Y.T.Zhu et al.Scripta Materialia 51 (2004)825-830 827 condition.It is reasonable to anticipate this will become 2.4.Corrosion resistance feasible within the next few years as facilities are established to produce larger samples through SPD Little information is available to date on the corro- techniques. sion resistance of NS materials.However,there is evi- The fatigue strength is another important mechanical dence that NS Ti has better corrosion resistance than property but the studies reported to date are fairly coarse-grained Ti [39].Several researchers have reported limited.Preliminary investigations suggest that most no significant difference in the corrosion resistance of SPD-processed metals have an enhanced high-cycle fa- NS materials by comparison with their coarse-grained tigue life but a shorter low-cycle fatigue life [33].The counterparts and the enhancement in the corrosion explanation for this trend lies in observations that the resistance of NS Ti is probably due to the more uniform high-cycle fatigue life correlates strongly with strength nature of the corrosion.In coarse-grained Ti the disso- whereas the low-cycle fatigue life correlates strongly lution of the material is heavily concentrated at the grain with ductility and,as already documented,NS metals boundaries because they have a higher energy than in usually have higher strength and lower ductility than in the grain interior;but in NS Ti the high defect density their coarse-grained counterparts.Moderate annealing inside the grains tends to equilibrate the energies across after SPD processing may improve the ductility without the material,leading to a more uniform corrosion. significantly sacrificing the strength,thereby improving the low-cycle fatigue life.Surface hardening techniques 2.5.Physical properties such as shot-peening are generally effective in improving the fatigue life of coarse-grained materials but appear to Amorphous and NS solids also have unique optical be ineffective in improving the fatigue life of NS mate- rials [20]. and magnetic properties [2].As the grain sizes change Since NS materials have a higher hardness than their from amorphous to the nanometer range,the solids will coarse-grained counterparts,it is reasonable to antici- change color and/or transparency [2,40].Amorphous Fe pate an increased wear resistance.This is supported by and Co-based alloys have very good soft magnetic recent experiments on NS low-carbon steel where the properties [2]and can be cast into cylinders or melt-spun wear resistance was increased [34].In addition,NS into ribbons [4].NS magnetic materials are found to materials have lower friction coefficients [34,35]. have lower Curie temperature and lower saturation magnetization than their CG counterparts [4,6]. Attractive soft magnetic properties are observed in NS 2.3.Thermal stability Fe-based alloys [4].They can be made with specific core loss,time variability of core-loss,and low magneto- striction that are desired for high frequency transform- NS materials are expected to have low thermal sta- bility because of their high densities of crystalline defects ers,magnetic heads,etc.[2]. such as grain boundaries and dislocations.Surprisingly, most NS metals produced by SPD exhibit relatively good thermal stability.For example,NS commercially- 3.Applications pure Ti processed by ECAP and cold rolling can be annealed at temperatures below 400 C without a sig- The potentials for using NS materials in structural nificant decrease in the strength [36].Thus,NS pure Ti is applications are being driven primarily by two separate sufficiently thermally stable for most applications factors:(1)superior properties and(2)superior manu- including for use as medical implants.Materials pro- facturability.NS materials produced by SPD have the duced by cryogenic ball-milling have even more stable greatest potential for large-scale industrial applications NS structures.For example,an NS Al-Mg alloy pro- because they make use of equipment that has many cessed by cryogenic ball-milling in liquid nitrogen has similarities with that used in conventional deformation been reported to maintain the NS structure after processing,thereby incurring only a relatively modest annealing at temperatures higher than 250 C [37], investment in capital equipment.It should be noted also which is remarkable considering the low melting tem- that,since SPD processing may produce metals having perature of Al alloys.It can be shown that low tem- characteristics that are only modestly different from perature annealing is beneficial in NS materials conventional metals,there is a low initial risk in the produced by SPD techniques because it significantly utilization of SPD metals although the payoff over time improves the ductility without markedly affecting the may be very high.Another significant advantage of the strength.This provides an opportunity to combine the SPD processing is its capability of producing bulk,large deformation and annealing to make stable and strong NS material stocks for real structural applications.For metals and alloys,as demonstrated in a recent example example,ECAP has been used to produce large Ti billets with Cu [38] (see Fig.1)[20]condition. It is reasonable