Development of Ultra-Fine Grained Structure in an Al-5.4%Mg-0.5%Mn Alloy Processed by ECAP

Alla Kipelovaa, Ilya Nikulinb, Sergey Malopheyevc, Rustam Kaibyshevd Laboratory of Mechanical Properties of Nanostructured Materials and Superalloys, Belgorod State

University, Pobeda 85, Belgorod 308015, Russia akipelova@bsu.edu.ru, bnikulin_ilya@bsu.edu.ru, cmalofeev@bsu.edu.ru,

drustam_kaibyshev@bsu.edu.ru

Keywords: aluminum alloy, grain refinement, continuous dynamic recrystallization, equal channel angular pressing, severe plastic deformation.

Abstract. Microstructural changes during equal channel angular pressing (ECAP) at the temperatures of 250 and 300°C to the strains ~4, ~8 and ~12 were studied in a coarse-grained Al-5.4%Mg-0.5%Mn-0.1%Zr alloy. At a strain of ~4, the microstructural evolution is mainly characterized by the development of well-defined subgrains within interiors of initial grains and the formation of fine grains along original boundaries. Further straining leads to increase in the average misorientation angle, the fraction of high-angle grain boundaries and the fraction of new grains. However, only at 300°C, the plastic deformation to a strain of Σε~12 leads to the formation of almost uniform submicrocrystalline (SMC) grained structure with an average crystallites size of ~ 0.5 µm. At 250°C, the microstructure remains non-uniform and consists of subgrains and new recrystallized grains. The mechanism of new SMC structure formation after ECAP is discussed.

Introduction

There is currently a significant commercial interest in the development of non-heat treatable aluminum alloys with a SMC structure for structural applications [1]. It is well-known that extensive grain refinement down to the submicrometer range leads to increased hardness and strength of these aluminum alloys at room temperature; ductility retains at sufficiently high level. At high and intermediate temperatures, the fine grained Al-Mg alloys may exhibit extraordinary high superplastic ductilities at high strain rates, typically in the range of ∼10-2-1 s-1 [1]. It is now well established that a substantial reduction in the grain size of aluminum alloys can be attained through severe plastic deformation (SPD) [2]. Several SPD techniques, such as equal channel angular pressing (ECAP) [3,4], accumulative roll bonding (ARB) [5] and multi-directional forging (MDF) [6], are mostly used for producing of SMC structures in aluminum alloys. These methods can impose a large amount of plastic deformation required for the occurrence of grain refinement in aluminum alloys.

The general features of the microstructure evolved during SPD in different aluminum alloys were examined in various works. It was reported [2] that the formation of a SMC structure can occur by the extension and compression of the initial grain boundaries followed by the discontinuous formation of transverse high-angle grain boundaries (HAGBs). Other investigations have emphasized the role of continuous dynamic recrystallization (CDRX) in grain refinement under SPD of aluminum alloys at elevated and high temperatures [3,6,7]. This process associates with the formation of low angle grain boundaries (LAGBs) at low strains and their gradual transformation into high angle ones with further straining due to accumulation of lattice dislocations [3,6-8]. New grains result from the gradual increase in misorientation between subgrains during plastic deformation [3,6,7]. It is worth noting that CDRX was found to be the mechanism of extensive grain refinement during ECAP [6,7] of Al-Mg-Sc containing nanoscale coherent Al3Sc particles. It is associated with the fact that the coherent dispersoids provide the excellent stability of deformation-induced arrays of LAGBs that is a prerequisite condition for the completion of CDRX process [6-8]. As a result, these alloys can be processed in semi-finished products with a very high strength and sufficient ductility. This unique

Materials Science Forum Vols. 667-669 (2011) pp 487-492Online available since 2010/Dec/30 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.667-669.487

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combination of strength and ductility, which typically have opposing characteristics, is attributed mainly to the formation of the SMC structure under SPD. In the same time the typical commercial alloys of 5XXX series belong to Al-Mg system and contain low amount Mn, Cr, Sc or Zr, additionally. It is extremely important to develop a commercial viable route that can provide the formation of a fully recrystallized structure in semi-finished products from these alloys.

The present study was initiated to demonstrate the potential of ECAP in producing a SMC structure in a commercial Al-5.4%Mg-0.5%Mn-0.1%Zr alloy designated as 1561 and denoted here as 1561 Al. Specific attention was paid to the effect of pressing temperature and strain on the microstructural evolution in the 1561 Al subjected to ECAP in an attempt to develop a simple processing method for extensive grain refinement in large scale billets of the 1561 Al that is highly important for commercial use in shipbuilding industry.

