REPORTS Hardening by Annealing and almost brittle.This is in contrast to the expected behavior after annealing-a decrease in strength and an increase in elongation or ductility Softening by Deformation in An increase in flow stress during annealing of a deformed metal is typical if the metal con- Nanostructured Metals tains alloying elements in solid solution that precipitate when the metal is annealed.This so- Xiaoxu Huang,1*Niels Hansen,1 Nobuhiro Tsuji? called precipitation hardening was not expected in the aluminum.which had a purity of 99.2% We observe that a nanostructured metal can be hardened by annealing and softened when However,other stable impurities might have subsequently deformed,which is in contrast to the typical behavior of a metal.Microstructural dissolved during processing and reprecipitated investigation points to an effect of the structural scale on fundamental mechanisms of dislocation- during annealing.To prove that this was not dislocation and dislocation-interface reactions,such that heat treatment reduces the generation the case,we carried out an experiment using and interaction of dislocations,leading to an increase in strength and a reduction in ductility. 99.99%pure aluminum as the starting material. A subsequent deformation step may restore the dislocation structure and facilitate the yielding As with the 99.2%pure aluminum,specimens process when the metal is stressed.As a consequence,the strength decreases and the ductility were produced by a six-cycle ARB processing increases.These observations suggest that for materials such as the nanostructured aluminum to an equivalent strain of 4.8,followed by an- studied here,deformation should be used as an optimizing procedure instead of annealing. nealing at 150C for 30 min.The specimens were tested under the same conditions.The rom the beginning of our civilization,the changes in properties and structure when results showed that,as expected,both the yield metalworkers have known that when a a nanostructured metal is annealed,and to use stress and the UTS were substantially reduced metal becomes too hard-for example, such findings to inspire the development of relative to the 99.2%Al,but the key phenome- when forged-it can be softened by annealing. new optimization processes. na (ie.,the hardening by annealing and the By choosing the right combination of annealing We investigated annealing behavior in a decrease in ductility)were reproduced.For temperature and time,a desired combination of fully dense,nanostructured aluminum of com- example,an increase of about 9%in the yield strength and ductility can be achieved.The cur- mercial purity (99.2%)that was prepared by a stress was observed after annealing.Therefore, rent focus is on nanostructured metals that have high-strain rolling deformation known as ac- the dissolution and reprecipitation of impurities, extreme strength but limited ductility and form- cumulative roll bonding (ARB)(/2).Alumi- if it occurred,did not contribute to the annealing- ability,which reduces their applicability.The num sheets of a final thickness of I mm were induced property changes. extreme strength is obtained through a structural produced by a six-cycle ARB processing to We used transmission electron microscopy refinement of the grains down to nanometer an equivalent strain of 4.8(/3).The ARB- (TEM)and high-resolution TEM(HRTEM)to dimensions,and an optimization of ductility has processed state showed a weak crystallo- characterize the structural parameters of ARB been sought through annealing.It has been graphic texture and a lamellar microstructure samples before and after annealing.The initial shown (/that when annealing under conditions of dislocation boundaries and grain boundaries structure (Fig.2A)is delineated by lamellar that produce a structure with bimodal distribu- characteristic of high-strain rolling of metals boundaries parallel to the rolling direction(RD) tion of grain sizes from nanometer to microme- and alloys.The lamellar boundaries are parallel and interconnecting boundaries parallel to the ter scales,the strength of nanostructured metals to the rolling plane,with an average spacing of normal direction (ND).Table 1 shows the quan- decreases slightly but the deformation-induced 180 nm.This lamellar morphology and the rel- tified structural parameters that are thought to hardening (i.e.,work hardening)of the coarse atively coarse boundary spacing ensure the contribute to the mechanical properties.Slight grains in the structure gives ductility.It has been elimination of grain boundary sliding during coarsening occurred during annealing (Fig.2B) also discovered that during annealing at low tensile testing.Tensile specimens of gauge di- for both the lamellar boundary spacing D and temperatures,which does not cause excessive mensions 10 mm by 5 mm were machined from the interconnecting boundary spacing Dic and heterogeneous coarsening of the nano- the sheets and tested at room temperature.The which produced a reduction of boundary