Superplasticity in a 5024 Aluminium Alloy Processed by Severe Plastic Deformation

Transcription

1 Materials Science Forum Online: ISSN: , Vol. 735, pp doi: / Trans Tech Publications, Switzerland Superplasticity in a 5024 Aluminium Alloy Processed by Severe Plastic Deformation Anna Mogucheva a, Diana Tagirova b, Rustam Kaibyshev c Belgorod State University, Pobeda 85, Belgorod, , Russia a mogucheva@bsu.edu.ru, b tagirovadiana@mail.ru, c rustam_kaibyshev@bsu.edu.ru Keywords: superplasticity, equal channel angular pressing, ultrafine grained structure, aluminium alloy Abstract. The superplastic behaviour of an Al 4.6%Mg 0.35%Mn 0.2%Sc 0.09%Zr alloy was studied in the temperature range ºC at strain rates ranging from 10-4 to 10-1 s -1. The AA5024 was subjected to equal channel angular pressing (ECAP) at 300 C up to ε~12. The highest elongation-to-failure of 3300% was attained at a temperature of 450 C and an initial strain rate of s -1. Regularities of superplastic behaviour of the 5024 aluminium alloy are discussed. Introduction Equal-channel angular pressing (ECAP) is a procedure whereby a material is subjected to severe plastic strain without any concomitant change in the cross-sectional dimensions of the work piece [1]. This processing method is capable of reducing the grain size of materials to, typically, the submicrometer or even nanometer scale level [1]. As a result, much interest has developed in using ECAP to fabricate reasonably large bulk samples with ultrafine grain sizes [2]. An AA5024 has recently become commercially available for the fabrication of airframes, stiffened panels and tanks for liquid gas storage used in aerospace industry at cryogenic temperatures. This alloy was initially developed in Russia and designated 1545K [3]. It was shown that ECAP is very effective in strengthening the 5024 aluminium alloy due to extensive grain refinement [4]; high strength thin sheets can be easily produced from this material by combination of ECAP with subsequent rolling. This process is capable of scale-up to produce billets with commercial dimensions. Al Mg Sc alloys are highly formable aluminium alloys with superior superplastic properties compared to conventional Al Mg alloys with the same magnesium content [5-7]. The uniformly distributed nanoscale coherent Al 3 Sc dispersoids in the alloys belonging to Al-Mg-Sc system are highly effective at stabilizing the ultrafine-grained (UFG) structure produced by ECAP [8-10] or other techniques based on severe plastic deformation [11,12]. As a result, these alloys exhibit superior superplastic ductilities at high strain rates and low temperatures [8-13]. However, there is no information on workability of the AA5024 subjected to ECAP. The aim of this work is to report superior superplastic behaviour of the 5024 alloy. Experimental Procedures The AA5024 with a chemical composition of Al-4.6Mg-0.35Mn-0.2Sc-0.09Zr (wt.%) was produced by continuous casting. Next, this alloy was hot rolled in the temperature interval o C with a total reduction of 70%. This processing resulted in the microstructure with an average grain size of about 17 µm and the dislocation density of m -2. The details of initial microstructure of the AA5024 were reported elsewhere [3,14]. Details of ECAP using a die with a rectangular cross-section of mm 2 and a height of 180 mm were reported in work [15]. The billets having plate-like shape were machined from the extruded ingot and pressed 12 times through the ECAP die at a temperature of 300 o C with approximate total accumulated strain of ~12. The samples were rotated by 90 o around the Z axis between each pass in the same direction. In addition, the samples were rotated by 180 o around the X axis, i.e. the modified route B CZ [1,16] was used. The pressing speed was approximately 3 mm/s. 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-06/03/16,16:28:15)

