The Effect of Texture on the Serrated Flow in Peak-Aged 2090 Al-Li Alloy

Transcription

1 Solid State Phenomena Vol. 105 (2005) pp online at Trans Teh Publiations, Switzerland The Effet of Texture on the Serrated Flow in Peak-Aged 2090 Al-Li Alloy Y.Z. Shen a, K.H. Oh b and D.N. Lee Researh Institute of Advaned Materials and Shool of Materials Siene and Engineering, Seoul National University, Seoul , Korea a shenliu8@snu.a.kr, b kyuhwan@snu.a.kr, dnlee@snu.a.kr Keywords: 2090 Al-Li alloy, solution-treatment, peak-aging, serrated flow, dynami strain aging, Portevin-Le Chatelier effet, texture. Abstrat. Tensile speimens ut from the surfae layer to the enter layer of a 12.7 mm thik 2090 Al-Li alloy plate were solution treated at 550 C for 30 min and subsequently peak-aged at 190 C for 18 h. They were tensile tested along the rolling diretion at 25 C at various strain rates. The solution-treated speimens gave rise to serrated flows at a strain rate of s -1. On the other hand, for the peak-aged alloy, the surfae-layer and subsurfae-layer speimens underwent omplex, serrated flows (fine and oarse types superimposed eah other), whereas the enter-layer and near-enter-layer speimens were devoid of serrated flows. The textures of the surfae-layer and subsurfae-layer speimens were approximated by the {001}<110> orientation, while those of the enter-layer and near-enter-layer speimens were approximated by the {011}<211> orientation. The different flow behaviors were disussed based on the rystallographi textures, mirostrutures and the strain rates. Introdution Explanation of the phenomenon of serrated flow, or the Portevin-Le Chatelier (PLC) effet in Al- Li alloys are based on two lines of thought: One is dynami strain aging (DSA) involving dynami interation between dissolved lithium solute atoms and mobile disloations [1-4] and another is shearing of δ (Al 3 Li) preipitates by mobile disloations [5-8]. In general, aluminum alloys are known to exhibit a onsiderable anisotropy of mehanial properties. This anisotropy is usually related to the rystallographi texture and in turn ative slip systems. Sine the PLC effet involves the interations between gliding disloations and other rystal defets, one should expet that its intensity may also exhibit anisotropy [9]. Suh anisotropy has been reported on a ommerial Al-Mg alloy by Cheng and Morris [10] and on an Al-Li-Cu-Zr model alloy by Mizera and Kurzydlowski [9]. They aounted for the observed differene in the PLC effet intensity in terms of anisotropy of the mirostruture or the morphology of the grains. On the other hand, little work has been made of the textural effet on the PLC effet. The previous study [11] indiated that serrated flow behavior in a peak-aged 2090 Al-Li alloy was strongly affeted by the textures of speimens. The purpose of this study is to investigate the flow behavior of peak-aged speimens ompared with that of solution-treated speimens. Experimental proedure The material used in this study was a near peak-aged 2090-T81 Al-Li alloy (2.05% Li, 2.86% Cu, 0.12% Zr, balane Al) plate of 12.7 mm in thikness, produed by Aloa, UK. Two types of speimens were prepared from the plate. One is RT-speimen, in whih the tensile axis is aligned with the rolling diretion (RD) while the width diretion is parallel to the transverse diretion (TD), and another one is RN-speimen, in whih the tensile axis is aligned with RD while the width diretion is parallel to the normal diretion (ND) as shown in Fig. 1. In order to investigate properties along the ND of the plate, it was slied into four sheets of about 1.2 mm in thikness from the surfae Liensed to D.N. (dnlee@snu.a.kr) - Korea 227 All rights reserved. No part of the ontents of this paper may be reprodued or transmitted in any form or by any means without the written permission of the publisher: Trans Teh Publiations Ltd, Switzerland, (ID: /05/05,02:50:54)

