Materials Science and Engineering A 463 (2007) 7477

High strength and ductility of nanostructured Al-based alloy, prepared by high-pressure technique
Nikolay A. Krasilnikov a, , A. Sharafutdiniv b
a

Ulyanovsk State University, 42 L. Tolstoy Street, Ulyanovsk 432970, Russia b Institute of Physics of Advanced Materials, USATU, Ufa, Russia

Received 14 March 2006; received in revised form 18 August 2006; accepted 19 August 2006

Abstract An Al-base alloy 2024 with homogeneous structure and grain size of approximately 70 nm was obtained using a high-pressure torsion process at 6 GPa pressure and ve torsional rotations between anvils at room temperature. The resulting structure and mechanical properties of this nanostructured alloy 2024 were investigated in this study. This nanostructured alloy demonstrated a very high ultimate tensile stress (>1 GPa) in testing at room temperature, and superplastic behavior at temperatures >300 C. After superplastic deformation, the microhardness of the nanostructured alloy was 1.5 GPa, 20% more than the microhardness of the coarse-grained parent material after standard treatment. The ability to achievement both high strength and good ductility in nanostructured metals and alloys makes them attractive for applications in industry, particularly, for micro-systems and for high strength details in components with complex geometry obtained with superplastic deformation. 2006 Elsevier B.V. All rights reserved.
Keywords: High-pressure torsion; Nanostructured alloy

1. Introduction Bulk nanostructured metals and alloys are characterized by a grain size in the nanometer (10100 nm) or submicrometer (100500 nm) range and attract considerable attention due to their excellent physical and mechanical properties [1]. As a rule, the aged Al-based alloys possess high strength properties after standard treatment of quenching and aging due to hardening by nanoparticles of second phase [2]. Applying torsional strain at high-pressure (about 10 GPa) produces nanostructured Al alloys with high strength due to signicant microstructure renement and the formation of nonequilibrium state of grain boundaries [3]. The inuence of severe plastic deformation (SPD) on a microstructure and the resulting mechanical properties of Al alloys are more complex than for pure metals because during SPD the phase transformations and processes of precipitation of a solid solution can take place [4]. It is known that even small deformations (1015%) at room temperature of the quench hardened Al alloys reduce the temperature for the precipitation sequence to occur [5]. Acceleration of precipitation kinetics, caused by deformation, can either increase or decrease the dura

bility of the some Al alloys depending on the conditions of deformation and thermal processing [2,4]. SPD changes the conditions for the phase transformations and kinetics of aging even more strongly. Thus, achievement of maximum strength during SPD processing of the aging Al alloys demands optimization of contributions from both dispersion hardening, and structure renement. The ductility of SPD processed alloys at room temperature is low and generally does not exceed 12% strain. After annealing the ductility of SPD processed alloy increases, which leads to a decrease in strength, intensive recrystallization processes and fast grain growth [5]. The structure and properties of SPD processed metals and alloys depend not only on the process parameters of pressure, strain degree and temperature, but also on the phase transformation during the SPD process and subsequent heat treatment. However, detailed investigations of inuence of SPD parameters and heat treatment on transformation of structure and properties of aging Al alloys are beyond the scope of this study. 2. Experimental In our experiments we used an industrial Al-based alloy 2024. Table 1 summarizes the chemical composition of this alloy.

Corresponding author. Tel.: +7 8422 320680; fax: +7 8422 412340. E-mail address: nick@sv.uven.ru (N.A. Krasilnikov).

