ULTRA-FINE GRAINED Al-Mg-Sc BASED ALLOYS STUDIED BY IN-SITU TRANSMISSION ELECTRON MICROSCOPY. Karel DÁM a, Pavel LEJČEK b

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1 ULTRA-FINE GRAINED Al-Mg-Sc BASED ALLOYS STUDIED BY IN-SITU TRANSMISSION ELECTRON MICROSCOPY Karel DÁM a, Pavel LEJČEK b a Department of Metals and Corrosion Engineering, Institute of Chemical Technology Prague, Technická 5, Prague 6, Czech Republic, damk@vscht.cz b Institute of Physics, ASCR, Na Slovance 2, Praha 8, Czech Republic Abstract Metallic materials with very fine structure exhibit enhanced properties compared to their coarse-grained equivalents. Moreover, certain chemical composition and deformation conditions can lead to superplastic behaviour of these alloys. One of the methods to produce this kind of materials is Equal Channel Angular Pressing (ECAP) which is based on the Severe Plastic Deformation (SPD) principle. Aluminium-magnesium alloys are widely commercially used, and addition of small amount of scandium improves their structural stability at increased temperatures which is essential for superplastic deformation ability. In this work Al-Mg- Sc alloy as well as Al-Mg and Al-Sc binary alloys (after ECAP) were submitted to in-situ transmission electron microscopy (TEM) examinations that contained annealing and straining. Dynamic changes in microstructure were observed and their influence on the deformation behaviour of the alloys was discussed. It is shown that considerable structural changes occur during the preheating before superplastic deformation. Processes such as grain coarsening, intermediary phase precipitation, (sub)grain boundary migration and dislocation movement were observed and discussed. Key words: ECAP, in-situ TEM, UFG materials, aluminium alloys 1. INTRODUCTION Development of metals with very fine structure has become a subject of great interest. This is because these materials have been confirmed to exhibit enhanced functional properties and also show some new qualities, e.g. superplasticity. There is number of methods used to produce nanocrystalline or ultra-fine grained (UFG) structures. One of the most promising methods is severe plastic deformation (SPD). It can be defined as metal forming procedure in which a very high strain is imposed on a bulk solid leading to the production of submicron-grain-sized or nanostructured metals. SPD includes several techniques with equal-channel angular pressing (ECAP) being the most popular representative. During this process high strains can be imposed on the worked material without change of its shape or dimensions. This procedure is especially attractive because it has some advantages compared to the other methods, e.g. it can be applied to fairly large billets and it is relatively simple method which can be performed on a wide range of alloys. [1-3] As was mentioned above some UFG metallic materials can have another interesting property superplasticity. This is an ability of polycrystalline material to exhibit very high uniform elongation without failure under deformation in a specific temperature range and relatively low strain rate ( s -1 ). Further grain refinement leads to the occurrence of superplasticity at lower temperatures and/or higher strain rates. This is the purpose of studies of UFG metallic materials with grain size below 1 μm [4]. Recent experiments have confirmed that superplasticity can be achieved in specimens subjected to ECAP but two basic aspects are required. First, the material must have very small grains and stable structure and second, superplasticity is achieved only at relatively high temperatures. Thus, materials suitable for this purpose are alloys containing a fine dispersion of precipitates that impede grain boundary mobility and restrict grain growth at elevated temperatures.

