《金属材料强韧化与组织调控》教学资源（参考文献）Performance and applications of nanostructured materials produced by severe plastic deformation

Available online at www.sciencedirect.com d Scripta MATERIALIA ELSEVIER Scripta Materialia 51 (2004)825-830 www.actamat-journals.com Performance and applications of nanostructured materials produced by severe plastic deformation Yuntian T.Zhua",Terry C.Lowe Terence G.Langdonb Los Alamos National Laboratory,Materials Science:Technology Division.MS G755,Los Alamos.NM 87545.USA bDepartments of Aerospace Mechanical Engineering and Materials Science,University of Southern California.Los Angeles.CA90089-1453.USA Accepted 4 May 2004 Available online 4 June 2004 Abstract Nanostructured materials produced by severe plastic deformation can be tailored to have both superior performance and superior properties.These materials are attractive for use in a range of applications from biomedical to aerospace industries. 2004 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved. Keywords:Applications;Manufacturing:Nanostructured materials;Severe plastic deformation 1.Introduction tion domains(crystallites),which are often smaller than 100 nm.Therefore,they can be called NS materials [5]. Nanostructured (NS)materials are defined as solids Several SPD processing methods are now available, having microstructural features in the range of ~1-100 including equal-channel angular pressing(ECAP)[6,7], nm in at least one dimension [1.Two complementary high-pressure torsion [5,8],accumulative roll-bonding approaches have been developed in attempts to synthe- (ARB)[9,10],repetitive corrugation and straightening size NS solids.The first is the "bottom-up"approach in [11,12],and friction stir processing (FSP)[13,14].At- which bulk NS materials are assembled from individual tempts have also been made to combine some of these atoms or from nanoscale building blocks such as nano- procedures such as ECAP and cold rolling [15],ARB particles.Techniques in this category include inert gas and FSP [16]or ECAP and HPT [17,18].An overall condensation [2],electrodeposition [3],and chemical and review of these various techniques suggests that,at least physical deposition [4]. in terms of the commercial viability of the processing The second approach is the "top-down"approach in route and the nature of the microstructures attained to which existing coarse-grained materials are processed to date,processing by ECAP has at least two advantages produce substantial grain refinement and a nanostruc- that favor its adoption into manufacturing practice. ture.The most successful "top-down"approach in- Firstly,it can be scaled up to produce relatively large volves the use of severe plastic deformation (SPD) bulk samples [19,20].Secondly,several groups have processing in which materials are subjected to the incorporated it into conventional rolling mills for con- imposition of very large strains without the introduction tinuous processing [21-23]. of concomitant changes in the cross-sectional dimen- Processing through the use of SPD techniques pro- sions of the samples.Materials produced by SPD tech- vides the capability of producing large,bulk NS mate- niques usually have grain sizes in the range of 100-1000 rials that may be utilized in structural applications.The nm.However,they have subgrain structures,such as incorporation of ECAP into continuous production subgrains,dislocation cells and X-ray coherent diffrac- techniques also holds out the promise of producing NS materials with a competitive low cost.Current costs to produce 5-20 mm diameter round bar of NS titanium "Corresponding author.Tel:+1-505-667-4029:fax:+1-505-667- and titanium alloys by non-continuous ECAP range 2264. between S50 and $150/kg.These costs are comparable to E-mail address:yzhu@lanlgov (Y.T.Zhu). those for intensive thermomechanical size reductions of 1359-6462/S-see front matter 2004 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved. doi:10.1016/j.scriptamat.2004.05.006

Performance and applications of nanostructured materials produced by severe plastic deformation Yuntian T. Zhu a,*, Terry C. Lowe a , Terence G. Langdon b a Los Alamos National Laboratory, Materials Science; Technology Division, MS G755, Los Alamos, NM 87545, USA b Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA Accepted 4 May 2004 Available online 4 June 2004 Abstract Nanostructured materials produced by severe plastic deformation can be tailored to have both superior performance and superior properties. These materials are attractive for use in a range of applications from biomedical to aerospace industries.
