Mechanical Properties of Ultra-Fine Grained Al-Li Alloys B. Adamczyk-Cieślak 1, a, M. Lewandowska 1, b, J. Mizera 1, c and K.J.

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1 Materials Science Forum Online: ISSN: , Vol. 513, pp doi: / Trans Tech Publications, Switzerland Mechanical Properties of Ultra-Fine Grained Al-Li Alloys B. Adamczyk-Cieślak 1, a, M. Lewandowska 1, b, J. Mizera 1, c and K.J. Kurzydłowski 1, d 1 Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska St. 141, Warsaw, Poland a badamczyk@inmat.pw.edu.pl, b malew@inmat.pw.edu.pl, c jmizera@inmat.pw.edu.pl, d kjk@inmat.pw.edu.pl Keywords: aluminium alloys, severe plastic deformation, grain refinement Abstract. The results obtained in the present study reveal the effect of equal channel angular extrusion (ECAE) on the grain size and mechanical properties of Al-Li alloys. During 8 passes of ECAE process, coarse grain microstructure in the initial state transforms into ultrafine grained. The final grain size depends on both total strain applied and Li content in the alloy. Due to the grain refinement the microhardness and yield stress increase by 100%. During compression deformation, the coarse grain alloys exhibit continuous hardening, whereas in the ultrafine-grained alloys, a stagnation of work hardening at the beginning of compression deformation is observed. This behaviour is related to the dynamic recovery of the severely deformed microstructure. Introduction The plastic deformation changes very strongly microstructure of materials. Severe plastic deformation (SPD) results in important evolution of sub-micron grain structure. There are numerous submicron-grained alloys, which exhibit high strength and toughness at room temperature. Small grain sizes at elevated temperatures, where diffusion is sufficiently fast, bring about good formability and superplastic properties [1]. A large majority of aluminium alloys after SPD are characterized by strongly banded structures with ultra-fine grains [2]. One of the methods, which can be used to obtain SPD, is ECAE (Equal Channel Angular Extrusion) [3, 4]. The aim of the present paper is to characterize the mechanical properties of model Al-Li alloys subjected to ECAE. Materials and Experimental Method In this experiment two model Al-0,7 % Li and Al-1,6 % Li alloys (concentrations of Li in Al- Li aloys are given in wt. %) were used in the form of hot extruded rods of 26 mm diameter. Microstructure of the alloys in the initial state was described elsewhere [5] and consisted of coarse grains of 300 µm diameter. The chemical composition of Al-0.7% Li alloy is situated in singlephase region of the Al-Li diagram, whereas that of Al-1.6% Li alloy on the boundary of two-phase region. TEM observation using SAED patterns revealed spots characteristic for δ precipitates in the later alloy in the initial state. These spots may be a result of short range ordering of the alloy structure rather, than the presence of precipitates. The coarse grain (CG) microstructures were refined by the ECAE process with a 90 angle between the channels (Fig. 1). The samples deformed to ε = 4.6 (4 passes) and ε = 9.2 (8 passes), were rotated by 90 around its longitudinal axis after each pass. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,14:15:07)

2 26 Advanced Materials and Technologies Fig. 1. Schematic diagram of the ECAE process. Optical microscopy with a polarized light and transmission electron microscopy were used to study the microstructural evolution. Observations were carried out on sections perpendicular to the extrusion direction. The mean grain size and shape were determined by the computer aided image analysis. Compression tests and microhardness measurements were carried out to characterize the mechanical properties of the Al-Li alloys after SPD and to compare it to that in the initial state. The compression tests were performed at room temperature. The compression direction was parallel to the longitudinal axis of the samples. Specimens were cut from the central part of the rods. The tests were conducted up to 10% and 50% deformation at a strain rate of s -1. The Vickers microhardness (HV0.2) was measured under a load of 200g, using the Vickers intender. The microhardness were carried out on the cross section plane of each sample before and after ECAE deformation. Results Microstructure Evolution. The microstructures of the investigated alloys after ECAE process are shown in Fig. 2. In these pictures, deformed and highly elongated grains are visible. Structures like these are usually observed in material formed with large reductions [6]. TEM observations (Fig. 3a) have revealed that the original grains are partitioned by extended dislocation boundaries, which form cell blocks with dislocation cells. As the number of passes increases from 4 to 8, a development of grain-like structure (Fig. 3b) and a slight refinement of the grain size are observed. In Al-0.7 % Li alloy, the grain size decreases from about 330 µm in the initial state to 1,3 µm after 4 passes and to 1.2 µm after 8 passes of ECAE. Similar results have been obtained for Al-1.6 % Li alloy, where the grain size decreases from 300 µm in the initial state to 0.7 µm after 4 passes and to 0.6 µm after 8 passes of ECAE. The grain size distributions in the severely deformed samples are shown in Fig. 4 and 5 for Al-0.7 % Li and Al-1.6 % Li alloy, respectively. It is worth to note that in Al-1.6 % Li alloy, spots characteristic for δ precipitates have disappeared that indicates the disordering of the alloy structure during SPD process.

