Keywords: Ultrafine-grained AZ31 alloy. Equal-channel angular pressing. High-pressure torsion. Microstructure evolution. Dislocation density.

Transcription

1 Materials Science Forum Vols (2014) pp (2014) Trans Tech Publications, Switzerland doi: / Microstructure and defect structure evolution in ultra-fine grained MgAlZn alloy Miloš Janeček 1, a, Jitka Stráská 1,b, Jakub Čížek 2,c and Hyoung-Seop Kim 3,d 1 Charles University, Department of Physics of Materials, Prague, Czech Republic 2 Charles University, Department of Low Temperature Physics, Prague, Czech Republic 3 POSTECH, Department of Materials Science and Engineering, Pohang, Korea a janecek@met.mff.cun.cz, b straska.jitka@gmail.com, c jakub.cizek@mff.cuni.cz, d hskim@postech.ac.kr Keywords: Ultrafine-grained AZ31 alloy. Equal-channel angular pressing. High-pressure torsion. Microstructure evolution. Dislocation density. Abstract. Commercial MgAlZn alloy AZ31 was processed by two techniques of severe plastic deformation, namely equal channel angular pressing (ECAP) and high pressure torsion (HPT). Microstructure evolution with strain due to ECAP and HPT was investigated by light and transmission electron microscopy (TEM). Significant grain refinement was observed in specimens processed both by ECAP and HPT. Moreover, HPT resulted in radial strain and microstructure inhomogeneity across the diameter of the sample disk. This inhomogeneity was continuously smeared out and almost homogeneous ultra-fine grained structure was observed in specimen subjected to 15 HPT rotations. Dislocation structure changes in individual specimens after different number of ECAP passes and HPT rotations were investigated by positron annihilation spectroscopy (PAS). Sharp increase of dislocation density occurred during the first two passes of ECAP, followed by the saturation and even a decline manifesting the dynamic recovery at higher strains. Introduction Due to low density, magnesium alloys are very attractive and promising materials for structural components in automotive and aerospace industries [1]. However, the use of magnesium alloys in more complex applications is limited because of the problems associated with low ductility, poor corrosion and creep resistance. The properties of magnesium alloys can be improved by refining the grain size to the submicrocrystalline or even nanocrystalline level [2]. A variety of new techniques have been proposed for the production of the ultra-fine grain (UFG) structure in materials. All these techniques rely on the imposition of heavy straining and thus introduction very high dislocation density in the bulk solid material resulting in strong grain refinement. Currently, ECAP [3] and HPT [4] are the most popular techniques of SPD. The objective of this work is to investigate microstructure and dislocation structure evolution in specimens processed by these two techniques. Experimental Commercial AZ31 alloy with a nominal composition of Mg-3%Al-1%Zn in the initial as cast condition (average grain size of μm) was used in this investigation. One set of the samples was prepared by hot extrusion (T = 350 C, extrusion ratio ER = 22) followed by 1, 2, 4, 8 and 12 passes of ECAP at 180 C, route B c (EX-ECAP). First results measured on AZ31 prepared by this two-step process could be find in Janeček et al. [5]. The other one was processed by HPT at room temperature with ¼, ½, 1, 5 and 15 rotations by applying the hydrostatic pressure of 2.5 GPa. Specimens for microstructural investigation were prepared by standard techniques (cutting, mechanical grinding and polishing or ion milling for TEM observations). A fast-fast spectrometer [6] with the time resolution of 150 ps (FWHM 22 Na) was employed for positron lifetime measurements. At least 10 7 positron annihilation events were accumulated in each positron lifetime 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 TTP, (ID: /05/14,15:38:50)

2 Materials Science Forum Vols spectrum which was subsequently decomposed into individual exponential components by a maximum likelihood procedure [7]. A 22 Na 2 CO 3 with the activity of 1.5 MBq was used as the source of positrons. Results and discussion Microstructure evolution was observed by light and transmission electron microscopy. Dislocation density variations in individual specimens were investigated by positron annihilation spectroscopy. The results will be presented separately for specimens processed by ECAP and HPT. Equal-channel angular pressing. Hot extrusion resulted in strong grain refinement. The microstructure of extruded material is rather inhomogeneous containing zones with coarse grains surrounded by fine grains. The typical example of the extruded specimen is shown in Fig. 1a. The bimodal character of the microstructure remained after the first two passes of the ECAP, see Figs. 1b and 1c. During further straining the fragmentation of coarse grains occurred resulting in almost homogeneous microstructure as shown in Fig. 1d which represents the typical microstructure in the specimen after 4 passes of ECAP. The homogeneous microstructure did not change significantly in specimens after 8 and 12 passes as presented on TEM micrographs in Fig. 2a and Fig. 2b, respectively, where almost equiaxed grains of the average size of approximately nm are clearly seen. (a) Extruded material (b) After 1 pass (c) After 2 passes (d) After 4 passes Fig. 1 Grain structure of AZ31 alloy processed by ECAP

