Effect of texture evolution on tensile properties of Al

Effect of texture evolution on tensile properties of Al–Ni eutectic alloy fabricated by equal channel angular pressing Z.-G. Zhang1,2, Y. Watanabe*3 and I. Kim1 The present study investigated in detail the effect of texture evolution on the mechanical properties of an Al–5.7 wt-%Ni eutectic alloy, which was subjected to severe plastic deformation by the equal channel angular pressing (ECAP) technique. The ECAP procedure was carried out using two strain introduction methods, route BC and route A, at a temperature of 298 K and a pressing rate of 0.33 mm s21. The as pressed microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicated that the Al–Ni eutectic alloy specimens after ECAP processing by route BC and route A methods had very different microstructures, which strongly affected the tensile properties of the specimens. It was demonstrated that after ECAP processing by route BC, fine Al3Ni particles of y300 nm were homogeneously dispersed in the aluminium matrix, and the specimens showed no clear anisotropy in tensile properties. After ECAP processing by route A, however, eutectic textures containing a-Al and Al3Ni fibrous dispersoids had a highly anisotropic distribution and were demonstrated to have significantly anisotropic tensile properties. Based on the experimental results, the fracture mechanism during tensile testing of the Al–Ni eutectic alloy using different strain induction methods is discussed. Keywords: Al–Ni eutectic alloy, ECAP, Microstructure, Anisotropy, Tensile properties

Introduction It is well known that aluminium has a low density which is only about one-third that of steel; therefore, its alloys are widely used in aero-, automobile and constructional engineering. The addition of alloying elements is made principally to improve the mechanical properties, such as tensile strength, hardness, rigidity and machinability, and sometimes to improve fluidity and other casting properties.1 Metallic materials with ultrafine grained (UFG) structures have been shown to have superior mechanical properties.2 Of several techniques to produce UFG materials, the equal channel angular pressing (ECAP) technique has been successfully applied to produce various bulk UFG materials.3,4 This is a process by which a material is subjected to a very intense plastic strain by being pressed through a special die and without any concomitant change in the cross-sectional dimensions of the specimen.5–7 1

Department of Functional Machinery and Mechanics, Shinshu University, 3-15-1 Tokida, Ueda 386–8567, Japan Corrosion Resistant Design Group, Steel Research Center, NIMS, Tsukuba, Ibaraki, Japan 3 Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Nagoya 466–8555, Japan 2

*Corresponding author, email [email protected]

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ß 2005 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 23 August 2004; accepted 28 September 2004 DOI 10.1179/174328405X43144

The principle of ECAP is illustrated schematically in Fig. 1 in the form of a section through a die. The die contains two channels, equal in cross-section, which intersect at an internal angle of w, and y is defined as the angle of the outer arc of curvature where the two channels intersect. It has been reported by Iwahashi et al.8 that the shear strain eN introduced during ECAPis given by N w y w y z z eN ~ 1=2 2 cot zycosec (1) 2 2 2 2 ð3Þ where N is the number of ECAP passes through the die. In the present study, w590u and y545u were applied to the die. Nakashima et al.9 documented that, regardless of the value of y, a single passage through a die with w590u will always give a strain close to y1.0. It is reported that the ECAP method is divided into four distinct processing routes, A, BA, BC and C.9 In route A, the specimen is not rotated between repetitive pressings; in route B, the specimen is rotated by 90u between each pressing; and in route C, the specimen is rotated by 180u between each pressing. A further possibility may be introduced when it is noted that route B can be undertaken either by rotating the specimen by 90u in alternate directions between each individual pressing, termed route BA, or by rotating the specimen by 90u in the same direction between each individual pressing, termed route BC. It is known that

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to provide further detailed investigation into the microstructural evolution using different strain induction methods, and to demonstrate highly anisotropic mechanical properties of Al–5.7 wt-%Ni eutectic alloy after ECAP by means of tensile specimens at various angles to the deformation direction.

