Solid State Phenomena Vol. 163 (2010) pp 106-109 Online available since 2010/Jun/07 at www.scientific.net (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/ssp.163.106 Characterization of β and Mg 41 Nd 5 equilibrium phases in Elektron 21 magnesium alloy after long-term annealing A. Kiełbus 1,a, T. Rzychoń 1,b, L.Lityńska-Dobrzyńska 2,c, G. Dercz 3,d 1 Silesian University of Technology, Katowice, Poland 2 Institute of Metallurgy and Materials Science Polish Academy of Sciences, Cracow, Poland 3 University of Silesia, Katowice, Poland a andrzej.kielbus@polsl.pl, b tomasz.rzychon@polsl.pl, c nmlityns@imim-pan.krakow.pl d grzegorz.dercz@us.edu.pl Keywords: Elektron 21, magnesium alloy, microstructure, long-term annealing, Rietveld method Abstract. The paper presents results of TEM and XRD investigations of Elektron 21 magnesium alloy in as cast condition and after long-term annealing at 250 and 350 C. In as cast condition Elektron 21 consists of primary α-mg solid solution with α-mg-mg 3 RE eutectic and regular precipitates of MgRE 3. Precipitation at 250 C causes formation of the equilibrium β phase. Annealing at 350 C caused precipitation of globular Mg 41 Nd 5 phases on solid solution grain boundaries. Also precipitates of MgRE 3 phase have been observed. Introduction The Elektron 21 is magnesium alloy containing neodymium and gadolinium planned for applications for up to 200 C. It has high strength, good corrosion resistance and excellent castability [1]. It s being used in both civil and military aircraft as well as in automobile (motor sport) industry [2]. Neodymium in Elektron 21 alloy has a positive effect on tensile strength at elevated temperatures and reduces porosity of casts. Gadolinium, like neodymium shows a decreasing solid solubility as temperature falls, indicating potential for precipitation strengthening. Addition neodymium to Mg-Gd alloys reduces the solid solubility of Gadolinium. It improves precipitation hardening response, at lower levels of gadolinium than the binary system offers [1]. The strength of Elektron 21 alloy is achieved essentially via precipitation strengthening. This alloy precipitate from the solid solution according to the sequence of phases: -Mg. The phase is fully coherent with the matrix and has a h.c.p. structure (a=0,642nm and c=0,521nm). The phase is semi coherent with the matrix. It has a b.c.o structure (a=0,640nm, b=2,223nm, c=0,521nm). The equilibrium (Mg 3 RE) phase is a f.c.c. structure (a=0,74 nm) [3]. The aim of this paper is to present the characterization of β and Mg 41 Nd 5 equilibrium phases in Elektron 21 magnesium alloy after long-term annealing. Experimental procedures The material for the research was the Elektron 21 magnesium alloy. It contained ~2.7 wt.% Nd, ~1.2 wt.% Gd, ~0.4 wt.% Zn and ~0.5 wt.% Zr. Sand casting was performed at 780 C temperature. The as-cast specimens were solution treated at 520 C during 8h and quenched in water. Ageing treatment was performed at 250 C and 350 C/4 1000h with air cooling. The microstructure was investigated by a Tecnai G2 transmission electron microscope (TEM). X-ray diffraction patterns were collected using X-Pert Philips diffractometer. The X-ray data collection was performed in the 10-140 2θ range. The Rietveld analysis was performed applying DBWS 9807 program [4]. The pseudo-voigt function appeared to be the most useful in the describing of diffraction line profiles. The R wp (weighed-pattern factor) and S (goodness of fit) parameters were used as numerical criteria of the quality of the fit of the calculated to experimental diffraction data [5]. 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, www.ttp.net. (ID: 79.191.230.195-22/01/11,16:28:34)
Solid State Phenomena Vol. 163 107 Results The TEM microstructures of the as-cast alloy are presented in Fig. 1. It can be seen that the Elektron 21 alloy composed mainly of a -Mg solid solution with -Mg + Mg 3 RE eutectic on the grain boundaries (Fig.1a). Fig. 1. Microstructure of the Elektron 21 alloy in as-cast condition. TEM image and corresponding diffraction pattern of a) Mg 3 Nd phase, b) MgGd 3 phase. The MgRE 3 precipitates of regular shape have been also observed (Fig.1b). After solution treatment the Mg 3 RE phase dissolved in the matrix. X-ray diffraction pattern of the solutioned sample reveals only the presence of -Mg phase (Fig. 2). TEM investigations showed that the microstructure of Elektron 21 alloy aged at 250 C for 4 h consisted of enlarged phase precipitates and the elongated, needle-shaped precipitates of equilibrium phase (Fig.2a). The precipitates of equilibrium phase were formed between the particles as they nucleated preferentially in the strain field of the β phase. Fig. 2. TEM image of microstructure of the Elektron 21 alloy aged at 250 C for a) 4h, b) 96h. The phase is non-coherent with the -Mg matrix and is identified as Mg 3 Nd phase. It has a face-centered cubic crystal structure (a=0.74 nm). Similarly in as-cast condition Mg 3 Nd phase has the form of (Mg) 3 (Nd,Gd). The diffractions lines of Mg 3 Gd phase were observed after ageing at 250 C for 4 h (Fig. 3). This compound is isomorphous with the equilibrium β phase. The intensity of peaks for equilibrium phase increases with continued ageing at 250 C (Fig. 3). This is in agreement with the increase intensity of the diffraction lines controlled by the increase in the volume fraction of β phase (Fig.2b). The lattice parameters determined by the Rietveld method for matrix are higher than the ones of pure magnesium. The values of the unit cell parameters decreased with increasing ageing time. It is connected with precipitation process and decomposition of a magnesium supersaturated solid solution.