2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Applications; Manufacturing; Nanostructured materials; Severe plastic deformation 1. Introduction Nanostructured (NS) materials are defined as solids having microstructural features in the range of
1–100 nm in at least one dimension [1]. Two complementary approaches have been developed in attempts to synthe￾size NS solids. The first is the ‘‘bottom–up’’ approach in which bulk NS materials are assembled from individual atoms or from nanoscale building blocks such as nano￾particles. Techniques in this category include inert gas condensation [2], electrodeposition [3], and chemical and physical deposition [4]. The second approach is the ‘‘top–down’’ approach in which existing coarse-grained materials are processed to produce substantial grain refinement and a nanostruc￾ture. The most successful ‘‘top–down’’ approach in￾volves the use of severe plastic deformation (SPD) processing in which materials are subjected to the imposition of very large strains without the introduction of concomitant changes in the cross-sectional dimen￾sions of the samples. Materials produced by SPD tech￾niques usually have grain sizes in the range of 100–1000 nm. However, they have subgrain structures, such as subgrains, dislocation cells and X-ray coherent diffrac￾tion domains (crystallites), which are often smaller than 100 nm. Therefore, they can be called NS materials [5]. Several SPD processing methods are now available, including equal-channel angular pressing (ECAP) [6,7], high-pressure torsion [5,8], accumulative roll-bonding (ARB) [9,10], repetitive corrugation and straightening [11,12], and friction stir processing (FSP) [13,14]. At￾tempts have also been made to combine some of these procedures such as ECAP and cold rolling [15], ARB and FSP [16] or ECAP and HPT [17,18]. An overall review of these various techniques suggests that, at least in terms of the commercial viability of the processing route and the nature of the microstructures attained to date, processing by ECAP has at least two advantages that favor its adoption into manufacturing practice. Firstly, it can be scaled up to produce relatively large bulk samples [19,20]. Secondly, several groups have incorporated it into conventional rolling mills for con￾tinuous processing [21–23]. Processing through the use of SPD techniques pro￾vides the capability of producing large, bulk NS mate￾rials that may be utilized in structural applications. The incorporation of ECAP into continuous production techniques also holds out the promise of producing NS materials with a competitive low cost. Current costs to produce 5–20 mm diameter round bar of NS titanium and titanium alloys by non-continuous ECAP range between $50 and $150/kg. These costs are comparable to those for intensive thermomechanical size reductions of * Corresponding author. Tel.: +1-505-667-4029; fax: +1-505-667- 2264. E-mail address: yzhu@lanl.gov (Y.T. Zhu). 1359-6462/$ - see front matter
2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.05.006 Scripta Materialia 51 (2004) 825–830 www.actamat-journals.com

826 Y.T.Zhu et al.Seripta Materialia 51 (2004)825-830 conventional metals as in,for example,the production techniques are contamination-free and porosity-free so of wire.However,they are not competitive for many that they usually have high strength and good ductility. larger-dimension higher-volume applications.As in any It can be shown that SPD processing decreases the commercialization,a successful combination of high ductility to a smaller extent than conventional defor- performance and low cost will be the factor that ulti- mation processes such as rolling,drawing and extrusion. mately determines whether NS materials move from the For example,experiments were conducted to compare laboratory to widespread industrial utilization.The the strength and ductility of the 3004 aluminum alloy fabrication rate is a key determinant of nanomaterial processed by ECAP and cold rolling [26].It was found cost,ranging from a fraction of a nanometer/second that processing by ECAP led to a greater retention of (nm/s)for synthesis using a scanning tunneling micro- ductility than cold rolling.In practice,the higher duc- scope,to hundreds of nm/s for electroless forming,to tility of materials processed by ECAP is a very attractive thousands of nm/s for photo-electroforming,to millions characteristic for structural applications. of nm/s for conventional machining and forming.Thus, Some NS materials produced by SPD have been SPD has a significant potential for producing NS found to have an extraordinary combination of both materials at rates,and therefore at costs,comparable to high strength and high ductility.For example,pure Cu conventional material production methods. processed via ECAP for 16 passes with a back-pressure In this paper,we shall focus on the performance and has a ductility close to that of coarse-grained Cu while at possible applications of NS materials produced via SPD. the same time having a yield strength that is several More specifically,we shall focus on their mechanical times higher [27].High strength and good ductility properties and structural applications.Nanostructured rarely exist simultaneously in any material.Therefore, and amorphous materials produced by other methods this combination is very attractive for advanced struc- and their physical properties will also be discussed tural applications in areas such as aerospace and briefly. sporting goods.Unfortunately,the mechanism for achieving