Experimental materials and procedures

1 Schematic illustration of equal channel angular pressing (ECAP) facility including definitions of coordinate planes in as pressed sample

homogeneous structures can be obtained by route BC, and inhomogeneous structures will be generated by route A.10,11 Eutectic materials show many advantages, such as good microstructural stability and long term property retention at elevated temperatures,12 and other special properties, such as good viscoelasticity,13 high conductivity,14 etc. Lemkey et al.15 reported that the microstructure of unidirectionally solidified Al–Al3Ni eutectic consisted of aligned Al3Ni whiskers in an aluminium alloy matrix, and the whiskers had a distorted hexagonal cross-section and exhibited a preferred crystallographic relationship with respect to the matrix. Meanwhile, it was reported that if these fibrous dispersoids of Al3Ni could be aligned in the aluminium matrix, they could significantly strengthen the matrix without considerably decreasing the electrical conductivity, leading to the development of high strength and high electrical conductivity alloys.16,17 Hertzherg et al.18 also investigated the effect of unidirectional solidification upon the microstructural, crystallographic and mechanical characteristics of Al–Al3Ni eutectic alloy specimens, and found that alignment of the Al3Ni phase by unidirectional solidification gave rise to a threefold increase in strength over that exhibited by specimens with an as cast microstructure. In the past decade, as a highly effective severe plastic deformation technique, ECAP has been widely used to obtain super grain refinement in bulk materials and produce materials having ultrafine grain sizes in the submicrometre or nanometre range.19,20 However, no fundamental investigation has been undertaken to date to examine the deformation structures introduced when ECAP is applied to a eutectic alloy. Zhang and Watanabe21 reported the grain refinement of Al–Ni eutectic alloy by equal channel angular pressing. It was demonstrated that, for as deformed specimens using route A, the a-Al crystals were distributed in parallel plates of below 5 mm in thickness, below 50 mm in width and limitless in length along the deformation direction, and the Al3Ni particles were mainly on a submicrometre scale except for a few particles on a micrometre scale. The purpose of the present study was

In the present study, rod shaped Al–5.7 wt-%Ni eutectic alloy was prepared by casting at 850uC using high purity (99.993 wt-%) aluminium and an Al–20 wt-%Ni industrial ingot. For ECAP, cylindrical pieces were machined to give rod shaped specimens with a diameter of 10 mm and a length of 60 mm. The ECAP procedure was conducted with a pressing speed of 0.33 mm s21 at room temperature (298 K) using MoS2 as a lubricant. In the present study, ECAP was conducted by routes BC and A, as explained above. Repeated pressings of the same specimen were carried out using up to eight passes through the die, since in the ninth pass cracks were observed on the surfaces of specimens. A strain of 8.0 would be introduced into specimens after ECAP of eight passes through the die. Following the ECAP procedure, a small coupon was prepared from the as pressed Al–Al3Ni rod specimens by electrical discharge machining. As illustrated in Fig. 1, two vertical sections, the x and y planes, were parallel to the deformation direction, and a cross-section, the z plane, was perpendicular to the deformation direction. The three planes of each specimen were mechanically polished, and then the surfaces were etched for 15 s with 0.2%HF acid solution. Examination of the specimens was carried out using a Hitachi S–3000 scanning electron microscope. For TEM observation, small pieces of 363 mm in cross-section and y0.8 mm in thickness were sectioned parallel to the longitudinal central axis of the specimens after ECAP. These pieces were polished to y150 mm in thickness by mechanical means and then thinned to perforation at 253 K using a twin jet electropolishing unit with a solution of 10%HCO4, 20%C3H8O and 70%C2H5OH. Then the specimens were examined using a Jeol 2010 transmission electron microscope operating at 200 kV. In addition, Vickers microhardness values were measured using an Akaishi MVK-C microhardness tester for each specimen deformed by ECAP. The reported values for the microhardness are the average of 10 separate measurements taken at randomly selected points by imposing a load of 350 g for 15 s. To evaluate the tensile properties of Al–Ni eutectic alloy prepared by ECAP, some small tensile testpieces were cut from the central region of as pressed rod specimens along the longitudinal axis using discharge electric machining. The gauge size of the fine tensile specimens was 0.361620 mm. In addition, other tensile specimens were also prepared along the planes at angles of 45, 75 and 90u to the deformation direction, to demonstrate anisotropy in the tensile properties related to the anisotropic texture. Since slices prepared directly from the as pressed rod specimens could not be obtained with sufficient length, the tensile specimens were adhered to same material slices using quick drying glue on the two ends. Tensile testing was conducted at room temperature using a versatile testing machine with


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2 Image (SEM) of Al–5.7 wt-%Ni eutectic alloy before ECAP procedure

a 500 N load cell at a constant rate of crosshead displacement equivalent to an initial stain rate of 3.361024 s21.