3 Materials Science Forum Vol Al-0.7 % Li a) Al-1.6 % Li a) 40 µ m a) b) 20 µ m a) 40 µ m 40 µ m b) b) Fig. 2. Microstructure of the investigated alloys: after four passes (a); after eight passes (b) (crosssections perpendicular to the extrusion direction). Quantitative analysis of the microstructures has also revealed the influence of ECAE process on the shape of grains. In this term, the grain elongation factor α, defined as a ratio of the maximum diameter of the grain to its equivalent diameter was analyzed. The results are presented in Table 1. In the initial state the grains are equiaxed (α = 1,3) in both alloys. ECAE deformation results in microstructure containing elongated grains, their α factor increases to 1.8 for Al-0.7 % Li alloy and 1,6 for Al-1.6 % Li alloy. The grains after severe plastic deformation are more elongated in the alloy with lower lithium content. To summarize this part of results, it should be noted that during 8 passes ECAE process, CG microstructure in the initial state transforms into ultrafine grained (UFG). The final grain size depends on both total strain applied and Li content in the alloy. The grain refinement is much more significant in the alloy with higher lithium content. In the next chapter the mechanical properties of CG and UFG microstructures will be compared.

4 28 Advanced Materials and Technologies Al-0.7 % Li Al-1.6 % Li 2µ m 2µ m a) a) 2µ m 1µ m b) b) Fig. 3. TEM micrographs of the investigated alloys: after four passes (a), after eight passes (b) (cross-sections perpendicular to the extrusion direction). Table 1. Grain elongation factors for Al-0,7 wt. % Li and Al-1,6 wt. % Li alloys before and after severe plastic deformation. CG alloys UFG alloys Initial state 4 passes of ECAE 8 passes of ECAE Al-0.7 % Li Al-1.6 % Li

5 Materials Science Forum Vol Fig. 4. Grain size distributions in Al-0,7 wt. % Li alloy: after four passes of ECAE (a), after eight passes of ECAE (b). Fig. 5. Grain size distributions in Al-1,6 wt. % Li alloy: after four passes of ECAE (a), after eight passes of ECAE (b). Mechanical properties. Microhardness measurements were performed to evaluate the mechanical properties resulting from multiple passes of ECAE. The microhardness profiles for the initial state and after eight passes of ECAE are shown in Fig. 6. It can be noted that 8 passes of ECAE results in microhardness increase by approximately two times. This is due to the very rapid refinement of structure in the deformed alloys. The microhardness is much higher in the alloy with higher Li content. The stress strain curves of the Al-0.7 % Li and Al-1.6 % Li alloys in the initial state and after 8 passes of ECAE are shown in Fig. 7. Both alloys compressed in the initial state exhibit continuous hardening, whereas after 8 passes of ECAE a stagnation of work hardening at the beginning of compression deformation is observed. From the stress strain curves the values of yield stresses were determined and they are shown in Table 2. The yield stress is more than two times higher in the UFG alloys, in comparison to the initial state. Discussion Effect of Li Content on Microstructure and Mechanical Properties of Ultrafine Grain Al-Li Alloys. One of the factors affecting the mechanical properties of materials is the grain size in

6 30 Advanced Materials and Technologies HV0.2 AV HV0.2 AV 60 HV0.2/15 40 HV0.2/ HV0.2 AV HV0.2 AV Center Distance from the edge [mm] Edge Center Distance from the edge [mm] Edge Fig. 6. Microhardness profiles: Al 0.7 % Li (a) and Al 1.6 % Li (b). Fig. 7. Compression curves of the investigated alloys: Al-0,7 wt. % Li (a), Al-1,6 wt. % Li (b). accordance with the Hall-Petch equation. In Al-Li alloys investigated here the degree of the grain refinement depends on Li content. The smaller grain size is achieved for the alloy with the higher lithium content. This can be due to the fact that substitutional Li atoms reduce the rate of dynamic recovery. Similar observation was reported for Al-Mg-Sc alloys [7] where at higher Mg content the grain refinement occurred faster and as a result the smaller grain sizes were achieved. Table 2. The value of the yield stress for investigated alloys. Sample Al-0.7 % Li alloy Al-1.6 % Li alloy CG UFG CG UFG Yield stress [MPa]