such good mechanical properties is not yet understood although it is generally recognized that the 2.Performance mechanical behavior of materials is determined by the deformation mechanisms.which in turn are controlled NS materials have unique mechanical and physical by the nature of the microstructures. properties which are derived from their unique micro- Some progress has been made recently in under- structures.These properties make them attractive for standing these deformation mechanisms.For example, many potential commercial applications. the emission of partial dislocations from grain bound- aries and the occurrence of stacking faults and defor- 2.1.Strength and ductility mation twinning in NS aluminum provides a sharp contrast to the behavior of coarse-grained aluminum The strength of a coarse-grained material usually where twinning is absent [28,29].Another example is NS follows the well-known Hall-Petch relationship, copper,which was found to twin abundantly when de- =0o+Kd-12,where o is the strength,d is the grain formed under HPT at room temperature and low strain size,and go and K are constants.NS materials deviate rate [30].In contrast,coarse-grained copper did not from this relationship.with slower strength increase deform by twinning under the same deformation con- (smaller K)as the grain size decreases.Below a certain dition [31].The low ductility of NS materials has been critical grain size,an inverse Hall-Petch relationship is attributed to their low work hardening because their observed [241.To have the desired combination of high small grain sizes do not accommodate further disloca- strength and high ductility for structural applications, tion accumulation [32].The twinning could be utilized to smaller grains are not always desired.Ductility usually increase work hardening of NS materials and to conse- decreases with decreasing grain size in NS materials.NS quently improve their ductility. metals and alloys with grain sizes less than 20 nm or amorphous alloys may have both lower strength and 2.2.Other mechanical properties lower ductility than materials with larger grain sizes. There exists an optimum grain size range in which a NS Although strength and ductility are the two most material has both high strength and good ductility. important mechanical properties,there are other The processing method also affects the strength and important properties for structural applications includ- ductility.NS materials produced by consolidation of ing fracture toughness,fatigue strength and wear resis- nanopowders usually are very brittle due to defects such tance. as oxidation,trapped gas and porosity [25].Electrode- To date.the fracture toughness has not been studied posited NS films may also be brittle due to impurities in NS samples because the measurements require large from the electrolyte.NS materials produced by SPD samples in order to reach the required plane strain

conventional metals as in, for example, the production of wire. However, they are not competitive for many larger-dimension higher-volume applications. As in any commercialization, a successful combination of high performance and low cost will be the factor that ulti￾mately determines whether NS materials move from the laboratory to widespread industrial utilization. The fabrication rate is a key determinant of nanomaterial cost, ranging from a fraction of a nanometer/second (nm/s) for synthesis using a scanning tunneling micro￾scope, to hundreds of nm/s for electroless forming, to thousands of nm/s for photo-electroforming, to millions of nm/s for conventional machining and forming. Thus, SPD has a significant potential for producing NS materials at rates, and therefore at costs, comparable to conventional material production methods. In this paper, we shall focus on the performance and possible applications of NS materials produced via SPD. More specifically, we shall focus on their mechanical properties and structural applications. Nanostructured and amorphous materials produced by other methods and their physical properties will also be discussed briefly. 2. Performance NS materials have unique mechanical and physical properties which are derived from their unique micro￾structures. These properties make them attractive for many potential commercial applications. 2.1. Strength and ductility The strength of a coarse-grained material usually follows the well-known Hall–Petch relationship, r ¼ r0 þ Kd1=2, where r is the strength, d is the grain size, and r0 and K are constants. NS materials deviate from this relationship, with slower strength increase (smaller K) as the grain size decreases. Below a certain critical grain size, an inverse Hall–Petch relationship is observed [24]. To have the desired combination of high strength and high ductility for structural applications, smaller grains are not always desired. Ductility usually decreases with decreasing grain size in NS materials. NS metals and alloys with grain sizes less than 20 nm or amorphous alloys may have both lower strength and lower ductility than materials with larger grain sizes. There exists an optimum grain size range in which a NS material has both high strength and good ductility. The processing method also affects the strength and ductility. NS materials produced by consolidation of nanopowders usually are very brittle due to defects such as oxidation, trapped gas and porosity [25]. Electrode￾posited NS films may also be brittle due to impurities from the electrolyte. NS materials produced by SPD techniques