2 354 Superplasticity in Advanced Materials - ICSAM 2012 After ECAP the tensile specimens with a 6 mm gauge length and cross-section of mm2 were cut parallel to the last extrusion axis of the pressed billets. Tensile tests were carried out using an Instron 5882 testing machine in the temperature interval C at strain rates ranging from to s-1. Each sample was held at the testing temperature for about 10 min in order to reach thermal equilibrium. The values of the strain rate sensitivity (m) were determined by strainrate-jump tests [17,18]. The strain hardening coefficient was determined as dσ Θ = 14 dε (1) The microstructure of the 1545Al was examined by techniques of transmission electron microscopy (TEM), electron backscattering diffraction (EBSD) analysis, which were described in previous papers in details [3,14,15]. The microstructures were examined in initial condition and after static annealing in grip sections of specimens tensioned at 450 and 500 C and ε = s 1. Results and Discussion Microstructure after ECAP. A typical microstructure of the AA5024 processed by ECAP is presented in Figure 1. It is seen that the formation of almost fully recrystallized structure is completed (Fig. 1). Most of the deformation-induced boundaries exhibit high angle misorientation (HAGBs), and recrystallized grains exhibit an equiaxed shape. The population of HAGBs is 90 pct; the average misorientation is 37º (Fig. 1(b)). Figure 1. Microstructures of the 5024 alloy subjected to ECAP.

3 Materials Science Forum Vol The grain size distribution is characterized by sharp peak for fine grain sizes below 2.25 µm (Fig. 1(c)). The mean grain size among these fine grains is 800 nm, and their area fraction comprises ~90 pct. Besides the fine grains the ECAPed microstructure includes relatively large grains with an average size of 4 µm, the area fraction of which does not exceed 10 pct. TEM studies showed that the ECAP resulted in the formation of fine crystallites with an average size of about 460 nm (Fig. 1(d)). Most of crystallites are true grains entirely delimited by HAGBs and containing high density of lattice dislocations (ρ m -2 ). Thus, ECAP up to ε~12 provides extensive grain refinement by a factor of about 20; density of lattice dislocations increases by a factor of ~10 2 in comparison with initially hot-rolled material [3]. Superplastic behaviour. Typical true stress - true strain curves for the ECAP processed AA5024 at initial strain rates of s -1, s -1, s -1 at temperatures ranging from C are shown in figure 2(a,b,c). At T 450 o C, an increase in temperature leads to decreasing flow stress. An exception is the temperature of 500 C, at which the flow stress are essentially the same with that at 400 C (Fig.2(c)) and higher then that at 450 C (Fig.2(a)). Commonly, the flow stress increases to its maximum and then decreases continuously up to the fracture with strain; extensive strain hardening takes place, initially. No well-defined steady-state flow was observed. Figure 2. Effect of temperature at ε= s 1 (a), ε= s 1 (b), ε= s 1 (c), and strain rate (d) at 450 o C on the true stress - true strain curves for the 5024 alloy. Increasing temperature or decreasing strain rate leads to a reduction in the strain-hardening coefficient and a shift of the peak stress to a higher strain. At 450 C and initial strain rates ranging from to s -1, the flow curves are characterised by a large duration of strain hardening stage, which extends up to true strains of about 1.5. The strain hardening coefficient tends to decrease with decreasing strain rate in the strain range of 0.2 to 1. It should be noted that

