strains, more dislocations are generated and annihilated in the subboundaries, raising up the energy of the subboundary and progressively increasing its misorientations. The orientations of the grains with respect to their neighboring grains become completely random, forming highly-misoriented grain boundaries. The increment of misorientations between neighboring grains can be realized by accumulating more dislocations with different Burgers vectors in grain boundaries, or alternatively, by rotation of grains (or grain boundary sliding) with respect to each other under certain strains. The grain rotation process would be much facilitated when the size of grains is reduced due to the obvious size dependence [18]. With further straining, DDWs and DTs could form inside the inner of the refined subgrains or grains, and these refined grains could be further subdivided following the similar mechanism. With increasing strain in the top surface layer, the subdivision takes place on a finer and finer scale. When dislocation multiplication rate is balanced by the annihilation rate, the increase of strains could not reduce the subgrain size any longer, and a stabilized grain size is resulted. In an Al alloy with a high SFE subjected to the SMAT, a similar grain refinement mechanism was found [15] Cu [19] Cu is an fcc metal with a medium SFE (about 78 mj/m 2 ). After the SMAT to a bulk Cu specimen, a nanostructured surface layer of about 35 m thick was formed. Microstructure examination by means of cross-sectional TEM observations revealed a different grain refinement mechanism: (1) Development of equiaxed dislocation cells (DCs): For the fcc structure, more dislocation slip planes exist compared with that in bcc Fe. The strain-induced dislocation activities lead to formation of equiaxed DCs (instead of DDWs or DTs as in Fe) where high density of dislocations locate at the cell boundary with few dislocations inside the cell. The cell size observed ranges from a few micrometers to several tens of nanometers, depending upon the strain and strain rate. With increasing strains, the DC size decreases and forming a DC network that subdivides the original coarse grains. (2) Formation of twins and subboundaries with small misorientations: At a certain strain and strain rate level, mechanical twinning is activated in some grains with favorable orientations. Twins appear at depth from 10 to 100 m from the top surface, implying the strains and strain rate at this depth is appropriate for formation of twins in Cu with the medium SFE. At the same time, in other grains, DCs transform into subboundaries with small misorientations with further straining, which is analogous to the transformation from DDWs as in Fe. These twin boundaries and subboundaries divide the original coarse grains into ultrafine (sub)grains. (3) Evolution of subboundaries to highly-misoriented grain boundaries: Increasing strains will drive the transformation from subgrain boundaries into conventional grain boundaries with large misorientations, similar to the process as in Fe AISI 304 stainless steel [20] AISI 304 stainless steel is a widely used engineering material with an fcc austenite structure and a very low SFE (about 17 mj/m 2 ). Due to the low SFE, dislocation activities in plastic deformation are much different from the above two cases. The grain refinement mechanism was found as follows: (1) Formation of planar dislocation arrays and twins: The strain-induced dislocations in the austenite phase slip mainly on their respective {111} planes, forming regular dislocation grids, instead of irregular DCs (as in fcc Al alloy, Cu) or DDWs (as in bcc Fe). Such a difference originates from the low SFE that makes it difficult for partial dislocations to cross-slip for forming DCs, rather favorites formation of dislocation arrays and twins in {111} planes. (2) Grain subdivision by twins and martensite transformation: With increasing strains, twins in different {111} planes intersect with each other and subdivide the original austenite grains into refined blocks. A strain-induced martensite transformation is a prevailing phenomenon in the plastic deformed AISI 304 stainless steel. In the SMAT-treated sample, we observed formation of martensite phase at intersections of the twins, of which the size ranges from several nanometers to submicrometers. Such a phase transformation is a unique process that facilitates grain refinement procedures. (3) Formation of nanocrystallites: In the top surface layer, manometer-sized martensite crystallites are formed by means of further twin-twin intersection and martensite phase transformation under high strains and strain rates. Analogous to other systems, randomly-oriented nanocrystallites are formed via grain rotation or other grain boundary activities (motion or sliding) to accommodate the high strains. 4. Properties of surface nanostructured layers Mechanical property measurements indicated a significant increment of hardness and strength in the surface layer with nanostructures after the SMAT. Fig. 5 shows a variation of hardness along the depth from the treated surface