3 392 THERMEC 2013 Fig. 2 TEM micrograph of the specimen after (a) 8 passes (left) and (b) 12 passes (right) The process of grain fragmentation in AZ31 alloy is similar as in other metals with hexagonal-close packed alloy. The grain refinement after processing by ECAP is complex resulting in the formation of a bimodal grain structure with high fraction of fine grains already after the first or second pass. During further processing and further accumulation of strain in the material only coarse grains are continuously refined while the size of fine grains remains almost unchanged. Similar course of grain fragmentation in the material with hexagonal lattice was reported by several authors, e.g. [8, 9]. Bimodal distribution of grains is probably due to limited active slip systems in magnesium, therefore only favourably oriented grains are deformed and refined during ECAP process and areas of less deformed and larger grains are left in microstructure. The plastic shear deformation by the ECAP causes accumulation of large plastic strain and increase the density of structural defects. Positron annihilation spectroscopy was employed to characterize the density of these defects. Dislocations were found to be the only defects present in specimens after ECAP. The mean dislocation density D can be calculated from positron lifetime data using the two-state simple trapping model (STM) [10] D I., (1) D 1 2 where 1 and 2 are measured lifetimes of free positrons and positrons trapped at dislocations, respectively. The symbol D stands for the specific positron trapping rate to dislocations which in the majority of metals falls into the range of m 2 s -1 [11]. Here we used D = m 2 s -1 since Mg exhibits low electron density in interatomic regions which makes the positron binding energy to open volume defects lower than in dense metals and, thereby, D is expected to be close to the lower limit of the aforementioned interval. The mean dislocation density D calculated from Eq. 1 is plotted in Fig. 3 as a function of the number of ECAP passes. One can see in Fig. 3 that D firstly increases during ECAP processing and reaches its maximum in the sample subjected to 2 ECAP passes. However, further ECAP processing leads to a gradual decrease of dislocation density indicating a recovery of dislocation structure connected with development of UFG structure. Most probably rearrangement of dislocations and mutual annihilation of dislocations with opposite sign takes place during further ECAP processing. Finally in the sample subjected to 12 ECAP passes D decreased to the similar value as in the extruded sample prior to ECAP processing.

4 Materials Science Forum Vols High-pressure torsion. Unlike ECAP the strain introduced to HPT specimens is inhomogeneous throughout the disk. This results in inhomogeneous microstructure in specimens after different number of rotations. Consequently, the grain fragmentation in the centre and near Fig. 3 Dislocation density in AZ31 specimens subjected to different number of ECAP passes the edge of the specimen differs with increasing number of HPT rotations. Examples of various microstructures are shown in Fig. 4. For lower strains (the specimen after 1 rotation) the microstructure in the centre and in the periphery is completely different. In the central regions (Fig. 4a) the microstructure is bimodal comprising large grains of the average size of several tens of micrometers and small grains with the size of approximately 500 nm (determined by TEM and EBSD). Extensive twinning was also observed in coarse grains (especially in those whose size exceeds 100 μm). On the other hand, in the regions near the periphery (Fig. 4b) the microstructure is entirely refined with grains of the size between 300 nm-500 nm. The difference in grain sizes is smeared out with increasing number of HPT rotations and nearly the same microstructure consisting of grains of the average size of 250 nm is observed in central (Fig. 4c) and peripheral region (Fig. 4d) of the sample after 15 HPT revolutions. Similarly as for ECAP specimens the dislocation density D was calculated from positron lifetime data using the two-state simple trapping model. Due to the inhomogeneous structure the measurement was done at the centre and near the periphery (in the distance 8 mm from the centre) of each specimen. The dependence of the mean dislocation density D (calculated from Eq. 1) on the distance r from the centre for the samples subjected to various number of HPT revolutions is plotted in Fig. 5. From inspection of the figure one can conclude that during HPT processing the dislocation density increases firstly at the periphery (c.f. samples subjected to N = ¼ and ½ rotation) which is subjected to the highest strain. For samples subjected to more (a) N=1, centre (LM) (b) N=1, periphery (LM)

5 394 THERMEC 2013 (c) N=15, centre (TEM) (d) N=15, periphery (TEM) Fig. 4 Microstructure of specimens processed by HPT than half of HPT rotation the dislocation density increases also in the centre. However, the difference between the centre and the periphery (i.e. lower D in the centre and higher at the periphery) remains also in the samples subjected to more HPT revolutions. This behaviour can be clearly seen in Fig. 6 showing the dislocation density in the centre (r = 0 mm) and at the periphery (r = 8 mm) as a function on the number of HPT revolutions. The highest dislocation density was observed at the periphery of the sample subjected to 15 HPT revolutions. This sample exhibits also the largest difference between the dislocation density in the centre and at the periphery. Fig. 5 Dislocation density at various distances r from the centre of the sample disk subjected to various number N of HPT revolutions Fig. 6 Dependence of the dislocation density D in the centre (r = 0 mm) and at the periphery (r = 8 mm) for samples subjected to various number of HPT revolutions Summary Microstructure and dislocation structure evolution in AZ31 alloy processed by ECAP and HPT were investigated. HPT was found to be more effective method of grain refinement and introducing higher density of dislocation in the AZ31 alloy than ECAP.

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