2 Several superplastic aluminium alloys have been developed but Al Mg based alloys belong to the group of most widely used for superplastic forming. This is because they have many interesting properties such as good weldability, good corrosion resistance, high strength and good formability which improve significantly in superplastic state [5, 6]. Recent studies also revealed that small addition of scandium significantly improves superplastic behaviour of the conventional Al Mg alloys. This is because of the presence of fine dispersion of coherent L1 2 phase (Al 3 Sc) [7]. The well-distributed, nanoscale-coherent Al 3 Sc precipitates are extremely thermodynamically stable, are very effective in dislocation pinning thus having a strengthening effect and it has been shown that they can restrict the grain grow which is suitable for superplasticity [8, 9]. A number of studies have confirmed that Al Mg Sc alloys subjected to severe plastic deformation exhibited high strain rate superplasticity with high ductility. The highest elongations to failure achieved for this alloy processed by ECAP exceeded 2000%. (e.g. [10]) In most of the cases, tensile testing for superplastic deformation contains step where specimens are held for a certain time (usually 15 min) at relatively high temperature before the straining starts. The aim of the present study is to provide additional information about structural processes which occur during the initial stage of superplastic deformation tests of the Al Mg Sc based alloys. Despite number of studies about their superplastic ability, these processes have not been discussed. During the starting period significant changes in microstructure can be present. For this purpose the in-situ transmission electron microscopy (TEM) was used. This allows observing dynamic changes in the very fine structure which is caused by annealing and straining. 2. EXPERIMENTAL PROCEDURE The used alloys had nominal compositions of Al 3%Mg 0.2%Sc, Al 0.2%Sc and Al 3%Mg (wt.%). They were prepared by induction melting in graphite crucible under an argon atmosphere (vacuum furnace Balzers VSG 02) followed by casting into ingots with dimensions of 14x14x120 mm 3. Al of % purity, a master alloy with composition of Al-2wt.%Sc and/or Mg of 99.9% purity were used as starting materials for melting. The alloys were homogenized in air for 24h at 753K (Al Mg Sc) and 913 K (Al Sc), solution treated for 1h at 863 K (Al Mg Sc) and 883 K (Al Sc) and eventually water quenched. The specimens for ECAP had the cross section of 10x10 mm 2 and the length of approx. 55 mm. The ECAP was conducted using a solid die fabricated from tool steel containing two internal channels of the same cross section having an angle Ф = 90 and an additional angle ψ = 45 representing the outer arc of curvature at the intersection of the two channels. The pressing was performed on the INSTRON 5882 machine. The specimens were pressed repetitively for up to 4 passes. Each specimen was rotated by 90 in the same direction after each pass in the procedure (route B C [3]). During the entire processing, the values of microhardness of the Al-3%Mg-0.2%Sc (wt.%) alloy were measured. It means after casting, homogenization, solution treatment, ECAP processing and after subsequent annealing (273K/15min) simulating the preheating stage of superplastic deformation.. Transmission electron microscopy was carried out on the JEOL 1200 EX operating at 120 kv. For the in-situ experiments a self-made TEM straining holder which allows heating a sample up to 573K was used. The processing of the specimens consisted of heating up from room temperature up to 573 K, with the heating rate of approximately 20 K.min. It took 15 min to reach the final temperature. Then, straining was initiated, while measuring the applied force. This was increased in pulses after relaxation processes were observed. 3. RESULTS AND DISCUSSION 3.1. Microhardness measurements Changes in mechanical properties of the Al 3%Mg 0.2%Sc alloy were monitored during the whole processing. Chyba! Nenalezen zdroj odkazů. shows the values measured after certain stages. It can be seen that the microhardness decreased after solutionizing which reveals the precipitation hardening effect of

3 the Al 3 Sc precipitates [11]. The significant increase after ECAP processing was caused by the grain refinement of the alloy. The final stage was annealing for 15 min at the temperature of 573K. This illustrates effect of preheating, which is applied before superplastic deformation, on the mechanical properties. It can be seen that the alloy lost most of the increment in hardness caused by ECAP. Although, the precipitation of the Al 3 Sc phase causes that the final value is at the same level as in the as cast condition In-situ TEM Fig. 1: Microhardness of the Al 3Mg 0.2Sc (wt.%) alloy during the processing Fig. 2 shows the microstructures of the Al Mg alloy during the in-situ TEM experiment. This is presented just to demonstrate the role of Mg in the Al Mg Sc alloy during the examined processing. At the beginning the alloys were in the as ECAPed condition (Fig. 2a). Grain size was reduced to ~ nm and the microstructure contained both equiaxed and elongated grains. Compared to pure Al, which has the grain size of ~1µm after the ECAP processing, the presence of magnesium in the solid solution reduces the recovery rates and leads to additional grain refinement [12]. Fig. 2b was taken after 15 min of annealing in the microscope. During this period dynamic structural changes were observed, particularly recrystallization, dislocation annihilation or subgrain boundary motion. The grains enlarged significantly to the value of ~2µm. Additional straining caused further grain coarsening and strain induced grain boundary motion was observed in the structure (Fig. 2c). Fig. 2: Microstructure of the Al 3Mg (wt.%) alloy after 4 passes by ECAP: (a) initial microstructure, (b) after annealing, (c) after additional straining.