surface structure,nanostructured metals may harden engineering stress-strain curve of the ARB area per unit volume. rather than soften-as observed,for example. sample (Fig.1,curve 1)shows a very high yield Statistical measurements of misorientation in metals produced by inert gas condensation stress (259 MPa)and ultimate tensile stress angles across the lamellar boundaries and inter- (24),electrodeposition (5,6),and plastic de- (UTS,334 MPa)and a reasonably good tensile connecting boundaries were made by Kikuchi formation to very high strains (7-10).Associ- ductility,as expressed by total elongation (7%) diffraction.Misorientation angles in both sam- ated with the hardening,a decrease in the tensile and uniform elongation(1.8%).This yield stress ples show a bimodal distribution,with one peak ductility has been reported (5,9,10)where ten- is nearly 10 times that of a coarse-grained ma- located in the range below 3 and the other sile tests were carried out to evaluate the mechan- terial with a grain size of 50 um (~28 MPa). located between40°and55°.More than60%of ical properties.This unusual,annealing-induced When the ARB sample was annealed at 150C the boundaries were high-angle boundaries hardening has been related to changes in struc- for 30 min,the yield stress increased by 8.9%to (>15)in the ARB samples both before and tural characteristics [e.g.,the grain boundary 281 MPa (curve 2,Fig.1)and the total elonga- after annealing.This high density,in combina- structure (//)]but various hypotheses have tion decreased markedly,making the material tion with the small boundary spacing.results in not been verified.The present work has two objectives:to improve our understanding of Table 1.Structural parameters in samples of different conditions.f,fraction of boundaries with Center for Fundamental Research:Metal Structures in misorientation angles less than 3;s fraction of boundaries with misorientation angles Four Dimensions,Materials Research Department,Rise between 3 and 15;fs,fraction of high-angle boundaries (>15). National Laboratory,DK 4000 Roskilde,Denmark.De- partment of Adaptive Machine Systems,Graduate School Sample P。(m-2) DLB (nm))Dice(nm）fes(%方-1s(%) f15(%） of Engineering,Osaka University,2-1 Yamada-Oka,Suita, ARB 1.33×1014 180 600 17.5 16.2 66.3 0saka565-0871,]apan. ARB annealed 0.53×1014 225 640 12.8 23.3 63.9 *To whom correspondence should be addressed.E-mail: at150°cfor30min xiaoxu.huang@risoe.dk www.sciencemag.org SCIENCE VOL 312 14 APRIL 2006 249Hardening by Annealing and Softening by Deformation in Nanostructured Metals Xiaoxu Huang,1 * Niels Hansen,1 Nobuhiro Tsuji2 We observe that a nanostructured metal can be hardened by annealing and softened when subsequently deformed, which is in contrast to the typical behavior of a metal. Microstructural investigation points to an effect of the structural scale on fundamental mechanisms of dislocation￾dislocation and dislocation-interface reactions, such that heat treatment reduces the generation and interaction of dislocations, leading to an increase in strength and a reduction in ductility. A subsequent deformation step may restore the dislocation structure and facilitate the yielding process when the metal is stressed. As a consequence, the strength decreases and the ductility increases. These observations suggest that for materials such as the nanostructured aluminum studied here, deformation should be used as an optimizing procedure instead of annealing. From the beginning of our civilization, metalworkers have known that when a metal becomes too hard—for example, when forged—it can be softened by annealing. By choosing the right combination of annealing temperature and time, a desired combination of strength and ductility can be achieved. The cur￾rent focus is on nanostructured metals that have extreme strength but limited ductility and form￾ability, which reduces their applicability. The extreme strength is obtained through a structural refinement of the grains down to nanometer dimensions, and an optimization of ductility has been sought through annealing. It has been shown (1) that when annealing under conditions that produce a structure with bimodal distribu￾tion of grain sizes from nanometer to microme￾ter scales, the strength of nanostructured metals decreases slightly but the deformation-induced hardening (i.e., work hardening) of the coarse grains in the structure gives ductility. It has been also discovered that during annealing at low temperatures, which does not cause excessive and heterogeneous coarsening of the nano￾structure, nanostructured metals may harden rather than soften—as observed, for example, in metals produced by inert gas condensation (2–4), electrodeposition (5, 6), and plastic de￾formation to very high strains (7–10). Associ￾ated with the hardening, a decrease in the tensile ductility has been reported (5, 9, 10) where ten￾sile tests were carried out to evaluate the mechan￾ical properties. This unusual, annealing-induced hardening has been related to changes