2 228 Texture and Anisotropy of Polyrystals II TD RT TD enter RT0 RT1 RT2 RT3 RD RD ND RN ND Fig. 1. Designation of tensile speimens. layer to the enter layer as shown in Fig.1. RT type tensile speimens of 1 mm 6 mm 25 mm (ASTM B557M-94) in gauge dimension were ut from the sheets. It is noted that RT0-speimen and RT3-speimen are from the enter and surfae layers, respetively, and RN-speimen is equivalent to a speimen onsisting of one RT0-speimen and about two RT1-speimens. The speimens were solution treated at 550 C for 30 min. Some of the solution treated speimens were peak-aged at 190 C for 18 h. All the treatments were performed in a salt bath. Tensile tests were arried out at initial strain rates of s -1 to 10-2 s -1 at 25 C. Thin foil speimens were examined using a Philips CM20 transmission mirosope operated at 200 kv. The textures of the speimens were measured with an x-ray texture goniometer in the bak refletion mode with Ni filtered Cu-K α radiation. The (111), (200), and (220) partial pole figures were measured and used to alulate the orientation distribution funtions (ODFs) by the WIMV method [12]. Other experimental details are desribed in Ref. [11]. Results and Disussion Mirostrutures. Fig. 2 shows an optial mirostruture and dark-field TEM mirographs of the solution-treated and peak-aged speimens. Elongated grains an be seen in the optial mirostruture. The small white partiles in the TEM mirograph of solution-treated speimen are thought to be δ (Al 3 Li) preipitates, beause δ preipitation takes plae even during quenhing from the solution temperature or rapidly at room temperature [13]. The white partiles in the TEM mirograph of the peak-aged speimen are δ (Al 3 Li) preipitates. The δ phase is ordered f and has an L1 2 rystal struture. It is also known that the δ phase is fully oherent and has a very small lattie mismath with the f α matrix [14]. Textures of surfae and enter layers. The textures of the solution treated speimens were the same as those of the peak-aged speimens. Fig. 3 shows the orientation distribution funtions (ODFs) of various speimens from the peak-aged Al-Li alloy plate. The textures of the RT3 and RT2 speimens are almost same and an be approximated by the {001}<110> orientation. The textures of the RT0 and RT1 speimens are almost the same and an be approximated by the {011}<211> orientation. The texture of the RN speimen an be approximated by the {111}<211> orientation. With referene to Fig. 1, the RT-speimen plane is normal to ND of the plate and ND is parallel to the width diretion of the RN-speimen. Therefore, the {111}<211> orientation of the RN-speimen is equivalent to the {011}<211> orientation of the RT-speimen. Sine the textures of the RT0 and RT1 speimens are almost same, the RN-speimen, whih omprises one RT0 speimen and about two RT1 speimens, has almost the same texture as that of the RT0-speimen. From the fat that the {001}<110> orientation is the major omponent in shear deformation texture of aluminum alloys, and the surfae layer of aluminum undergo shear deformation (e.g., [15]), the surfae and subsurfae layers (RT3 and RT2) of the Al-Li alloy plate must have been shear deformed during rolling. The shear texture remains little hanged even after annealing [16]. On the other hand, the enter and near-enter layers (RT0, RT1, and RN), whih undergo plane strain ompression, show a texture approximated by the {011}<211> orientation, whih is obtained in many opper alloys with low staking fault energies [17] and Cu-Mn alloys with high strainhardening rates [18]. These results indiate that the deformation texture of the starting plate 228

3 Solid State Phenomena Vol a b 100nm 100nm 500 μm Fig.2. (a) Optial mirostruture and dark-field TEM mirographs of (b) solution-treated, () peak-aged 2090 Al-Li alloy speimens. L, LT, and ST stand for longitudinal (rolling), long transverse (transverse), and short transverse (normal) diretions of plate. remained almost unhanged even after the solution heat treatment and subsequent aging treatments. Flow stresses of surfae and enter layers. Fig. 4 shows the flow urves of RT0, RN, and RT3-speimens taken from the solution-treated Al-Li alloy speimen. The RT0-speimen has the highest flow stress, and the RT3-speimen has the lowest flow stress. The flow urve of the RNspeimen is loated between the RT0 and RT3-speimens. This behavior is understandable from the fat that the RN-speimen is equivalent to a speimen onsisting of one RT0 and about two RT1-speimens. Fig. 5 shows the flow urves of various speimens taken from the peak-aged Al- Li alloy plate. The differenes in flow urves of speimens from different depth layers of the same plate ould be aused by different rystallographi textures. The solution-treated speimens and the peak-aged speimens showed the same texture. It indiates that the texture of matrix did not hange by preipitation. The flow stress of eah speimen dereases with inreasing strain rate, implying a negative strain rate sensitivity. Regardless of heat-treatments (solution treatment, peak-aging), the enter-layer speimens show about times higher flow stresses than the surfae-layer ones at a given strain rate. This an be qualitatively explained using the approximate orientations of the surfae and enter layers, whih are the (001) orientation and the ( 011) [211] orientation, respetively. The tensile diretions of the (001) and ( 011) [211] oriented speimens are and [211], respetively. For an f single rystal, the yield stresses under a tensile stress along the and [112] axes will be same, beause the largest Shmid fators for the two axes are all 6 / 6 (Table 1). However, for an f polyrystalline material with a well developed texture, its yield behavior will not be ontrolled only by slip systems with the largest Shmid fators. Lee and Oh [19] alulated the plasti anisotropy of polyrystalline metals by assuming that all the slip systems ontributed to the deformation but that their ontributions were proportional to their Shmid fators. Similarly, the yield stress or flow stress of a polyrystalline may be alulated by τ σ = mi = τ M avg (1) n i where σ, τ, n, and m i are the flow stress, the ritial resolved shear stress, the total number of ative slip systems, and the reiproal of the Shmid fator on the ith slip system, respetively. The flow stress for the tensile axis is given by σ = τ / 6 (2) 6 If all the slip systems with non-zero Shmid fators are assumed to be ative, the flow stress for the [112] tensile axis is given by 229