7 Materials Science Forum Vol Fig. 8 shows the Hall-Petch relation for both investigated alloys. The line for Al-1.6 % Li is shifted to the higher values of microhardness HV0.2. The differences between both alloys are more significant for large grains. This may be explained by the presence of ordered regions in Al-1.6 % Li alloy before ECAE. Severe plastic deformation of this alloy leads to disordering of the alloy structure (spots characteristic for δ precipitates were not visible) and resulting strengthening is due to both the small grain size and Li atoms in solid solution. [µm 1/2 ] Fig. 8. The plots of the Hall - Petch relation of the investigated alloys. Deformation Behaviour of CG and UFG Al-Li Alloys. In order to explain the different behaviour of the CG and UFG alloys during compression deformation, the test was stopped at 10% (the deformation level within the range of flow stress plateau in the samples after 8 passes of ECAE). TEM observations of microstructure evolution were carried out after 10 and 50% of compression deformation. In CG alloy (before ECAE) the uniform distribution of dislocations within grains was observed after 10% of deformation. An example of this microstructure is presented in Fig. 9a. In UFG alloy (after 8 passes of ECAE), the microstructure consists of small grains (Fig. 9b). When compared to the microstructure directly after ECAE, grains are more equiaxed (the extended dislocation boundaries have disappeared) and contain smaller density of free dislocations. Quantitative analysis of this microstructure (Fig. 10) has shown that the mean grain diameter slightly increases, whereas the grain elongation factor α decreases from 1.73 to This observation suggests that the observed plateau on the stress-strain curve of ultrafine grain alloys isrelated to dynamic recovery of the severely deformed microstructure. After 50% of compression deformation, the microstructures of CG alloys consist of high density of dislocation tangles, which form cell structure (Fig. 11a), whereas in the UFG alloys further grain refinement can be observed (Fig. 11b). Conclusions Ultrafine-grained structure in Al-Li alloys can be obtained by ECAE process. The resulting grain size depends on the total plastic strain and Li content in these alloys. The mechanical properties (microhardness and yield stress) increase by 100% after 8 passes of ECAE due to grain size refinement. Additional strengthening is due to Li atoms in solid solution.

8 observation suggests that the observed plateau on the stress-strain curve of ultrafine grain alloys isrelated to dynamic recovery of the severely deformed microstructure. 32 Advanced Materials and Technologies 1 µm Fig. 9. Microstructures after 10% compression deformation: CG Al-1.6 % Li alloy (a) and UFG Al- 1.6 % Li alloy (b). 2 1,73 1,8 1,6 1,49 1,6 1,4 Grain size d 2 [µm] 1,2 0,8 0,4 0,6 0,66 1,2 1 0,8 0,6 0,4 Grain elongation α 0,2 0 d 2 α 0 after 8 passes of ECAE after 8 passes of ECAE + 10% compression Fig. 10. Influence of 10% compression on the grain size and shape in Al-1.6 % Li alloy Ultrafine-grained alloys exhibit stress plateau on their stress strain curves. This behaviour is related to the dynamic recovery of the severely deformed microstructures. Acknowledgements This work was supported by the State Committee for Scientific Research (grant Nr 4T08A 02724).

9 Materials Science Forum Vol µm 1 µm Fig. 11. Microstructures after 50% compression deformation: CG Al-1.6 % Li alloy (a) and UFG Al-1.6 % Li alloy (b). References [1] R.Z.Valiev, A.V. Korznikov, R.R. Mulynkov: Mater. Sci. Eng. A Vol. 168 (1993) p [2] P.L. Sun, P.W. Kao, C.P. Chang: Mat. Sci. Eng. A Vol. 283 (2000) p. 82. [3] L.R. Cornwell, K.T. Hartwig; R.E. Goforth, S.L. Semiatin: Materials Characterization Vol. 37 (1997) p [4] S.M. Sivakumar, M. Ortiz: Comput. Methods Appl. Mech. Eng. Vol. 193 (2004) p [5] J. Mizera, M. Lewandowska, A.V. Korznikov, K.J. Kurzydłowski: Solid State Phenomena Vol (2005) p. 73. [6] V.M. Segal: Mater. Sci. Eng. A Vol. 197 (1995) p [7] M. Furukawa, A. Utsunomiya, K. Matsubara, Z. Horita T.G. Langdon: Acta Mater. Vol. 49 (2001) p

10 Advanced Materials and Technologies / Mechanical Properties of Ultra-Fine Grained Al-Li Alloys / DOI References [4] S.M. Sivakumar, M. Ortiz: Comput. Methods Appl. Mech. Eng. Vol. 193 (2004) p /j.cma [5] J. Mizera, M. Lewandowska, A.V. Korznikov, K.J. Kurzydłowski: Solid State Phenomena ol (2005) p /