4 356 Superplasticity in Advanced Materials - ICSAM 2012 the rate of strain hardening for each sample does not vary remarkably in this strain range, leading to almost linear increase in the flow stress with strain. The extension of the strain hardening stage provides uniform elongation and highly promotes achieving superior superplastic ductilities [17]. At T 350oC, the AA5024 alloy demonstrates high ductility (>500% elongation-to-failure) at a low initial strain rate of s-1. Increasing strain rate leads to a reduction in ductility at these temperatures (Fig. 2(b,c)). On the other hand, at T 350 C, an increase in the strain rate slightly affects the elongation-to-failure; ductility tends to increase with increasing strain rates. In the temperature interval oC, the tensioned specimens demonstrate a very uniform elongation irrespective of the strain rates; the fracture occurs without necking. At T<300oC and 500oC, the fracture occurs by unstable plastic flow, while at 500oC, however, plastic deformation within the gauge section is reasonably uniform. The peak stress, the coefficient of strain-rate sensitivity, m, and elongation-to-failure, δ, as functions of strain rate are plotted on a double-logarithmic scale (Fig. 3(a,b,c)). The AA5024 exhibits a sigmoidal relationship between the flow stress and strain rate with maximum strain rate sensitivity coefficient m 0.65 at 500oC and at ε = s-1; three well-known regions of superplastic deformation can be identified [17,18]. In the temperature interval oC the peak stress decreases with increasing temperature at all strain rates examined, while upon further temperature increase to 500oC only at ε = s-1. At higher strain rates, the peak stress increases with increasing temperature from 450 to 500oC. Figure 3. The variation of true stress (a), the coefficient of strain rate sensitivity (b), elongation-tofailure (c) with strain rate and (d) temperature dependencies of the elongation-to-failure for the 5024 alloy. The maximum m and δ values are found to occur in the second region (where m 0.4 and δ 900%) and tend to decrease on either side of the strain rate associated with these maximum values (Fig. 4(b) and (c)) at T 300oC. At 250oC, no decrease in elongation-to-failure with

5 Materials Science Forum Vol decreasing strain rate was found due to the fact that experimental data at ε < s -1 were omitted from the present work. It is worth noting at 450 o C, the highest m value of 0.55 is observed at ε = s -1, while highest elongation-to-failure of 3300% appeared at ε = s -1 corresponding with unreasonable low value of m 0.2 [17]. In the temperature interval o C the highest δ values are observed at strain rates, which are higher by a factor of 10 than strain rates, at which the highest m values attained. In contrast, at 500 o C, the difference between strain rates corresponding with the highest m and δ values is lowest. In general, an increase in temperature from 250 C to 450 C leads to a shift of the optimum strain-rate interval of superplastic deformation [17,18] to higher strain rates, while under further temperature increase to 500 C the opposite trend is observed (Fig. 3(b) and (c)). At 250 C, ductility increases from 95 to 885% with decreasing strain rates from s -1 to s -1. In the temperature range of C, superior ductility ( 1000%) is observed in the wide strain rate range s -1. A higher ductility ( 1000%) is observed also at m 0.4. At ε = s 1, an increase in temperature from 250 C to 450 C leads to an increase in δ (Fig. 3(d)). Upon further temperature increase to 500 o C, the elongation-to-failure drops rapidly in the strain rate region s -1, while atε s -1 the δ value increases with increasing temperature (Fig.3(d)). Figure 4. Microstructure in grip sections of samples tested at ε = s -1 and 450 C (a) and 500 C (b) Microstructure after static annealing. It is seen that in the AA5024 alloy, the UFGs produced by ECAP processing exhibit a superior stability under static annealing up to 450 o C (Fig.4(a)). In contrast, at 500 C, the static annealing leads to extensive discontinuous grain coarsening (Fig.4(b)) [19]; the size of coarse grains ranges from 10 to 18 µm. Continuous coarsening of fine grains also takes place leading to their growth from 1 to 4 µm. Thus, positive temperature dependence of peak stress at T 450 o C and ε s 1 is attributed to extensive grain coarsening. Superplastic properties at high strain rates are very sensitive to stability of UFG structure under static annealing condition. In the same time, at moderate strain rates, the superplastic ductility tends to increase with increasing temperature from 450 to 500 o C despite extensive static grain coarsening. It seems that the last is caused by the occurrence of continuous dynamic recrystallization (CDRX) during superplastic deformation resulting in refinement of coarse grains within gauge length of samples [6,7]. It is known [6,7] that CDRX could provide high superplastic properties in alloys belonging to Al-Mg-Sc system with initial unrecrystallized structure. Thus, ECAP processing of the 5024 alloy provides achieving extraordinary high superplastic elongations due to uniformity of recrystallized structure and its very high resistance to grain coarsening at T 450 o C. This alloy exhibits superior elongation of 3300% at a very high strain rate of s 1 and T=450 o C due to high stability of UFG structure produced by ECAP. In addition,