are contamination-free and porosity-free so that they usually have high strength and good ductility. It can be shown that SPD processing decreases the ductility to a smaller extent than conventional defor￾mation processes such as rolling, drawing and extrusion. For example, experiments were conducted to compare the strength and ductility of the 3004 aluminum alloy processed by ECAP and cold rolling [26]. It was found that processing by ECAP led to a greater retention of ductility than cold rolling. In practice, the higher duc￾tility of materials processed by ECAP is a very attractive characteristic for structural applications. Some NS materials produced by SPD have been found to have an extraordinary combination of both high strength and high ductility. For example, pure Cu processed via ECAP for 16 passes with a back-pressure has a ductility close to that of coarse-grained Cu while at the same time having a yield strength that is several times higher [27]. High strength and good ductility rarely exist simultaneously in any material. Therefore, this combination is very attractive for advanced struc￾tural applications in areas such as aerospace and sporting goods. Unfortunately, the mechanism for achieving such good mechanical properties is not yet understood although it is generally recognized that the mechanical behavior of materials is determined by the deformation mechanisms, which in turn are controlled by the nature of the microstructures. Some progress has been made recently in under￾standing these deformation mechanisms. For example, the emission of partial dislocations from grain bound￾aries and the occurrence of stacking faults and defor￾mation twinning in NS aluminum provides a sharp contrast to the behavior of coarse-grained aluminum where twinning is absent [28,29]. Another example is NS copper, which was found to twin abundantly when de￾formed under HPT at room temperature and low strain rate [30]. In contrast, coarse-grained copper did not deform by twinning under the same deformation con￾dition [31]. The low ductility of NS materials has been attributed to their low work hardening because their small grain sizes do not accommodate further disloca￾tion accumulation [32]. The twinning could be utilized to increase work hardening of NS materials and to conse￾quently improve their ductility. 2.2. Other mechanical properties Although strength and ductility are the two most important mechanical properties, there are other important properties for structural applications includ￾ing fracture toughness, fatigue strength and wear resis￾tance. To date, the fracture toughness has not been studied in NS samples because the measurements require large samples in order to reach the required plane strain 826 Y.T. Zhu et al. / Scripta Materialia 51 (2004) 825–830

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

828 Y.T.Zhu et al.Seripta Materialia 51 (2004)825-830 822…25%1里-502% 660.68Mp 6 25 Los Alamos Los Alamos Fig.2.Plate-implants for bone osteosynthesis made of nanostructured Fig.1.An ECAP-processed Ti rod,which is about 50 mm in diameter Ti. and 170 mm in length.The unit on the ruler is inch. a new material.One specific example is the use of NS 3.1.Applications driven by superior properties pure Ti for dental implants.Typical implants have diameters greater than 3 mm because the cyclic loads It is generally easier to process the lower strength associated with chewing can reach levels that push metals such as Al or Cu.Also,a more thorough conventional Ti towards the fatigue performance limit. However.the small lower front teeth can be so closely knowledge base is currently available for these metals [18].For the SPD processing of Al and Cu,special spaced that they require smaller diameter implants.By applications requiring limited-production volumes will introducing NS Ti possessing significantly higher fatigue strength,it will be possible to use 2 mm or smaller undoubtedly initiate the first applications of these materials.Later,after the economics of SPD processing diameter implants to replace the lower front teeth.For are well established,it is reasonable to assume that high- dental implants,and many other medical devices,the volume production will become attractive.Lightweight virtues of superior material properties are determined structures of Al alloys,as in weight-sensitive products not through specific materials properties requirements or testing but through the fabrication of devices from such as aircraft,bicycles,automobiles and boats,will probably appear first.Limited production demonstra- the new material and the subsequent simulated or in- tions for components manufactured in small volumes service evaluation of the performance of the device. will lead the way.One example is bicycle components. Thus it is critical to be able to fabricate manufacturing- For both mountain bikes and road racers,the added scale sizes and quantities of NS materials for evaluation strength achievable in the SPD-processed alloy equates in specific applications. directly to weight savings in the structural frame tubing The high strength of NS materials also makes them and hardware components such as the gearing,pedals, ideal for making micro-devices and this is an exciting shifters,rims and spokes.Since the cost of bicycle and rapidly developing field.For example,high strength frames purchased by enthusiasts can often exceed $3000, micro springs and gears have been made of NS Ni-Mn there is ample opportunity for incorporating high-per- alloys via electrodeposition [3]. formance materials into these specialist applications. Other early uses of SPD metals will develop for 3.2.Applications driven by superior manufacturability applications where there are strong market drivers.For example,there is a high level of competition and inno- 3.2.1.Machinability and forgability vation with advanced materials in the medical device For products that are directly machined to shape industry and this is supported by the growing societal from SPD-processed mill products,the feed rates and interest in products that extend or enhance the quality of cutting depths can be increased because of the manner in life.One example is plate-implants for bone osteosyn- which the deformation occurs under the machining thesis made of NS Ti (see Fig.2)[20].The enhanced conditions.Processing by SPD leads to a smoother fatigue performance that is possible in SPD metals is surface finish and a reduction in tool wear.For some particularly attractive for Ti and Ti alloys used for specialized products,the largest portion of the cost may prosthetics.Specific requirements for medical devices be associated with the machining.For example,the ratio are wide ranging,depending upon the impact that the of the machining cost to the material cost is greater than enhanced properties may have on the market advantage a factor of 10 for some sporting goods products,thereby that is imparted to a given product.Generally speaking, creating a significant incentive for reducing the an enhancement of greater than 25%over the properties machining cost through material substitution.In some of conventional metals is necessary to motivate adopting cases the superior surface finish from machining of the

3.1. Applications driven by superior properties It is generally easier to process the lower strength metals such as Al or Cu. Also, a more thorough knowledge base is currently available for these metals [18]. For the SPD processing of Al and Cu, special applications requiring limited-production volumes will undoubtedly initiate the first applications of these materials. Later, after the economics of SPD processing are well established, it is reasonable to assume that high￾volume production will become attractive. Lightweight structures of Al alloys, as in weight-sensitive products such as aircraft, bicycles, automobiles and boats, will probably appear first. Limited production demonstra￾tions for components manufactured in small volumes will lead the way. One example is bicycle components. For both mountain bikes and road racers, the added strength achievable in the SPD-processed alloy equates directly to weight savings in the structural frame tubing and hardware components such as the gearing, pedals, shifters, rims and spokes. Since the cost of bicycle frames purchased by enthusiasts can often exceed $3000, there is ample opportunity for incorporating high-per￾formance materials into these specialist applications. Other early uses of SPD metals will develop for applications where there are strong market drivers. For example, there is a high level of competition and inno￾vation with advanced materials in the medical device industry and this is supported by the growing societal interest in products that extend or enhance the quality of life. One example is plate-implants for bone osteosyn￾thesis made of NS Ti (see Fig. 2) [20]. The enhanced fatigue performance that is possible in SPD metals is particularly attractive for Ti and Ti alloys used for prosthetics. Specific requirements for medical devices are wide ranging, depending upon the impact that the enhanced properties may have on the market advantage that is imparted to a given product. Generally speaking, an enhancement of greater than 25% over the properties of conventional metals is necessary to motivate adopting a new material. One specific example is the use of NS pure Ti for dental implants. Typical implants have diameters greater than 3 mm because the cyclic loads associated with chewing can reach levels that push conventional Ti towards the fatigue performance limit. However, the small lower front teeth can be so closely spaced that they require smaller diameter implants. By introducing NS Ti possessing significantly higher fatigue strength, it will be possible to use 2 mm or smaller diameter implants to replace the lower front teeth. For dental implants, and many other medical devices, the virtues of superior material properties are determined not through specific materials properties requirements or testing but through the fabrication of devices from the new material and the subsequent simulated or in￾service evaluation of the performance of the device. Thus it is critical to be able to fabricate manufacturing￾scale sizes and quantities of NS materials for evaluation in specific applications. The high strength of NS materials also makes them ideal for making micro-devices and this is an exciting and rapidly developing field. For example, high strength micro springs and gears have been made of NS Ni–Mn alloys via electrodeposition [3]. 3.2. Applications driven by superior manufacturability 3.2.1. Machinability and forgability For products that are directly machined to shape from SPD-processed mill products, the feed rates and cutting depths can be increased because of the manner in which the deformation occurs under the machining conditions. Processing by SPD leads to a smoother surface finish and a reduction in tool wear. For some specialized products, the largest portion of the cost may be associated with the machining. For example, the ratio of the machining cost to the material cost is greater than a factor of 10 for some sporting goods products, thereby creating a significant incentive for reducing the machining cost through material substitution. In some cases the superior surface finish from machining of the Fig. 1. An ECAP-processed Ti rod, which is about 50 mm in diameter and 170 mm in length. The unit on the ruler is inch. Fig. 2. Plate-implants for bone osteosynthesis made of nanostructured Ti. 828 Y.T. Zhu et al. / Scripta Materialia 51 (2004) 825–830

