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 strainconventional 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