SERBIATRIB th International Conference on Tribology. Kragujevac, Serbia, May 2011

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1 Serbian Tribology Society SERBIATRIB th International Conference on Tribology Kragujevac, Serbia, 3 May 0 Faculty of Mechanical Engineering in Kragujevac INHOMOGENITIES OF PLASTIC DEFORMATION SERRATIONS IN COMERCIAL Al-Mg ALLOYS 3 Lj.Radovi, M.Nika evi, M.Popovi, E.Romhanji, B.Jordovi Military Technical Intitute, Belgrade, Serbia; ljmradovic@gmail.r, banikac@ptt.r Faculty of Technology and Metallurgy, Belgrade, Serbia, amiljana@tmf.bg.ac.r, bendre@tmf.bg.ac.r 3 Technical Faculty a ak, Serbia, bjordovic@tfc.kg.ac.r Abtract: Platic deformation of olid olution i often accompanied by platic intabilitie due to dynamic train aging (DSA) and dilocation interaction. The dilocation interaction lead to tre erration and localized train (deformation band) in tenile tet, known a the Portevin-Le Chatelier (PLC) effect. The intable PLC deformation i cloely connected with a localization of train within the front of (propagating) deformation band. Thee band ignificantly limit the heet formability leading to unacceptable urface roughne or premature fracture. The characteritic of errated yielding of commercial Al-Mg heet (3-6%Mg) have been tudied uing room temperature tenile teting. Tenile tet were performed at train rate of and. The initiation of errated flow wa found to depend on Mg content a well a train rate. The type of the erration alo, wa governed by the train rate and Mg content. The amplitude of the erration () increaed with increaing train and decreaing train rate. The noticed dependence of the Mg content reflect the role of Mg atom in locking/pinning the dilocation. The train rate howed no influence on the yield tre, while the general tre level and ultimate tenile tre have increaed with decreae of train rate. Therefore, it lead to increae the difference between ultimate and yield trength (Rm-R 0. ), and tronger work hardening effect, becaue of uppreion of the dynamic recovery in Al-Mg alloy. Keyword: Al-Mg alloy, errated yielding, Mg content, train rate, The Portevin-Le Chatelier effect (PLC).. INTRODUCTION Lightweight Al-Mg alloy have been widely ued in many field becaue of their optimal combination of trength, formability, corroion reitance and weldability. However, their untable platic flow retrict their application. Untable flow appear a a yield point elongation (or Lüder elongation) within the few percent of deformation, and dicontinuou or errated yielding at higher train, tipical for indutrial production. The Lüder elongation during uniaxial tretching caue the appearance of urface relief known a "A" ("flamboyant"), a dicontinuou yielding caue the appearance yielding "B" ("parallel band") type urface marking -4. Both tretcher marking are harmful becaue they caue pronounced urface roughne. 5 Dicontinuitie in the flow tre a a manifetation of dynamic train ageing, are commonly oberved during deformation of f.c.c. olid-olution aluminum alloy under certain circumtance. It ha been extenively invetigated in aluminium alloy containing magneium [5-0]. In the mot accepted model errated flow i related to the dynamic interaction between diffuing olute atom and mobile dilocation, i.e. dynamic train ageing (DSA). Solute atom retrict the mobility of the dilocation and make dilocation rearrangement and annihilation more difficult. Mg atom are particularly effective in thi becaue of their large atomic ize difference with aluminium. Solute preferentially concentrate at dilocation due to the energetic interaction of the two ource of field, forming olute atmophere at mobile dilocation and arret them. When the applied force i raied high enough, the mobile dilocation th International Conference on Tribology Serbiatrib