0921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.08.117

N.A. Krasilnikov, A. Sharafutdiniv / Materials Science and Engineering A 463 (2007) 7477 Table 1 Chemical composition of 2024 alloy Element Si Actual (wt.%) Nominal (wt.%) 0.07 < 0.5 Fe 0.17 <0.5 Cu 4.6 3.84.9 Mn 0.64 0.30.9 Mg 1.5 1.21.8 Cr 0.01 <0.10 Zn 0.15 <0.25 Ti

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0.03 <0.15

One disadvantage of specimens prepared by high-pressure torsion (HPT) processing has been their small thickness. The possibility of increasing the thickness of the nanostructured sample of Al alloy 2024 was investigated using a modied HPT die under a pressure of 6 GPa as illustrated in Fig. 1. The pressure was selected to avoid sliding between the surfaces of the anvils and sample during torsion. The bottom anvil was made with a recess of 0.5 mm (Fig. 1) to accommodate a 10 mm diameter sample. Initially the samples were subjected to annealing at a temperature of 495 C for 30 min with subsequent immersion in water at room temperature. Then, ve torsional turns were made at a pressure of 6 GPa at room temperature to impart a strain of e = 6 to the sample. With the exception of aging, the time between water quenching and HPT was short (about 30 min). The nal procedure consisted of aging the samples for 96 h at room temperature. The dimension of the samples was approximately 0.7 mm thick with a diameter of 10 mm. The microstructure of the samples was examined using transmission electron microscopy (TEM). A JEM-100 electron microscope was operated at an excitation voltage of 100 kV. The foils for TEM were prepared by jet polishing using a Tenuipol3 at a voltage of 20 V and a solution of 20% HNO3 and 80% CH3 OH at a temperature of 0 C. The grain size and structural constituents were measured in dark-eld TEM images and in high-resolution scanning electron micrographs of surfaces at a magnication of 50,000. For each datum point, 10 structure images were used for the measurement of at least 70 grains. The measuring error was 15% at a condence probability of 50%. The diffraction patterns were obtained using a 0.5 m diaphragm. Vickers microhardness HV was measured using a PMT3 tester at a load of 0.2 kg. The measuring error was 5%. The samples were annealed in a SNOL-type furnace, which maintained the temperature to an accuracy of 3 K. The room temperature tensile properties of the samples were measured, using samples with a gage length of 2.5 mm and a width of

1 mm, on the universal dynamometer at an initial strain rate of 2 103 s1 . Three samples were tested at each condition. The values of the yield stress and elongation showed a scatter of 5%. 3. Results and discussion Investigations of the mechanical properties indicated that the various combinations of deformation followed by heat treatment resulted in different levels of hardening of the alloy 2024. The HPT processed Al-based alloy possessed the maximum strength, which exceeded the strength of the parent material, after the standard procedure of quenching and aging as summarized in Table 2. The most suitable scheme for achieving the maximum strength was found to be consecutive use of quenching, HPT and natural aging, that lead to microhardness of 2.6 GPa with a corresponding ultimate tensile stress >1000 MPa (Table 2). Transmission electron microscopy data of the 2024 alloy shows great structural renement as a result of HPT. Already after 0.5 turns the alloy has a microstructure with a grain size of about 160 nm, the appearance of high-angle grain boundaries, and the characteristic spreading of spots on the diffraction pattern (Fig. 2a). After HPT at ve turns the structure becomes homogeneous with grain size of 70 nm and mainly high-angle misorientations (Fig. 2b). The investigated microstructure shows curved and irregular grain boundaries, which indicates a disordered and nonequilibrium structure. The diffraction contrast inside grains is nonuniform and changes from dark to bright contrast within one grain, which demonstrates a high level of internal stresses and signicant lattice distortions. Great azimuthally spreading of spots on the diffraction pattern conrms this observation [2,3]. The combination of thermal treatment and HPT processing resulted in signicant hardening of the alloy due to microstructural renement of the aluminum matrix. This is effective for material only after HPT processing at room temperature of the quenched alloy. It is theorized that during HPT, small amounts of disperse particles of strengthening phases (Al2 Cu) and S (Al2 CuMg) bring an additional contribution to hardening. Although the number of these particles is negligible, the Cu and Mg present in the Al matrix after SPD is in a supersaturated solid solution state. The effect of strengthening of the quenched samples after SPD and aging has shown that the basic amount of particles was precipitated during aging. An increase in the microhardness of quenched samples after HPT from 2.3 up to 2.6 GPa, due to natural aging, supports this conclusion [6].

Fig. 1. A schematic of the high-pressure torsional (HPT) process.