Y.T.Zhu et al.Scripta Materialia 51 (2004)825-830 829 SPD-processed metals may obviate the need for sub- perature superplasticity [45]in bulk nanostructured sequent surface finishing steps.This will reduce the materials,where high strain rates refer to the tensile manufacturing cost by eliminating or simplifying the testing of samples at rates at and above 10-2s-!and low processing steps.Forging is used to create product temperatures refer to tensile testing at homologous shapes in the aerospace and automotive industry and temperatures below 0.5Tm.For example,there are re- there is evidence that the forging temperatures can be ports of tensile elongations of up to >2000%at a strain significantly reduced when forging SPD-processed alu- rate of 1 s-in a Zn-Al alloy processed by ECAP [46] minum alloys for aerospace applications.In addition, and the occurrence of superplasticity at homologous the times for subsequent heat treatments may be re- temperatures as low as ~0.36Tm for an electrodeposited duced by as much as 50%.Thus,for alloys with heat nickel [45]. treatments in excess of 12 h,the energy and time savings The production of bulk nanostructured alloys in are substantial. sheet form,with ultrafine grains that are fairly stable at elevated temperatures,has the potential to expand the 3.2.2.Formability through superplasticity superplastic forming niche into a processing regime that The NS alloys processed by SPD can be formed su- will be effective in producing components for a very wide perplastically at lower temperatures and faster rates range of commercial applications.The recent demon- than is possible in conventional superplastic alloys. stration of the ECAP processing of plate samples [47] Superplasticity is a flow process in which polycrystalline suggests that it may be a fairly easy task to produce materials exhibit high elongations prior to ultimate superplastic NS materials that can be readily utilized in failure.This type of flow is the characteristic feature of forming operations.However,even in the absence of the superplastic forming industry in which complex sheet production,there are several potential applications components,often having multiple curved surfaces,are for these materials in bulk form:an example of current formed from superplastic sheet metals.The essential interest is the production of superplastic seismic damp- requirements for achieving a superplastic forming ing devices [48]. capability are small grain sizes,typically less than ~10 um,and high forming temperatures,typically above 0.5 Tm,where Tm is the absolute melting point of the material.At the present time,the superplastic forming 4.Summary and conclusions industry occupies a small but viable cost-effective niche through the production of high-cost low-volume com- 1.Processing through the application of SPD is attrac- ponents associated primarily with the aerospace,archi- tive for the production of bulk NS materials.These tectural and sports industries [41].Expansion beyond materials can be tailored to exhibit both superior per- this niche,into automotive and other high-volume formance and superior properties. applications,is currently restricted by the slow strain 2.A primary advantage of SPD is the development of rates involved in the forming process(typically ~10-3 materials having good machinability,forgability, s-)and the consequent long forming times (~20-30 and formability at potentially low processing cost. min)associated with the production of each separate This makes these NS materials especially attractive component. for use in specialized structural applications such The introduction of bulk NS materials provides a as medical implants,biomedical devices,and high- potential for overcoming the inherent limitations asso- performance bicycles.In the longer term,when ciated with conventional coarse-grained superplastic continuous-processing methods are developed,it is materials.Thus,it is now well established,both theo- reasonable to anticipate large-scale applications in retically and experimentally [42,43],that the rate of flow the automotive and other fields. within the superplastic regime varies inversely with the 3.Amorphous and NS materials also have unique phys- grain size raised to a power that is close to ~2.It is ical properties that are attractive for optical and elec- anticipated,therefore,that a decrease in the grain size trical applications.The high strength of NS materials by one order of magnitude will lead to an increase in the makes them ideal for micro-devices optimal superplastic forming rate by approximately two orders of magnitude and thus,in effect,the total forming time will be reduced to ~20-30 s.It can be shown also that this reduction in grain size will lead to the advent of Acknowledgements a superplastic forming capability which occurs at lower temperatures than those generally associated with con- This work was supported by the US Department of ventional superplastic flow.Early experimental results Energy IPP program(YTZ TCL)and by the National provided very clear demonstrations of the occurrence of Science Foundation under Grant No.DMR-0243331 both high strain rate superplasticity [44]and low tem- (TGL)