2 break away from the atmophere and advance to the other obtacle. Repetition of thi proce i manifeted a the errated flow oberved in the tre-train curve. The amplitude of the erration, defined a the tre change from the highet to the lowet point of the erration, i ued for evaluated /the main meaure of the errated yielding. Different erration type can be identified from macrocopic tre-train curve a a reult of difference in microtructural train localization./microtructure and teting condition. Reearche claify erration a type A, B, C D or E, which can occur ometime imultaneouly, depending on the teting condition [,0-]. Different type of erration can overlap each other. Type A erration are characterized by an abrupt rie of load (tree) above the general level of the load-extenion (tre-train) curve, type B are marked by fluctuation load around the general level of curve, and type C erration are characterized by fall of load below the general level of curve. In many cae it i not clear to make ditinction between B and C erration. It eem that C erration drop to lower tree more abruptly than B erration and A and B erration do not occur under the ome tet condition [6,9,] More recently a laer canning extenometer wa ued to identify erration type baed on deformation band propagation [3-5]. Detailed decription of erration type wa done in ref 7. In order to decribe the amplitude of the erration (), defined a the tre change from the highet to the lowet point of the erration, ha been invetigated [6,6-0]. The trength of Al-Mg alloy arie from everal factor. The Mg in olution provide olution hardening, and thee alloy alo have a ignificant Hall-Petch lope compared with other Al alloy, o grain ize trengthening i important. The train hardening ignificant contribute to the trength becaue of the high work hardening rate. Apart well known influence Mg content on the mechanical propertie, it would be expected that Mg content ha influence on deformation behaviour and the characteritic of errated yielding. Further, it i known that deformation behaviour and mechanical propertie of Al-Mg alloy are enitive to train rate. The preent paper i focued on ome experimental obervation of characteritic of errated yielding in three commercial magneium-containing aluminium alloy a function of deformation variable.. EXPERIMENTAL To invetigate the influence of Mg content and the train rate on the deformation behaviour, three annealed commercial Al-Mg alloy, with average grain ize of approximately 5 m were teted. The chemical compoition of the alloy are: AlMg3 (3. Mg, 0.03 Mn, 0.3 Fe, 0.09 Si); AlMg4.5Mn (4.55 Mg, 0.47 Mn, 0.4 Fe, 0.6 Si), AlMg6Mn (5.95 Mg, 0.54 Mn, 0.36 Fe, 0. Si). Tenile tet were carried out at room temperature on a Zwick teting machine, uing mall ASTM tenion pecimen with a 5 mm gauge length. To invetigate the influence of the train rate on the mechanical propertie and errated yielding, different initial train rate of and were applied RESULTS The et of typical deformation curve of annealed Al-Mg alloy with grain ize of about 5 m, are hown in Fig. and. Well developed erration have been oberved in all invetigated alloy. The critical train for errated yielding c =0, i.e. erration began immediately after the Lüder elongation and continued until failure. The amplitude of the erration beyond the Lüder train increae with train. During initial deformation at lower train rate (6, ) A+B type of erration were oberved, and with increaing train they changed to type B. Thi behaviour wa oberved in both AlMg3 and AlMg4.5 alloy. Type C wa oberved in AlMg6Mn alloy at the ame train rate (load fall below the general level of the curve). With increaing train rate, at train rate of 6, , in AlMg3 alloy the type A erration were oberved, while A, A+B and B erration were oberved in the other two alloy (Fig. ). Depite that A and B erration occur in both alloy, type A erration i le prominent in AlMg6Mn than in AlMg4.5Mn alloy, i.e. type B dominate in the AlMg6Mn alloy. The amplitude of the erration teadily increae a a function of train in all cae. Apart from difference in the type of the erration, there i alo a difference in the amplitude of erration, (Fig. 3) how the maximum amplitude of the erration,, a a function of train rate and Mg content. depend trongly on both Mg content and train rate. A the Mg content increae, amplitude of the erration raie almot linearly, while the increae of train rate decreae it. th International Conference on Tribology Serbiatrib 53

3 The effect of the train rate and Mg content on the mechanical propertie are hown in Fig. 4. Ultimate tenile trength lightly increae with decreaing train rate and increae with increae in Mg content. 4. DISCUSSION Lüder yielding and errated flow were etablihed to confirm the inhomogenou deformation during uniaxial tenion. The Lüder yielding at the onet of platic flow in the annealed condition in teted Al-Mg alloy indicate a low initial denity of mobile dilocation. The Lüder yielding in ubtitutional alloy, like Al-Mg, occure when concentration of olute atom i enough to form atmopere around the mobile dilocation and block them. To continue deformation the dilocation have to break away from thee atmophere and multiply. Figure. Deformation curve of Al-Mg alloy at train rate Figure. Deformation curve of Al-Mg alloy at train rate egment with erration. 54 [MPa] ,7 0 6,7 0 3,0 4,5 6,0 Mg [%] Figure 3. The influence of magneium content and train rate on amplitude of errated yielding (). So, the Lüder yielding fenomenon depend on relation between concentration of olute atom and dilocation denity [], and become more pronounced with increaing of thi relation [0]. The abence of Lüder yielding wa oberved only in AlMg6Mn alloy with coare grain [] at both train rate (imilar reult wa publihed earlier for AlMg6.5 alloy with grain ize of m ref. ). Thi apparently i due to both the higher Mg content and large grain. The addition of Mg increae denity of mobile dilocation, produced in activated dilocation ource [8,0]. Thi activation of dilocation ource i enhanced in large grain tructure, together with the preading of deformation to neighbouring grain. Thi ugget that the high denity of dilocation in coare grained AlMg6Mn alloy i ufficient to remove yield point (inhomogenitie) oberved in the other two alloy. The errated yielding ha occurred in all invetigated alloy under the teting condition and alo increae from AlMg3 to AlMg4.5Mn and than AlMg6Mn alloy (Fig and ). Mg content and train rate affect the type and frequency of the erration, but the influence of the grain ize wa not oberved. At low train rate tet (~0-4-) type A+B and B erration occure in both AlMg3 and AlMg4.5Mn alloy, but in AlMg6Mn alloy only C type occure. At higher train rate (~0-3-), while the A type erration dominate in AlMg3 alloy, with further increae in Mg content type A become le prominent, reulting in the domination of type B erration. Thi difference i related to the Mg content. Obviouly, the increae of the Mg content a well a decreae of the train rate lead to change type of erration on the tre-train curve from A B C. It eem that when the arret mobile dilocation become more effectively, a a reult of addition Mg and lower train rate, it lead to change of type of erration from A via B to C. C type of th International Conference on Tribology Serbiatrib