in struc￾tural characteristics Ee.g., the grain boundary structure (11)^, but various hypotheses have not been verified. The present work has two objectives: to improve our understanding of the changes in properties and structure when a nanostructured metal is annealed, and to use such findings to inspire the development of new optimization processes. We investigated annealing behavior in a fully dense, nanostructured aluminum of com￾mercial purity (99.2%) that was prepared by a high-strain rolling deformation known as ac￾cumulative roll bonding (ARB) (12). Alumi￾num sheets of a final thickness of 1 mm were produced by a six-cycle ARB processing to an equivalent strain of 4.8 (13). The ARB￾processed state showed a weak crystallo￾graphic texture and a lamellar microstructure of dislocation boundaries and grain boundaries characteristic of high-strain rolling of metals and alloys. The lamellar boundaries are parallel to the rolling plane, with an average spacing of 180 nm. This lamellar morphology and the rel￾atively coarse boundary spacing ensure the elimination of grain boundary sliding during tensile testing. Tensile specimens of gauge di￾mensions 10 mm by 5 mm were machined from the sheets and tested at room temperature. The engineering stress-strain curve of the ARB sample (Fig. 1, curve 1) shows a very high yield stress (259 MPa) and ultimate tensile stress (UTS, 334 MPa) and a reasonably good tensile ductility, as expressed by total elongation (7%) and uniform elongation (1.8%). This yield stress is nearly 10 times that of a coarse-grained ma￾terial with a grain size of 50 mm (È28 MPa). When the ARB sample was annealed at 150-C for 30 min, the yield stress increased by 8.9% to 281 MPa (curve 2, Fig. 1) and the total elonga￾tion decreased markedly, making the material almost brittle. This is in contrast to the expected behavior after annealing—a decrease in strength and an increase in elongation or ductility. An increase in flow stress during annealing of a deformed metal is typical if the metal con￾tains alloying elements in solid solution that precipitate when the metal is annealed. This so￾called precipitation hardening was not expected in the aluminum, which had a purity of 99.2%. However, other stable impurities might have dissolved during processing and reprecipitated during annealing. To prove that this was not the case, we carried out an experiment using 99.99% pure aluminum as the starting material. As with the 99.2% pure aluminum, specimens were produced by a six-cycle ARB processing to an equivalent strain of 4.8, followed by an￾nealing at 150-C for 30 min. The specimens were tested under the same conditions. The results showed that, as expected, both the yield stress and the UTS were substantially reduced relative to the 99.2% Al, but the key phenome￾na (i.e., the hardening by annealing and the decrease in ductility) were reproduced. For example, an increase of about 9% in the yield stress was observed after annealing. Therefore, the dissolution and reprecipitation of impurities, if it occurred, did not contribute to the annealing￾induced property changes. We used transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) to characterize the structural parameters of ARB samples before and after annealing. The initial structure (Fig. 2A) is delineated by lamellar boundaries parallel to the rolling direction (RD) and interconnecting boundaries parallel to the normal direction (ND). Table 1 shows the quan￾tified structural parameters that are thought to contribute to the mechanical properties. Slight coarsening occurred during annealing (Fig. 2B) for both the lamellar boundary spacing DLB and the interconnecting boundary spacing DICB, which produced a reduction of boundary surface area per unit volume. Statistical measurements of misorientation angles across the lamellar boundaries and inter￾connecting boundaries were made by Kikuchi diffraction. Misorientation angles in both sam￾ples show a bimodal distribution, with one peak located in the range below 3- and the other located between 40- and 55-. More than 60% of the boundaries were high-angle boundaries (915-) in the ARB samples both before and after annealing. This high density, in combina￾tion with the small boundary spacing, results in 1 Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Risø National Laboratory, DK 4000 Roskilde, Denmark. 2 De￾partment of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan. *To whom correspondence should be addressed. E-mail: xiaoxu.huang@risoe.dk Table 1. Structural parameters in samples of different conditions. f G3- , fraction of boundaries with misorientation angles less than 3-; f 3–15- , fraction of boundaries with misorientation angles between 3- and 15-; f 915- , fraction of high-angle boundaries (915-). Sample r0 (mj2) DLB (nm) DICB (nm) f <3- (%) f 3–15- (%) f >15- (%) ARB 1.33
1014 180 600 17.5 16.2 66.3 ARB annealed at 150-C for 30 min 0.53
1014 225 640 12.8 23.3 63.9 REPORTS www.sciencemag.org SCIENCE VOL 312 14 APRIL 2006 249