6 358 Superplasticity in Advanced Materials - ICSAM 2012 this material exhibits a high superplastic ductility of 885% at T=250 o C and ε= s 1. These superplastic ductilities are superior among all alloys belonging to Al-Mg-Sc system and subjected to ECAP. Therefore, superplastic forming of the AA5024 can be used for the fabrication of highvalue components additionally strengthened by ECAP processing [3] for use in the aerospace industry. Resistance of UFG structure against coarsening under static conditions is a prerequisite condition for attaining exceptionally high superplastic ductilities at ε>10 2 s 1, at which CDRX has no time to refine coarsening initial grains [8]. Uniformity of UFG structure and low average size of recrystallized grains are very important for achieving high superplastic elongations at low temperatures (T 250 o C). Acknowledgments This study was supported by Federal Agency for Science and Innovations, Russia, under grant No The main results were obtained by using equipment of Joint Research Centre, Belgorod State University. References [1] R.Z. Valiev, T.G. Langdon, Prog.Mater.sci. Vol. 51 (2006), p [2] S. Ferrasse, V.M. Segal, F. Alford, J. Kardokus, S. Strothers, Mater.Sci.Eng.: Vol. A 493 (2008) p [3] Yu.A. Filatov, V.I. Yelagin, V.V. Zakharov, Mater.Sci.Eng. Vol. A280 (2000) [4] R. Kaibyshev, A. Mogucheva, A. Dubyna, Mater. Sci. Forum Vol (2012), p. 55. [5] Y.-Y. Li, W.-H.Wang, Y.-F. Hsu, Sh. Trong, Mater.Sci.Eng.: Vol.A 497 (2008) p.10. [6] T.G.Nieh, L.M.Hsiung, J.Wadsworth and R.Kaibyshev, Acta Mater. Vol.46 (1998) p [7] R.Kaibyshev, E.Avtokratova, A.Apollonov, R. Davies. Scr. Mater.: Vol.54 (2006) p [8] F.Musin, R.Kaibyshev, Y.Motohashi, G.Itoh, Metall.Mater.Trans.: Vol.35A (2004) p [9] Z. Horita, M. Furukawa, M. Nemoto, A. J. Barnes, T. G. Langdon, Acta Mater. Vol.48 (2000) p [10] S. Lee. A. Utsunomiya, H. Akamatsu, K. Neishi, M. Furukawa, Z. Horita, T.G. Langdon, Acta Mater. Vol.50 (2002) p.553. [11] F.C. Liu and Z.Y. Ma, Scr. Mater. Vol.59 (2008) p.882. [12] F.C. Liu, Z.Y. Ma, L.Q. Chen, Scr.Mater. Vol. 60 (2009) p.968. [13] S. Ota, H. Akamatsu, K. Neishi, M. Furukawa, Z. Horita, T.G. Langdon: Mater.Trans.: Vol. 43 (2002) p [14] A. Mogucheva and R. Kaibyshev, Mater. Sci. For. Vols (2011) p.949. [15] A. Mogucheva, R. Kaibyshev, Adv. Mater. Res.: Vols (2010) p 389. [16] M. Kamachi, M. Furukawa, Z. Horita, T.G. Langdon: Mater. Sci. Eng.A. 361 (2003), p [17] J. Pilling and N. Ridley: Superplasticity in Crystaline Solids, The Institute of Metals, London, 1989, p [18] O.A. Kaibyshev: Superplasticity of Alloys, Intermetallides, and Ceramics, Springer-Verlag, Berlin, 1992, p [19] M. Ferry, N.E. Hamilton, F.J. Humphreys, Acta Mater.: Vol. 53 (2005) p.1097.

7 Superplasticity in Advanced Materials - ICSAM / Superplasticity in a 5024 Aluminium Alloy Processed by Severe Plastic Deformation / DOI References [3] Yu.A. Filatov, V.I. Yelagin, V.V. Zakharov, Mater. Sci. Eng. Vol. A280 (2000) /S (99)