6 K. Lu, J. Lu / Materials Science and Engineering A (2004) as-treated as-annealed at 923 K Hardness (GPa) Depth from surface (µm) Fig. 5. Measured hardness as a function of depth from the top surface for the SMAT Fe sample and for the annealed (at 593 K for 30 min) sample. as determined by using nanoindentation tests in the SMAT Fe sample. In the top surface nanostructured layer, hardness reaches as high as 3.8 GPa, which is about twice that for the coarse-grained matrix. There is no change in hardness profile after the sample was annealed 593 K for 1 h for residual stress relaxation, implying that the high hardness in the surface layer is not resulted from the residual stress induced in the SMAT. After annealing at 923 K for 1 h for recrystallization of the nanostructures and forming coarse grains, hardness in the surface layer drops to that of the coarse-grained matrix. This observation indicates that the hardness increment is due to grain refinement into the nanometer scale, instead of the alloying (contamination) effect from the SMAT media (balls and other impurities). Plotting the hardness values as a function of measured grain size along the depth, one may check the validity of Hall-Petch relation in a wide grain size range (from 10 nm to 50 m) in one sample. A normal Hall-Petch relation is observed in Fe as far as the grain size is around 10 nm. Tensile yield strength of a SMAT low-carbon steel sheet (1.5 mm thick) was found to be enhanced by about 35% relative to the untreated sample, while the elongation-to-failure remains unchanged. Such a large increment in yield strength can be reasonably attributed to the strong nanostructured surface layers (about 20 m thick) on both sides of the sample. The yield strength of the surface nanostructured layer is estimated to be about 2 3 times that of the coarse-grained form. A similar phenomenon was observed in a 316L stainless steel sample in which both the yield strength and the fracture strength are enhanced. Fig. 6 shows a tensile stress-strain curve for a SMAT 316L stainless steel sample (1 mm thick with 10 m thick nanostructured surface layers on both sides, gauge length of 46 mm and width of 5 mm), in comparison with an untreated original sample with the same geometry. The yield strength of the sample at a strain rate of 10 3 s 1 increases from 280 MPa (original) to 550 MPa after the SMAT, while the ultimate tensile stress is increased by about 13% (from 620 to 700 MPa). Analysis of the fracture mechanism of the SMAT-treated sample revealed that the nanostructured surface layers on both sides of the sample, which are much stronger than the coarse-grained matrix, obstruct the slip bands developed in the coarse grains in the inner part of the sample. So that crack nucleation will not occur in the sample surface due to the much high yield strength of the surface layer, but in the sub-surface layers. Therefore, the stress -to-failure is much enhanced for the sample with nanostructured surface layers. Wear and friction properties of the low-carbon steel sheet after the SMAT were measured by using a reciprocating friction tester with a diamond tip in comparison with that of the original steel sheet [21]. As shown in Fig. 7, the wear volume loss of the nanostructured surface layer in the SMAT sample is lower than that for the untreated original one. The friction coefficient values at different applied loads for the as-treated sample are evidently smaller (about a half) than those for the original sample. These observations indicate that the friction and wear properties of the low-carbon steel can be improved by means of formation of the nanostructured surface layer. The improved wear and fraction properties can be attributed to the strong surface layer with nanograins and a gradient variation in the microstructure and properties along the depth from top surface. As grain boundaries may act as fast-diffusion channels, atomic diffusivity in the nanostructured surface layer will be considerably enhanced compared to the conventional polycrystalline materials. This behavior can be utilized to upgrade the traditional surface chemical treatment techniques, such as nitriding process of steel, by enhancing the diffusion