Results and discussion Microstructure observation To understand the microstructural evolution of Al– 5.7 wt-%Ni eutectic alloy during ECAP, a specimen before the ECAP procedure was examined using SEM. Figure 2 shows an SEM image of the Al–5.7 wt-%Ni eutectic alloy, which was cast at 850uC. From Fig. 2, it can be seen that the eutectic lamellar microstructure contains two phases. One is the (grey) a-Al matrix, and the other is Al3Ni intermetallic phase alternating with the a-Al matrix. Three-dimensional SEM images of Al–Ni eutectic alloy specimens fabricated by ECAP using route BC are constructed in Fig. 3. It can be seen that the Al–Ni eutectic microstructures containing a-Al crystals alternating with fine Al3Ni particles had no obvious

anisotropic distribution trends, and most Al3Ni particles were cracked and distributed in an irregular arrangement. The longitudinal sections, x and y planes, indicate that the microstructure contained short fibrous Al3Ni dispersoids uniformly distributed in the aluminium matrix, and some of the Al3Ni particles were generally aligned parallel to the deformation direction. The transverse section, z plane, indicates a uniform distribution of the short Al3Ni particles through the crosssection. The Al3Ni particles were refined to a large degree, and distributed on a submicrometre scale. It can also be seen from Fig. 3 that in the microstructure after eight passes of ECAP, Al3Ni particle sizes were further refined compared with those in the microstructure after four passes of ECAP. Three-dimensional SEM images of Al–Ni eutectic specimens after four passes and eight passes of ECAP by route A are constructed in Fig. 4. In the x and y planes, parallel to the deformation direction, the aligned texture containing a-Al and Al3Ni dispersoids had a highly anisotropic distribution. The (grey) a-Al crystals and the short fibrous dispersoids of Al3Ni uniformly dispersed in the aluminium matrix were rearranged and elongated at an angle of about 0–15u to the deformation direction. In the z plane, it can be seen that the a-Al crystals were distributed in parallel platelets of less than 5 mm in thickness, below 50 mm in width and limitless in length at an angle of y15u to the deformation direction. Simultaneously, it can be seen that the Al3Ni particles were mainly on a submicrometre scale except for a few particles of micrometre scale. The coarse Al3Ni particles in the microstructure after eight passes of ECAP were further refined compared with those of the microstructure after four passes of ECAP. Figure 5 shows TEM images of y planes, parallel to the deformation direction, of as pressed Al–5.7 wt-%Ni specimens after eight passes of ECAP by route A and route BC. As described above for the SEM results, elongated Al3Ni dispersoids of 200 nm in width and 400 nm in length were distributed in planes at angles of

3 Images (SEM) constructed to represent three-dimensional sections of Al–5.7 wt-%Ni eutectic alloy after ECAP by route BC

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4 Images (SEM) constructed to represent three-dimensional sections of Al–5.7 wt-%Ni eutectic alloy after ECAP by route A

0–45u to the deformation direction for the specimen fabricated by route A (Fig. 5a). Moreover, the distance between two nearby elongated aligned Al3Ni dispersoids was y500 nm. On the other hand, for the specimen obtained by route BC (Fig. 5b), fairly equiaxed ultrafine

Al3Ni particles of y400 nm in diameter were dispersed in the aluminium matrix. After ECAP processing of the controlled microstructure material (route A), most grain boundaries were of the low angle type with few high angle grain boundaries. In the other material (route BC), most high angle grain boundary dislocations were piled up near secondary Al3Ni particles and transgranular cracks were generated.

Microhardness measurement

a route A; b route BC 5 Images (TEM) of Al–5.7 wt-%Ni eutectic alloy after eight passes of ECAP by given routes