4 230 Texture and Anisotropy of Polyrystals II σ = 10.5τ / 6 (3) Max: 9.5 Max:10.2 Max: Max:11.8 RT0 RT1 RT2 RT3 RN 1,3,5,7,9 max :10.3 Fig. 3. ODFs of RT0, RT1, RT2, RT3, RN speimens from peak-aged Al-Li alloy plate. Textures of RT0 and RT1 are approximated by {011}<112>. Textures of RT2 and RT3 are approximated by {001}<110>. Texture of RN is approximated by {111}<112>. Here {hkl} indiates lattie planes parallel to speimen plane and <uvw> indiates lattie diretions parallel to tensile diretion. Plane Diret /6 6 / /6 6 /6 [112] 0 6 /9 6 /9 6 /9 6 /6 6 /18 6 /9 6 /18 6 / Table 1. Calulated Shmid fators for tensile axes of and [112] of f rystal having {111}<110> slip systems. If the slip systems with the Shmid fator of 6 / 18 are assumed to be non-ative, beause the fator is small ompared with 6 / 9 and 6 / 6, σ = 8τ / 6 (4) Thus, σ / σ = for σ [112] in Eq. (3) and σ / σ = for σ [112] in Eq. (4). It an be seen that σ σ is in qualitative agreement with the experimental results. / = Types of serrated flows. The experimental results in Figs. 4 and 5 show two different types of serrated flows, fine (F) type and oarse (C) type (Fig. 6). The flow urves of the solution-treated speimens show F-type serration (Fig. 4). On the other hand, F-type serration is drastially redued and C-type serration appears in the flow urves of the peak-aged speimens (Fig. 5). The peakaging brings about the preipitation of δ (Al 3 Li) and redues the onentrations of dissolved solutes. It follows from this result that F-type serration is assoiated with dynami strain aging (DSA) (or Portevin-Le Chatelier effet) due to repeated loking of moving disloations by solute atoms, espeially Li, and C-type serration seems to be assoiated with shearing of δ preipitates. When a speimen ontaining preipitates is elongated, disloations pile up on the slip planes at the preipitates. If the stress on the disloation at the head of the disloation pile-up reahes the shear yield stress of preipitates, the preipitates are sheared and a stress drop an take plae. Suh proesses are repeated to form C-type serration. If the stress on the disloation at the head of disloation pile-up reahes the frature stress of preipitates, raking of the preipitates takes 230

5 Solid State Phenomena Vol plae. The preipitate raking will likely to bring about frature of speimen, resulting in a redution of its elongation. Stress(MPa) RT0 RN RT Al-Li Alloy Solution-treated : 550 o C x 30 min Tension : 2 x 10-4 s -1, 25 o C Strain (%) Fig. 4. Flow urves of RT0, RN, TR3 speimens from solution-treated 2090 Al-Li alloy plate at tensile strain rate of s -1. Stress (MPa) (7) (6) (8) (4) (3) (2) (1) (1) RT3, 2 x 10-4 s -1 (2) RT3, 10-3 s -1 ; (3) RT3, 2 x 10-3 s -1 ; (6) RT1, 2 x 10-4 s -1 (4) RT2, 2 x 10-4 s -1 ; (7) RT0, 2 x 10-4 s -1 (5) RT2, 10-2 s -1 ; (8) RN, 2 x 10-4 s Strain (%) Fig. 5. Flow urves of RT3, RT2. RT1, and RN speimens from peak-aged 2090 Al-Li alloy at various strain rates at 25 C. (5) F C Fig. 6. Shemati of F and C types of flow urves. However, these two types of serrations are not independent eah other, beause solute atoms an pin stagnant disloations surrounding the preipitates, if any. This makes it impossible to simply add the effets of solute atoms and preipitates on serration. Nevertheless, suh a rough lassifiation is onvenient for understanding the differene in serration before and after peakaging. Texture effet on serration. For the peak-aged alloy plate, the surfae-layer and subsurfae-layer speimens show serration at strain rates from s -1 to 10-2 s -1 (Curves 1-5 in Fig. 5), whereas the enter-layer and near-enter-layer speimens are almost devoid of serration (Curves 6-8 in Fig. 5). When the (001) oriented surfae-layer and subsurfae-layer speimens are elongated, it an be supposed that the ( 111)[10 1], ( 111)[01 1], ( 111)[101], and ( 111)[011] slip systems are equally ativated, beause the Shmid fators on the systems are same (Table 1), and disloations pile up on the slip planes at δ spherial preipitates (Fig. 2). When the stress on the disloation at the head of the disloation pile-up reahes the shear yield stress of the preipitates, the preipitates are sheared, resulting in the stress drop (C-type). However, tensile straining of the ( 011) [211] oriented enter-layer and near-enter-layer speimens ativates the (111) [ 10 1], (111) [ 011], ( 111), ( 111) [101], ( 111), and ( 111) [011] slip systems (Table1). The slip systems having 6/18 are negleted. Thus, the Shmid fators for the ative slip systems are different unlike the (001) oriented surfae-layer and subsurfae-layer speimens. The slip systems on whih the Shmid fator is 6/6 will first be ativated. If the slip systems of whih the Shmid fator is 6/9 are ativated by ross slip before the stress on the disloation pile-up on the first slip systems reahes the shear yield stress of the preipitates, partiles are not likely to be sheared. In this ase, no C-type serration will take plae. This may be the ase of the flow urve of the enter layer speimen, whih is devoid of serration. This does not negate the possibility of shearing of the preipitates. If a suffiient number of preipitates are not sheared, the serration annot be deteted. The smaller elongations of the peak-aged enter-layer speimens imply that disloations on the slip systems with the smaller Shmid fators are diffiult to shear δ preipitates without raking, 231