108 Applied Crystallography XXI Fig. 3. X-ray diffraction patterns of solid samples after solution treatment and ageing at 250 C. The diffraction patterns of Elektron 21alloy aged at 250 C for 96 h was analyzed by the Rietveld refinement method (Fig.4). Fig. 4. X-ray powder diffraction pattern fitting by Rietveld refinement of Elektron 21 alloy after ageing at 250 C for 96 h. In the present study the Rietveld method had one objective: to confirm that the phases identified by transmission electron microscopy observations are really present in the alloy. In multiphase alloys, some diffraction peaks are superposed by Bragg diffraction peaks from two or more phases. Therefore, the visual inspection of the diffraction pattern may not allow identifying all the phases present in the alloy. The structure was refined to R wp = 0.74 % and S = 2.11 %. The refined of the unit cell parameter of β phase was a=0.7326 nm. After annealing at 350 C the microstructure consists of Mg 41 Nd 5 phase precipitates on grain boundaries (Fig.5a). After ageing at 350 C for 5000h the precipitates of this phase create the characteristic network on grain boundaries. Also the regular precipitates of MgRE 3 phase have been observed (Fig.5b). The diffraction patterns of Elektron 21alloy aged at 350 C for 500 h was analyzed by the Rietveld refinement method (Fig. 6). The structure was refined to R wp = 10.85 % and S = 1.87 %. The refined of the unit cell parameter of Mg 41 Nd 5 phase was a=1.4719 nm and c=1.0429nm.
Solid State Phenomena Vol. 163 109 Fig. 5. Microstructure of the Elektron 21 aged at 350 C/500h: a) Mg 41 Nd 5 phase, b) MgRE 3 phase. Fig. 6. X-ray powder diffraction pattern fitting by Rietveld refinement of Elektron 21 alloy after ageing at 350 C for 500 h. Summary Precipitation sequence from supersaturated solid solution of Elektron 21 proceeds as follows: α-mg β β β (Mg 3 RE) Mg 41 Nd 5. Ageing at 250 C for four hours causes formation of the equilibrium β phase. This equilibrium phase has face centered cubic crystal structure with the lattice parameter a = 0.7326 nm, which makes it isomorphous with the Mg 3 Nd compound. With the prolonged ageing time the volume fraction of the β phase increases. Annealing at 350 C/500h/air caused precipitation of globular Mg 41 Nd 5 phases on solid solution grain boundaries. This phase has bct crystal structure with the lattice parameters a = 1.4719 nm and c = 1.0429 nm. Also precipitates of MgRE 3 phase have been observed. The longer annealing time caused formation of the network of Mg 41 Nd 5 precipitatates on grain boundaries. Acknowledgments The present work was supported by the Polish Ministry of Science and Higher Education under the research project No N N507 451334. References [1] P. Lyon, T. Wilks, I. Syed, in: Magnesium Technology, edited by Neal R. Neelameggham, TMS Publications (2005), p.303. [2] B. Smola, I. Stulikova, F. von Buch, B. Mordike, Mat. Sci. and Eng. Vol. A 324 (2002) p.113. [3] X. Gao, S.M.He, X.Q. Zeng, L.M. Peng, W.J. Ding, J.F. Nie, Mat. Sci. and Eng. Vol. A 431 (2006) p.322. [4] R.A. Young, A. Skahivel et al. J. Appl. Cryst. Vol. 28 (1995) p. 366. [5] R. A. Young, in: The Rietveld Method, IUCr Monogr. on Crystallogr. Vol. 5, Oxford, (1995).
Applied Crystallography XXI doi:10.4028/www.scientific.net/ssp.163 Characterization of β and Mg<sub>41</sub>Nd<sub>5</sub> Equilibrium Phases in Elektron 21 Magnesium Alloy after Long-Term Annealing doi:10.4028/www.scientific.net/ssp.163.106