to anticipate this will become feasible within the next few years as facilities are established to produce larger samples through SPD techniques. The fatigue strength is another important mechanical property but the studies reported to date are fairly limited. Preliminary investigations suggest that most SPD-processed metals have an enhanced high-cycle fa￾tigue life but a shorter low-cycle fatigue life [33]. The explanation for this trend lies in observations that the high-cycle fatigue life correlates strongly with strength whereas the low-cycle fatigue life correlates strongly with ductility and, as already documented, NS metals usually have higher strength and lower ductility than in their coarse-grained counterparts. Moderate annealing after SPD processing may improve the ductility without significantly sacrificing the strength, thereby improving the low-cycle fatigue life. Surface hardening techniques such as shot-peening are generally effective in improving the fatigue life of coarse-grained materials but appear to be ineffective in improving the fatigue life of NS mate￾rials [20]. Since NS materials have a higher hardness than their coarse-grained counterparts, it is reasonable to antici￾pate an increased wear resistance. This is supported by recent experiments on NS low-carbon steel where the wear resistance was increased [34]. In addition, NS materials have lower friction coefficients [34,35]. 2.3. Thermal stability NS materials are expected to have low thermal sta￾bility because of their high densities of crystalline defects such as grain boundaries and dislocations. Surprisingly, most NS metals produced by SPD exhibit relatively good thermal stability. For example, NS commercially￾pure Ti processed by ECAP and cold rolling can be annealed at temperatures below 400 C without a sig￾nificant decrease in the strength [36]. Thus, NS pure Ti is sufficiently thermally stable for most applications including for use as medical implants. Materials pro￾duced by cryogenic ball-milling have even more stable NS structures. For example, an NS Al–Mg alloy pro￾cessed by cryogenic ball-milling in liquid nitrogen has been reported to maintain the NS structure after annealing at temperatures higher than 250 C [37], which is remarkable considering the low melting tem￾perature of Al alloys. It can be shown that low tem￾perature annealing is beneficial in NS materials produced by SPD techniques because it significantly improves the ductility without markedly affecting the strength. This provides an opportunity to combine the deformation and annealing to make stable and strong metals and alloys, as demonstrated in a recent example with Cu [38]. 2.4. Corrosion resistance Little information is available to date on the corro￾sion resistance of NS materials. However, there is evi￾dence that NS Ti has better corrosion resistance than coarse-grained Ti [39]. Several researchers have reported no significant difference in the corrosion resistance of NS materials by comparison with their coarse-grained counterparts and the enhancement in the corrosion resistance of NS Ti is probably due to the more uniform nature of the corrosion. In coarse-grained Ti the disso￾lution of the material is heavily concentrated at the grain boundaries because they have a higher energy than in the grain interior; but in NS Ti the high defect density inside the grains tends to equilibrate the energies across the material, leading to a more uniform corrosion. 2.5. Physical properties Amorphous and NS solids also have unique optical and magnetic properties [2]. As the grain sizes change from amorphous to the nanometer range, the solids will change color and/or transparency [2,40]. Amorphous Fe and Co-based alloys have very good soft magnetic properties [2] and can be cast into cylinders or melt-spun into ribbons [4]. NS magnetic materials are found to have lower Curie temperature and lower saturation magnetization than their CG counterparts [4,6]. Attractive soft magnetic properties are observed in NS Fe-based alloys [4]. They can be made with specific core loss, time variability of core-loss, and low magneto￾striction that are desired for high frequency transform￾ers, magnetic heads, etc. [2]. 3. Applications The potentials for using NS materials in structural applications are being driven primarily by two separate factors: (1) superior properties and (2) superior manu￾facturability. NS materials produced by SPD have the greatest potential for large-scale industrial applications because they make use of equipment that has many similarities with that used in conventional deformation processing, thereby incurring only a relatively modest investment in capital equipment. It should be noted also that, since SPD processing may produce metals having characteristics that are only modestly different from conventional metals, there is a low initial risk in the utilization of SPD metals although the payoff over time may be very high. Another significant advantage of the SPD processing is its capability of producing bulk, large NS material stocks for real structural applications. For example, ECAP has been used to produce large Ti billets (see Fig. 1) [20]. Y.T. Zhu et al. / Scripta Materialia 51 (2004) 825–830 827