7 44 K. Lu, J. Lu / Materials Science and Engineering A (2004) Treated material 600 Base Material Stress (MPa) Strain (%) Fig. 6. Tensile test stress-strain curves for the SMAT 316L stainless steel sheet sample (1.5 mm thick) with nanostructured surface layers (of about 15 m thick) on both sides, and for the untreated sample (base material) Wear Volume Loss, mm as-treated original Friction Coefficient as-treated original (a) Load, N (b) Load, N Fig. 7. Variations of the wear volume loss with load (a) and variations of the coefficient of friction with load (b) for the SMAT and the original low-carbon steel samples. kinetics or reducing the diffusion temperature. Our preliminary results showed that the nitriding temperature of iron can be reduced to about 300 C after the SMAT to form a nanostructured surface layer, in contrast to more than 500 C for conventional polycrystalline Fe and steels [22]. Enhanced diffusivity of other elements (Cr, Al) in the nanostructured surface layer is also observed experimentally. 5. Summary and prospects The evidences obtained so far have already indicated that the nanostructured surface layer synthesized by means of the SMAT to metallic materials provides a plenty of unique opportunities in both basic scientific research and technological applications as well, including: (i) To enhance the surface mechanical, tribological, chemical and corrosion properties of bulk materials; (ii) To investigate the strain-induced grain refinement mechanism in a wide grain size scale (micro-nanometer); (iii) To study the structure property relationship of solid in a wide grain/cell size range (several nanometers micrometers) in ONE sample. The gradient structure in the surface layer allows one to prepare porosity-free microsamples for property measurements with different

8 K. Lu, J. Lu / Materials Science and Engineering A (2004) grain sizes (in surface layers at different depths) but the same composition. (iv) New alternative approaches to functional surface structures by means of nanostructure-selective reaction due to the much enhanced atomic diffusivity and chemical reactivity of the nanostructured surface layer. (v) Flexibility and low-cost procedures of the SMAT will greatly facilitate the widespread application of this technique to various industry areas. The process is also feasible to obtain a localized nanostructured surface layer in bulk materials and to realize surface nanocrystallization of components with complex shapes. Other advantages of this technique include the fact that many existing processes can be used to obtain the nanostructured surface layer with a high productivity. Surface nanocrystallization of metallic materials will certainly provide a complementary process to the nanocrystallization process for bulk materials. With increasing investigations on the processing and properties of the nanostructured surface layer, industrial applications of this new technique to upgrade the traditional engineering materials and technologies in the near future can be anticipated. Acknowledgements Financial support from the National Science Foundation of China, the Ministry of Science and Technology of China (Grant G ), NEDO International Joint Research Grant Program (01MB5), and Ministry of Research of France (Grant , CPER EN2040) is acknowledged. The authors thank G. Liu, N.R. Tao, M.L. Sui, H.W. Zhang, K. Wang, and W.P. Tong for their valuable contributions to the present work. References [1] H. Gleiter, Prog. Mater. Sci. 33 (1988) 223. [2] C. Suryanarayana, Int. Mater. Rev. 40 (1995) 41. [3] K. Lu, Mater. Sci. Eng. R16 (1996) 161. [4] D.G. Morris, Mechanical Behaviour of Nanostructured Materials, Trans. Tech. Publications Ltd., Switzerland, 1998, p. 70. [5] S.X. McFadden, R.S. Mishra, R.Z. Valiev, A.P. Zhilyaev, A.K. Mukherjee, Nature 298 (1999) 684. [6] L. Lu, M.L. Sui, K. Lu, Science 287 (2000) [7] H. Gleiter, in: R.W. Cahn, P. Haasen (Eds.), Physical Metallurgy, Elsevier, Amsterdam, 1996, p [8] C.C. Koch, Nanostruct. Mater. 2 (1993) 109. [9] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103. [10] D.A. Hughes, N. Hansen, Phys. Rev. Lett. 87 (2001) [11] K. Lu, J. Lu, J. Mater. Sci. Technol. 15 (1999) 193. [12] N.R. Tao, M.L. Sui, J. Lu, K. Lu, Nanostruct. Mater. 11 (1999) 433. [13] G. Liu, J. Lu, K. Lu, Mater. Sci. Eng. A286 (2000) 91. [14] G. Liu, S.C. Wang, X.F. Lou, J. Lu, K. Lu, Scripta Mater. 44 (2001) [15] X. Wu, N. Tao, Y. Hong, B. Xu, J. Lu, K. Lu, Acta Mater. 50 (2002) [16] N.R. Tao, Z.B. Wang, W.P. Tong, M.L. Sui, J. Lu, K. Lu, Acta Mater. 50 (2002) [17] B. Bay, N. Hansen, D.A. Hughes, D. Kuhlmann-Wilsdorf, Acta Metall. Mater. 40 (1992) 205. [18] H. van Swygenhoven, D. Farkas, A. Caro, Phys. Rev. B 62 (2000) 831. [19] K. Wang, G. Liu, J. Lu, K. Lu, in press. [20] H.W. Zhang, Z.K. Hei, G. Liu, J. Lu, K. Lu, Acta Mater. 51 (2003) [21] Z.B. Wang, X.P. Yong, N.R. Tao, S. Li, G. Liu, J. Lu, K. Lu, Acta Metall. Sinica 37 (2001) [22] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Science 299 (2003) 686.