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N.A. Krasilnikov, A. Sharafutdiniv / Materials Science and Engineering A 463 (2007) 7477

Table 2 The inuence of SPD and heat treatment on mechanical properties of alloy 2024 States of Al-based alloy Initial coarse-grained (CG) CG + quenching (495 C) CG + quenching + aging CG + quenching + aging + HPT CG + HPT CG + HPT + aging CG + quenching + HPT CG + quenching + HPT + aging HV (GPa) 0.60 0.85 1.20 1.65 1.62 1.65 2.30 2.60 UTS (MPa) 200 300 420 1070 YS (MPa) 100 220 310 1010 Ductility (%) 25 23 16 2

Fig. 2. TEM structure images of alloy 2024 after quenching HPT was applied at a pressure of 6 GPa for (a) 0.5 turns and (b) 5 turns.

Investigation of the mechanical properties show a small increase in the microhardness of the alloy to 2.6 GPa after being subjected to torsional loading in more than ve turns of anvil as summarized in Table 3. After 15 turns and aging, HV decreases. This is probably due to microstructural recovery during high strain degree of HPT. At the same time the plasticity of nanostructured alloy is small and elongation to rupture is about 12%, but can be increased up to 35% after long aging (about a month) at room temperature. This is connected with the transformation of grain boundaries to more equilibrium state.

The obtained ultimate stress of more than 1000 MPa (Table 3, Fig. 3a) is a record for industrial Al-based alloys. The reasons of this effect are the contributions to hardening from small grain size, nonequilibrium state of grain boundaries and presence of nanoparticles with homogeneous distribution in volume sample [4,5]. As a rule, plasticity increases are observed after annealing as well as in high temperature testing [6,7]. Annealing at low homological temperature leads to transformation of grain boundaries to equilibrium state with small grain growth, resulting in

Fig. 3. The engineering stressstrain curves of 2024 alloy after quenching. HPT was applied for ve rotational turns and aged. Tension test results are shown for (a) 20 C temperature test and (b) 325 C temperature test in which superplastic deformation was observed.

N.A. Krasilnikov, A. Sharafutdiniv / Materials Science and Engineering A 463 (2007) 7477 Table 3 Mechanical properties of Al-based alloy as a function of processing routes States of Al-based alloy Quenching + aging Quenching + HPT at 0.5 turns + aging Quenching + HPT at 1 turn + aging Quenching + HPT at 5 turns + aging Quenching + HPT at 10 turns + aging Quenching + HPT at 15 turns + aging Quenching + HPT at 5 turns + aging + superplastic deformation at 325 C HV (GPa) 1.20 1.70 2.10 2.60 2.60 2.45 1.45 UTS (MPa) 900 1030 1070

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an increase of ductility with a small decrease in strength. The nanostructured alloy after tension testing at 325 C shows superplastic behavior and elongation to rupture of approximately 230% (Fig. 3b). Note that after superplastic deformation, this alloy demonstrated microhardness of approximately 1.5 GPa, which is higher than the microhardness of the parent material after standard treatment of quenching and aging as summarized in Table 3. Such an unusual combination of mechanical properties of this nanostructured alloy has many technological applications. An example would be in obtaining details in a complex form processed by superplastic forming, which would retain high strength properties at room temperature. 4. Conclusions (i) A commercial aluminum alloy, Al-2024, was consistently processed by quenching, HPT, and aging at room temperature. Microstructural examination showed the formation

of high-angle grain boundaries with a reduced grain size of 70 nm. (ii) Achieving high ultimate tensile stress, greater than 103 MPa, breaks the record for industrial Al-based alloys, and is caused by several microstructural factors including: small grain size, nonequilibrium state of grain boundaries, and strengthening inuence of disperse particles of second phases. (iii) Microhardness values for the nanostructured alloy 2024 after superplastic deformation at 325 C are 20% higher than that of the parent alloy (coarse-grained) after standard treatment of quenching and aging. The combination of high strength and good plasticity is very promising for microsystem applications, and for obtaining complex-shape machine parts by superplastic forming, whose components should have high strength properties at ambient temperatures. References
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