Material and Experimental Procedure

The commercial 1561 Al with a chemical composition of Al–5.43%Mg–0.52%Mn–0.1%Zr–0.12%Si–0.014%Fe (in weight %) was manufactured by direct chill casting and subjected to homogenization annealing at 360°C for 6 hours. Next, the 1561 Al was machined to rods with dimension of 20× 20× 100 mm3. These rods were deformed by ECAP at temperatures of 250 and 300°C using an isothermal die with a square cross-section of 20× 20 mm. The channel had an L-shaped configuration with an angle of intersection of 90° and that at the outer arc curvature of 1°. Deformation through this die produced a strain of ~1 in each passage [9]. The rods were pressed repeatedly to several cumulative strains (Σε) of ~4, ~8 and ~12 using route BC. The pressing speed was approximately 3 mm/s. Finally, the samples were water quenched.

Fig. 1. Microstructure of the 1561 Al after annealing at 360°C. Optical (a) and TEM (b) micrographs.

The specimens for microstructural examination were cut from the central area of pressed rods parallel to the pressing direction. For the electron-backscattering diffraction (EBSD) analysis these specimens were slightly electropolished at –30°C in a solution of 30% HNO3 and 70% CH3OH to give a strain-free surface. EBSD orientation maps were recorded using a FEI Quanta 600FEG scanning electron microscope equipped with a high-resolution EBSD analyzer. The arbitrary area was automatically scanned with a step size of ~0.1 µm. The boundaries with misorientation less than 2° were not taken into account. In the data presented, HAGBs were defined as θ ≥ 15° in misorientation and LAGBs as 2° < θ < 15°. HAGBs and LAGBs are depicted in EBSD maps as black and white lines, respectively. The TEM foils were prepared by double jet electro-polishing using above mentioned solution. The foils were examined using a JEM-2100 transmission electron microscope (TEM) operating at 200 kV that was equipped with an INCA energy-dispersive X-ray spectrometer. Particles were identified by semiautomatic energy dispersive X-ray spectroscopy (EDX)-analysis.

488 Nanomaterials by Severe Plastic Deformation: NanoSPD5

Experimental Results

A granular structure with an average grain size of ∼250 µm was observed in the 1561 Al after annealing (Fig. 1a). Relatively large Mg2Si, Al3Mg2 second phase particles with an average size of ~ 200 nm and nanoscale Al6Mn dispersoids with an average size of about 25 nm, were observed in initial structure (Fig. 1b). The present alloy was careful examined in TEM with respect to Al3Zr formation during annealing at 360°C. However, no Al3Zr dispersoids were found.

Fig.2. Typical EBSD maps of the 1561 Al deformed by ECAP to cumulative strain Σε~ 4 (a, b), Σε~ 8 (c, d) and Σε~ 12 (e, f) at temperatures 250°C (a, c, e), 300°C (b, d, f).

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The coarse-grained 1561 Al was subjected to ECAP at temperatures of 250 and 300°C to several strains. Typical EBSD maps of the structure developed at ECAP are shown in Fig. 2. It is clearly seen that the new equiaxed grains outlined by HAGBs alternated with the original grains containing subgrains in interior are developed at both temperatures and strain of ~ 4 (Fig. 2a and 2b). New grains mainly evolved in the vicinity of original boundaries suggesting deformation gradient near such places. The TEM observation of the 1561 Al deformed to Σε~ 4 at 250 and 300°C shows that microstructure mainly consists of deformation bands with a moderate-to-high dislocation density (Fig. 4a, b). The selected area electron diffraction (SAED) patterns form nets indicating that the boundaries having low angle misorientation (Fig. 4a, b). The average misorientation angle, θav, the fractions of HAGBs (VHAGBs) and the fractions of new grains (Vrec) are almost the same (Fig. 3) at both temperatures. It is evident (Figs. 2 a, b and 4 a, b) that the ECAP to cumulative strain Σε~ 4 leads to development of essentially similar microstructures at examined temperatures.

Fig. 3. Effect of cumulative strain on (a) the average misorientation angle (circle symbols) and the fractions of HAGBs (square symbols); (b) the average size of crystallites (circle symbols) and the fractions of new grains (square symbols).

Further straining to Σε ~8 at 250°C does not change deformed microstructure significantly (Fig.

2c). The microstructure becomes aligned in shear direction (Fig. 2c) and θav, VHAGBs, and Vrec are slight increase (Fig. 3). Increased fraction of HAGBs is attributed to extension of original grain boundaries in shear direction. On the other hand, at 300°C the increase in VHAGBs and θav (Fig. 3) is related to formation of new grains instead of subgrains (Fig. 2d). The fraction of new equiaxed grains increased in two times as the strain rise from 4 to 8 due to the transformation of deformation-induced LAGBs into HAGBs (Figs. 2d and 3b). The average size of crystallites at 250 and 300°C almost unchanged and was of ~0.47 and ~0.56 µm, respectively.