4 In Fig. 3 results of the binary Al Sc alloy are presented. The grain size in as ECAPed state was ~1µm which supports the theory of the Mg effect on grain refinement (Fig. 3a). During the in-situ annealing, the process of precipitation of the Al 3 Sc intermediate phase was observed; Fig. 3b shows the microstructure after this stage. The dispersion of fine precipitates started to occur at approximately 160 C. Some dislocation annihilation was also observed but no significant grain coarsening was detected, either during additional straining (Fig. 3c). Presence of the coherent precipitates of the Al 3 Sc phase in the structure has a strong impact on the stability of the structure. Some authors have suggested that it slows down the grain boundary movement but during our experiment almost no movement was detected [1, 13]. Fig. 3: Microstructure of the Al 0.2Sc (wt.%) alloy after 4 passes by ECAP: (a) initial microstructure, (b) after annealing, (c) after additional straining. The structural development of the superplastic ternary Al Mg Sc alloy is depicted in Fig. 4. The grain size after the ECAP was evaluated to ~300 nm (Fig. 4a) and contained both equiaxed and elongated grains, which was similar to the Al Mg alloy (Fig. 2a). Fig. 4b shows the alloy after the in-situ annealing, during which several processes were observed. It started with dislocation annihilation. Then subgrain boundaries migrated to form equiaxed grains which was followed by further grain coarsening. Meanwhile, the dispersion of Al 3 Sc phase precipitated to restrict the grain growth. It can be seen that after 15 min of annealing the grain size increased to ~1µm. It should be taken into account that the initial microstructure of this superplastic alloy could vary from that observed in the as-ecaped condition because its potential influence on the further superplastic deformation. Fig. 4c shows that additional annealing with subsequent straining does not cause any further grain coarsening. Uniformly distributed dispersion of the fine, coherent precipitates is visible in the presented micrographs (Fig. 4b,c). Fig. 4: Microstructure of the Al 3Mg 0.2Sc (wt.%) alloy after 4 passes by ECAP: (a) initial microstructure, (b) after annealing, (c) after additional straining. 4. CONCLUSIONS The microhardness measurements showed how the mechanical properties of the Al-Mg-Sc alloy vary during the processing leading to superplasticity of this alloy and revealed the significant decrease during the preheating before deformation. The in-situ experiments were used to examine the role of the additional elements. Substantial changes in the microstructure of the Al Mg, Al Sc and Al-Mg Sc were indicated. The

5 experiments simulated the microstructural development of the Al-Mg-Sc system during the early stages of superplasticity. The observations pointed out that considerable structural changes occur during the preheating period prior superplastic deformation. ACKNOWLEDGEMENTS The authors would like to thank to the Academy of Sciences of the Czech Republic (grant No. KAN ) and to the Specific University Research (MSMT No. 21/2012) for the financial support. The possibility of using experimental equipment of the department of metals at the Institute of Physics AS CR, is gratefully acknowledged. LITERATURE [1] LEE, S., UTSUNOMIYA, A., et al. Influence of scandium and zirconium on grain stability and superplastic ductilities in ultrafine-grained Al-Mg alloys. Acta Materialia, 2002,vol. 50, nr. 3, pp [2] PEREVEZENTSEV, V., SHCHERBAN, M., et al. High-strain-rate superplasticity of nanocrystalline aluminum alloy Technical Physics Letters, 2007,vol. 33, nr. 8, pp [3] VALIEV, R.Z., LANGDON, T.G. Principles of equal-channel angular pressing as a processing tool for grain refinement. Progress in Materials Science, 2006,vol. 51, nr. 7, pp [4] VALIEV, R., ISLAMGALIEV, R., et al. Superplasticity of nanostructured metallic materials obtained by methods of severe plastic deformation. Metal Science and Heat Treatment, 2006,vol. 48, nr. 1, pp [5] MUSIN, F., KAIBYSHEV, R., et al. High strain rate superplasticity in a commercial Al-Mg-Sc alloy. Scripta Materialia, 2004,vol. 50, nr. 4, pp [6] PARK, K.-T., HWANG, D.-Y., et al. High strain rate superplasticity of submicrometer grained 5083 Al alloy containing scandium fabricated by severe plastic deformation. Materials Science and Engineering A, 2003,vol. 341, nr. 1-2, pp [7] RØYSET, J., RYUM, N. Kinetics and mechanisms of precipitation in an Al 0.2 wt.% Sc alloy. Materials Science and Engineering: A, 2005,vol. 396, nr. 1-2, pp [8] HORITA, Z., FURUKAWA, M., et al. Superplastic forming at high strain rates after severe plastic deformation. Acta Materialia, 2000,vol. 48, nr. 14, pp [9] LEE, S., FURUKAWA, M., et al. Developing a superplastic forming capability in a commercial aluminum alloy without scandium or zirconium additions. Materials Science and Engineering A, 2003,vol. 342, nr. 1-2, pp [10] LIU, F.C., MA, Z.Y. Achieving exceptionally high superplasticity at high strain rates in a micrograined Al-Mg-Sc alloy produced by friction stir processing. Scripta Materialia, 2008,vol. 59, nr. 8, pp [11] VENKATESWARLU, K., RAJINIKANTH, V., et al. The characteristics of aluminum-scandium alloys processed by ECAP. Materials Science and Engineering: A, 2010,vol. 527, nr. 6, pp [12] FURUKAWA, M., UTSUNOMIYA, A., et al. Influence of magnesium on grain refinement and ductility in a dilute Al-Sc alloy. Acta Materialia, 2001,vol. 49, nr. 18, pp [13] KOMURA, S., BERBON, P.B., et al. High strain rate superplasticity in an Al-Mg alloy containing scandium. Scripta Materialia, 1998,vol. 38, nr. 12, pp