Figure 6 shows the variation of Vickers microhardness with number of passes through the die for Al–Ni eutectic alloy specimens after ECAP by routes BC and A. Data for pure aluminium specimens after ECAP by route BC are also shown in Fig. 6. It can be seen that the hardness values for pure aluminium even after eight passes of ECAP increased to only y40 MPa, because there were no reinforcement particles in the aluminium matrix. However, for the Al–Ni eutectic alloy specimens after ECAP, the hardness was obviously enhanced even after only one pass. When the specimens were subjected to further repeated ECAP processing, the hardness values increased slightly, reaching the highest value after eight passes of ECAP. In the present study, ultrafine microstructures of submicrometre scale were obtained after repeated ECAP, and the sizes of the fine grains were dependent on the intervals of the subgrain bands introduced into the specimen. For the pure aluminium specimens, it is considered that abundant low and high angle boundaries generated within the a-Al crystals were the main reinforcement during ECAP. Therefore, the hardness increment was not significant.18 On the other hand, during ECAP of the Al–Ni eutectic alloy, the hard Al3Ni particles played a crucial role in the hardness increment, and the microhardness values increased to y120 MPa after eight passes of ECAP from 60 MPa before the ECAP procedure. It can also be seen from Fig. 6 that the hardness values of as pressed specimens


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6 Variation of Vickers microhardness with number of passes through die for Al–5.7 wt-%Ni eutectic alloy specimens after ECAP by routes BC and A and pure aluminium by route BC

after ECAP by route A were slightly higher than those of specimens after route BC. The reinforcing mechanism during ECAP by route A and route BC is now discussed. At first, according to equation (1), a single ECAP pass through a die with w590u will always give a strain close to y1.0. In Ref. 19, the authors depicted the shearing strain characteristics within a crystalline sample when it was subjected to ECAP. During refining by ECAP, after one pass, subgrain bands were generated within the crystalline sample. When repeated ECAP was carried out in the present work, changing the introduction direction using route BC, the disorientations between the subgrains increased rapidly, eventually forming ultrafine grains surrounded by high angle boundaries. In this case, at least four passes of ECAP were necessary to obtain equiaxed grain structures. In the case of route A, the shearing characteristics were similar during repeated ECAP and increasing distortions of the original phases were generated. This resulted in refined Al3Ni dispersoids being distributed along the deformation direction and most a-Al matrix crystals being elongated to a large degree. Second, because a large amount of shear strain was introduced into the interior of specimens during the ECAP procedure, Al3Ni particles were cracked and refined to a submicrometre or nanometre scale, and they were distributed more uniformly than before. Third, as a result of the large strain introduced into the specimens, much work hardening was generated. However, with further ECAP passes, the interior of specimens was subjected to cracking along grain boundaries or within grains.

7 Variation of ultimate tensile stress (UTS) with equivalent strain introduced into Al–5.7 wt-%Ni eutectic alloy specimens after ECAP by routes A and BC

increased abruptly after two passes through the die, but thereafter the increase was small. As a result, after eight passes of ECAP, the UTS increased to y235 MPa by route BC and 275 MPa by route A from y133 MPa before ECAP. Furthermore, the UTS values using route A were higher than those using route BC. This is because route A resulted in aligned textures with high anisotropy along the deformation direction. To demonstrate further the anisotropic tensile properties with respect to the deformation microstructures after ECAP, tensile tests were also carried out at different angles to the deformation direction for the same as pressed specimens. Figure 8 shows UTS values for tensile specimens as a function of angle to the deformation direction during ECAP. It is clear that the UTS values after ECAP increased markedly. Furthermore, after ECAP by route A and route BC, UTS values for the Al–Ni eutectic alloy tensile specimens showed different reduction trends with an increase of angle to the deformation direction. In the case of specimens processed by route A,

Tensile properties Tensile properties of as deformed Al–Ni eutectic alloy specimens at room temperature were measured. Values of ultimate tensile stress (UTS) along the deformation direction plotted against the equivalent strain imposed by ECAP using routes BC and A are shown in Fig. 7, where the equivalent strain corresponds directly to the number of passes through the die. The point recorded at an equivalent strain of zero represents testing of the specimen before ECAP. It can be seen that the UTS

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8 Variation of UTS with angle to deformation direction of Al–5.7 wt-%Ni eutectic alloy specimens after ECAP