SPD-processed metals may obviate the need for sub￾sequent surface finishing steps. This will reduce the manufacturing cost by eliminating or simplifying the processing steps. Forging is used to create product shapes in the aerospace and automotive industry and there is evidence that the forging temperatures can be significantly reduced when forging SPD-processed alu￾minum alloys for aerospace applications. In addition, the times for subsequent heat treatments may be re￾duced by as much as 50%. Thus, for alloys with heat treatments in excess of 12 h, the energy and time savings are substantial. 3.2.2. Formability through superplasticity The NS alloys processed by SPD can be formed su￾perplastically at lower temperatures and faster rates than is possible in conventional superplastic alloys. Superplasticity is a flow process in which polycrystalline materials exhibit high elongations prior to ultimate failure. This type of flow is the characteristic feature of the superplastic forming industry in which complex components, often having multiple curved surfaces, are formed from superplastic sheet metals. The essential requirements for achieving a superplastic forming capability are small grain sizes, typically less than
10 lm, and high forming temperatures, typically above
0.5 Tm, where Tm is the absolute melting point of the material. At the present time, the superplastic forming industry occupies a small but viable cost-effective niche through the production of high-cost low-volume com￾ponents associated primarily with the aerospace, archi￾tectural and sports industries [41]. Expansion beyond this niche, into automotive and other high-volume applications, is currently restricted by the slow strain rates involved in the forming process (typically
103 s1) and the consequent long forming times (
20–30 min) associated with the production of each separate component. The introduction of bulk NS materials provides a potential for overcoming the inherent limitations asso￾ciated with conventional coarse-grained superplastic materials. Thus, it is now well established, both theo￾retically and experimentally [42,43], that the rate of flow within the superplastic regime varies inversely with the grain size raised to a power that is close to
2. It is anticipated, therefore, that a decrease in the grain size by one order of magnitude will lead to an increase in the optimal superplastic forming rate by approximately two orders of magnitude and thus, in effect, the total forming time will be reduced to
20–30 s. It can be shown also that this reduction in grain size will lead to the advent of a superplastic forming capability which occurs at lower temperatures than those generally associated with con￾ventional superplastic flow. Early experimental results provided very clear demonstrations of the occurrence of both high strain rate superplasticity [44] and low tem￾perature superplasticity [45] in bulk nanostructured materials, where high strain rates refer to the tensile testing of samples at rates at and above 102 s1 and low temperatures refer to tensile testing at homologous temperatures below 0.5Tm. For example, there are re￾ports of tensile elongations of up to >2000% at a strain rate of 1 s1 in a Zn–Al alloy processed by ECAP [46] and the occurrence of superplasticity at homologous temperatures as low as
0.36Tm for an electrodeposited nickel [45]. The production of bulk nanostructured alloys in sheet form, with ultrafine grains that are fairly stable at elevated temperatures, has the potential to expand the superplastic forming niche into a processing regime that will be effective in producing components for a very wide range of commercial applications. The recent demon￾stration of the ECAP processing of plate samples [47] suggests that it may be a fairly easy task to produce superplastic NS materials that can be readily utilized in forming operations. However, even in the absence of sheet production, there are several potential applications for these materials in bulk form: an example of current interest is the production of superplastic seismic damp￾ing devices [48]. 4. Summary and conclusions 1. Processing through the application of SPD is attrac￾tive for the production of bulk NS materials. These materials can be tailored to exhibit both superior per￾formance and superior properties. 2. Aprimary advantage of SPD is the development of materials having good machinability, forgability, and formability at potentially low processing cost. This makes these NS materials especially attractive for use in specialized structural applications such as medical implants, biomedical devices, and high￾performance bicycles. In the longer term, when continuous-processing methods are developed, it is reasonable to anticipate large-scale applications in the automotive and other fields. 3. Amorphous and NS materials also have unique phys￾ical properties that are attractive for optical and elec￾trical applications. The high strength of NS materials makes them ideal for micro-devices. Acknowledgements This work was supported by the US Department of Energy IPP program (YTZ & TCL) and by the National Science Foundation under Grant No. DMR-0243331 (TGL). Y.T. Zhu et al. / Scripta Materialia 51 (2004) 825–830 829

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