6 232 Texture and Anisotropy of Polyrystals II beause the preipitates assoiated with the smaller Shmid fators are subjeted to higher tensile stresses than those with the smaller Shmid fators. Strain rate effet on serration. Aording to flow urves at different strain rates of the peakaged speimens, the serration harateristis of the surfae-layer speimen (RT3), appear to be omposed of C-type and F-type. At a strain rate of s -1, the flow urve ontains both C-type and F-type serrations, but as the strain rate inreases to the order of 10-3 s -1, F-type serration disappears. For the subsurfae-layer speimens (RT2), C-type and F-type serrations are observed at a strain rate of s -1, whereas F-type serrations disappear as the strain rate inreased to 10-2 s -1. That is, for the peak-aged surfae-layer (RT3) and subsurfae-layer (RT2) speimens, F-type serration dereases with inreasing strain rate while C-type serration is insensitive to strain rate, as shown in Curves 1-5 of Fig. 5. The evolution of DSA is a funtion of strain rate. If the disloation veloity is too fast ompared with the diffusion speed of solute atoms, the solute atoms annot ath and lok disloations, resulting in no DSA. If the disloation veloity is too slow ompared with the diffusion speed of solute atoms, the solute atoms an ath but move together with disloations, resulting in no DSA. Aknowledgement This study has been supported by Texture Control Laboratory (NRL), Seoul National University. Referenes [1] L.P. Kubin, A. Styzynski, Y. Estrin: Sripta Metall. Mater. Vol. 26 (1992), p [2] J.C. Huang, G.T. Gray: Sripta Metall. Mater. Vol. 24 (1990), p. 85. [3] S. Kumar, J. Król, E. Pink: Sripta Mater. Vol. 35 (1996), p.775. [4] N. Ilić, D.J. Drobnjak, V. Radmilović, M.T. Jovanović, D. Marković: Sripta Mater. Vol. 34 (1996), p [5] S.J. Zambo, J.A. Wert: Sripta Metall. Mater. Vol. 29 (1993), p [6] S. Kumar, E. Pink: Sripta Metall. Mater. Vol. 32 (1995), p [7] F. Chmelík, E. Pink, J. Król, J. Balík, J. Pešička, P. Lukáč: Ata mater. Vol. 46 (1998), p [8] S. Kumar, E. Pink: Ata mater. Vol. 45 (1997), p [9] J. Mizera, K.J. Kurzydlowski: Sripta Mater. Vol. 45 (2001), p [10] X.-M. Cheng, J.G. Morris: Sripta Mater. Vol. 43 (2000), p [11] Y.Z. Shen, K.H. Oh and D.N. Lee: Sripta Mater. Vol. 51 (2004), p [12] S. Matthies: Phys stat sol. Vol. 101 (1980), p 111. [13] M.H. Tosten, A.K. Vasudevan, P.R. Howell: Metall. Trans. Vol. 19A (1988), 51. [14] D.B. Williams, J.W. Edington: Metal Si. Vol. 9 (1975), p.529. [15] K.-H. Kim and D.N. Lee: Ata mater. Vol. 49 (2001), p [16] D.N. Lee and K.-H. Kim, Innovations in Proessing and Manufaturing of Sheet Materials (edited by M.Y. Demeri, TMS, 2001), p [17] P.A. Bek and H. Hu: Trans. AIME Vol. 194 (1952), p. 83. [18] O. Engler: Ata mater. Vol. 48 (2000), p [19] D.N. Lee and K.H. Oh: J Mater. Si. Vol. 20 (1985), p