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the UTS value decreased abruptly from 275 MPa for tensile specimens parallel to the deformation direction to 210 MPa for those at an angle of 90u to the deformation direction, after eight passes of ECAP. After four passes of ECAP the UTS value was reduced to 165 MPa from y225 MPa. However, for the specimens after four and eight passes of ECAP by route BC, the UTS values hardly changed and maintained values of y210 and 240 MPa, respectively. It is considered that in the microstructure produced by route A the a-Al crystals and Al3Ni dispersoids showed higher anisotropy trends than those in specimens processed by route BC. Figure 9 shows values of elongation to failure of tensile specimens after ECAP by route BC and route A as a function of angle to the deformation direction. After ECAP, it can be seen that the elongation to failure of specimens increased by a large amount. Furthermore, after the same number of passes of ECAP, specimens processed by route A had higher elongations to failure than those of route BC specimens. With an increase of angle to the deformation direction, for Al–Ni eutectic alloy specimens processed by route A, the elongations to failure presented a highly anisotropic distribution, which showed a threefold decrease compared with an angle perpendicular to the deformation direction of the tensile specimen. However, for Al–Ni eutectic alloy specimens processed by route BC, the elongations to failure showed no obvious anisotropic distribution. This is because, in the Al–Ni eutectic microstructure after ECAP by route A, there was a significant anisotropic texture which could endure a large external force in the deformation direction but could endure only a lower external force in the perpendicular direction. In contrast, in the microstructure obtained by route BC, fine Al3Ni particles were homogeneously distributed in the a-Al crystal matrix, and therefore the uniform texture could endure a large external force in any direction. The results indicate that the anisotropic tensile properties are in agreement with the highly anisotropic textures after ECAP. In the present study, it is considered that two types of fracture mechanism, i.e. ductile fracture and brittle fracture, were operating in the two Al–Ni eutectic alloy microstructures obtained by ECAP using routes A and BC. After ECAP by route A, the microstructure showed a highly anisotropic distribution, and during tensile testing the elongated textures consisting of Al3Ni dispersoids and a-Al crystals could be further elongated and endure a large external force. Therefore, when the tensile specimens were stretched along the deformation direction of the ECAP procedure this would lead to ductile fracture. However, when the tensile specimens were stretched along planes at an angle of 90u to the deformation direction, the connection between the Al3Ni dispersoids and a-Al matrix would be weak, so fracture would be generated at the interface between the Al3Ni dispersoids and a-Al matrix and also at grain boundaries of Al3Ni particles and a-Al crystals. This would result in brittle fracture, the tensile specimens not being able to endure a large external force. In contrast, after ECAP by route BC, the fine Al3Ni particles of submicrometre order were dispersed along the a-Al grain boundaries and the dislocations formed during ECAP were piled up near the fine dispersed particles. When the tensile specimens were stretched, because the


9 Variation of elongation to failure with angle to deformation direction of Al–5.7 wt-%Ni eutectic alloy specimens after ECAP

fine dispersed Al3Ni particles were homogeneously distributed in the aluminium matrix, grain boundary fracture was generated with the development of microcracks near the particles. As a result, ductile fracture and brittle fracture was always simultaneously generated at the grain boundaries of a-Al crystals and Al3Ni dispersoids.

Summary In the present study, Al–5.7 wt-%Ni eutectic alloy was severely deformed by ECAP processing using the route BC and route A methods. It was demonstrated that for as pressed specimens processed by route BC, the eutectic microstructure had no obvious anisotropic distribution trend and consisted of a-Al crystals alternating with fine Al3Ni phases, because most Al3Ni particles were cracked and distributed in an irregular arrangement. It was shown that in planes at angles of 0, 45, 75 and 90u to the deformation direction, UTS values did not differ significantly and all were y235 MPa. For as pressed specimens processed by route A, the aligned texture containing a-Al and Al3Ni fibres had a highly anisotropic distribution. The a-Al crystals and short fibrous Al3Ni dispersoids uniformly dispersed in the aluminium matrix were rearranged and elongated at an angle of about 0–15u to the deformation direction. It was shown that in different planes at angles of 0, 45, 75, and 90u to the deformation direction, the stress–strain curves exhibited similar shapes but significantly anisotropic tensile properties, the UTS values decreasing from y275 MPa at an angle of 0u to y210 MPa at an angle of 90u. During tensile testing of microstructures with a highly anisotropic distribution obtained by ECAP using route A, ductile fracture was generated along the deformation direction and ductile fracture and brittle fracture were generated at the grain boundaries. In tensile testing of microstructures obtained after ECAP using route BC along any direction, ductile fracture and brittle fracture were simultaneously generated at the grain boundaries of a-Al crystals and Al3Ni dispersoids.


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Acknowledgement This study is supported by 21st Century COE Research by the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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