At high strains and temperature of 250°C, the microstructure still is non-uniform and consists of colonies of recrystallized grains and subgrains (Fig. 2e). The fraction of HAGBs increases insignificantly while pronounced increase in volume fraction of new grains to ~0.63 was found (Fig. 3). The new grains are evolved inside original grain and in the vicinity of original grain boundaries. The SAED patterns shows a spreading of diffraction spots with increase cumulative strain to Σε~ 12 (Fig. 4c) suggesting an increment in overall misorientation of the subgrains. On the other hand, at 300°C, the deformation to a strain of Σε~ 12 leads to the formation of almost uniform SMC grained structure (Figs. 2f and 4d). Most of the deformation-induced boundaries have a high-angle origin (Fig. 2f). As a result, most of crystallites being true grains are entirely delimited by HAGBs (Fig. 4d). And a well-defined recrystallized structure is evolved at 300°C (Fig. 2f); the fraction of recrystallized grains of ~0.9 exceeds the fraction of grains developed at 250°C. Thus, it can be concluded that the ECAP of the 1561 Al to a cumulative strain of Σε~12 at 300°C provides formation of fully

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recrystallized structure with average crystallites size of ~ 0.5 µm. It is also to note that at both temperatures, numerous Al6Mn dispersoids precipitated in deformed structure at moderate to high strains additionally to those observed after homogenization annealing at 360°C.

Fig.4. Microstructures and associated SAED patterns of the 1561 Al deformed by ECAP to cumulative strain Σε~ 4 (a, b) and Σε~ 12 (c, d) at temperatures 250°C (a, c), 300°C (b, d).

Discussion

Inspection of experimental data shows that extensive grain refinement in the 1561 Al occurs through continuous dynamic recrystallization, which was elsewhere found to be operative in an Al-6%Mg-0.35%Sc alloy designated as 1570Al [6-8]. It is caused by the fact that homogenization annealing at 360°C provides the formation of incoherent nanoscale Al6Mn particles. It is obvious that the presences of these dispersoids are a prerequisite condition for occurrence of CDRX in Al-Mg alloys. No evidence for precipitation of Al3Zr phase was found both under homogenization annealing and severe plastic deformation as well. Therefore, Zr retains within solid solution hinders diffusion, significantly. This fact results in two sequences affecting positively CDRX occurrence. First, ability of dislocation to climb controlled by lattice diffusion is decreased. Second, dispersoids of Al6Mn phase showed high resistance to coarsening under annealing and severe plastic deformation as well. As a result, these dispersoids exert high Zener drag force [10], providing final grain size less than in an Al-Mg-Sc alloy [6, 7].

It is obvious that at Σε > 4 the recrystallized grains persistently replace subgrains through continuous transformation of their boundaries to HAGBs. Increasing temperature promotes the dislocation rearrangement within the interiors of initial grains due to acceleration of diffusion

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processes. Mobile dislocations move across subgrains and are trapped by deformation-induced LAGBs resulting in an increase in their misorientation (Fig. 3a). This can result in the formation of HAGBs. At 300°C, the kinetics of transformation of LAGBs into HAGBs increases with strain. It seems that this fact is associated with significant boundary migration at this temperature. The fraction of recrystallized grains of ~ 0.9 exceeds a threshold value of ~ 0.6 for conventional grained structures [2]. The average misointation angle of ~30º is the same as in fcc materials processed by severe plastic deformation [11]. Thus, the fully recrystallized structure is developed in the 1561 Al under ECAP at 300°C to a cumulative strain of ~12. Decrease in deformation temperature to 250°C reduces the rate of CDRX and even a cumulative strain ~ 12 provides the formation of a partially recrystallized SMC structure (Fig. 2e).

Summary

The present study demonstrates the feasibility of achieving grain refinement in the commercial 1561 aluminum alloy. It was established that ECAP is a very effective processing method to introduce SMC structure with average size of crystallites of ~ 0.5 µm into bulk billets of the 1561 Al in as-cast state. At temperature of 300°C, extensive grain refinement takes place in the 1561 Al subjected to ECAP. The average crystallites size with increasing temperature and strain almost unchanged and was of∼ 0.5 µm. The ECAP of the 1561 Al to a cumulative strain of Σε~12 at 300°C provides the formation of a fully recrystallized SMC structure. The ECAP of the 1561 Al to a cumulative strain of Σε~12 at 250°C provides the formation of a partially recrystallized SMC structure.

Acknowledgements

This study was supported by Department of Education and Science, Russian Federation, under grant No. Р1735. Authors are grateful to staff of Joint Research Center, Belgorod State University, for their assistance with computational and instrumental analysis.

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Nanomaterials by Severe Plastic Deformation: NanoSPD5 10.4028/www.scientific.net/MSF.667-669 Development of Ultra-Fine Grained Structure in an Al-5.4%Mg-0.5%Mn Alloy Processed by ECAP 10.4028/www.scientific.net/MSF.667-669.487

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