4 the erration only fall below the general tretrain dependence and it i conidered that thi type refer to the unlocking of dilocation from olute pinning and called unlocking erration. On the other hand type A or B erration refer to the locking of dilocation by olute and called locking erration. That kind of relationhip ha been found in ref. [9,,0]. It eem that the mot effective impede of dilocation and conequently the highet flow tre (Fig. 4) correpond to the appearance of the type C of erration, but/although the exact mechanim leading to either A and B or to C erration are till hardly known. The intenity of the errated yielding, evaluated by the amplitude of the erration, ha howed a relatively trong dependency of the Mg content and train rate (Fig. 3). The moving dilocation are temporary held at obtacle, o Mg atom than diffue to thee dilocation and arret them. It wa hown that both the mobile and foret dilocation denitie increae with increaing Mg content [8,0 Horvath, Robinon]. Moreover, the increae of the mobile dilocation i higher than foret dilocation, becaue Mg enhane trongly the multiplication of mobile dilocation. The addition of Mg decreae the mean free path for dilocation motion, o the dilocation interect each other more frequently. Alo, the addition of Mg uppree the recovery of the Al-Mg alloy becaue the tocking fault energy decreae and cro-lip become more difficult []. Thi enhance trongly the multiplication of the mobile dilocation, leading to their higher denity. The more dilocation temporary held at obtacle, the higher force i needed for dilocation unpinning, reulting in higher amplitude of the erration on the tre-train curve,5,0,0,3-5. Strain rate analyi give a good indication of the intenity of dynamic train ageing, too. It i aumed to be due to lower rate of mobile dilocation, which i proportional to train rate []. Therefore, the waiting time for the mobile dilocation at obtacle i longer which i manifeted a an increae of the amplitude of the erration. The mobile dilocation can be arreted more effectively by olute atom at lower train rate (~0-4 - ) than at higher (~0-3 - ), leading to the increae of the amplitude of the erration. It wa found that the yield tre i rather train rate independent in annealed Al-Mg alloy. On the other hand the ultimate trength exhibit invere train rate dependence. At higher train rate the level of flow curve decreae. The minor influence the train rate on the yield tre indicate that the concentration of the olute /Mg atom / in annealed Al-Mg alloy (3-6 wt.%mg) i ufficient to form atmophere and block mobile dilocation at the onet of the deformation and the tre for unpinning dilocation i.e. multiplication mobile dilocation i independent of the train rate. The trength of the teted Al-Mg alloy in annealed condition i affected primarily by olution trengthening/hardening from the Mg (and/or other) atom and grain ize trengthening, according to the well known Hall-Petch relationhip []. According to the previouly reported reult, the frictional tre 0 in the Petch equation, which reflect olution hardening due to Mg in olution, increae linearly with Mg content [3]. The addition of approximately 0.5wt. % Mn increae trength of AlMg4.5Mn and AlMg6Mn alloy, mainly due to olid olution trengthening. 5. CONCLUSIONS Deformation behaviour ha been invetigated in three annealed commercial Al-Mg alloy (AlMg3, AlMg4.5Mn, and AlMg6Mn) at room temperature and train rate of 6, and 6, Well developed erration have been oberved in all invetigated alloy. The intable PLC deformation i cloely connected with a localization of train within the front of (propagating) deformation band. Thee band ignificantly limit the heet formability leading to unacceptable urface roughne or premature fracture. Serration of A, B, A+B and C type were oberved on tre-train curve. The type and frequency of erration were governed by Mg content and train rate. While the A type erration dominate in AlMg3 alloy, further increae in Mg content decreae it, reulting in the domination of type B erration in higher train rate or tet C type in low train rate tet in AlMg6Mn alloy. The intenity of the errated yielding, evaluated by the amplitude of the erration/tre drop, (), increaed with increaing train and decreaing train rate. The noticed dependence amplitude of the erration of the Mg content reflect the role of Mg atom in locking/pinning the dilocation. REFERENCE [] Ildong Choi at al. Scripta Mat. Vol. 38, No.6, 998, [] H.Fujita and T.Tabata, Acta Met., Vol. 5, 977, [3] D.J.Lloyd, Metall. Tran., Vol. A, 980, [4] E.Romhanji et al., MJoM Metalurgija - Journal of Metalurgy, Vol 0 No 3, 004, [5] Wei Wen, J.G.Morri, Mat.Sci.Eng A354, 003, th International Conference on Tribology Serbiatrib 55

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