Structure Refinement of S-Phase Precipitates in Al-Cu-Mg Alloys by Quantitative HRTEM
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ABSTRACT The crystal structure of the AI2 CuMg S-phase precipitate in an Al matrix has been determined by quantitative high resolution electron microscopy. This work combines techniques of image processing and quantitative comparison between experimental and simulated images with automatic refinement of imaging and structural parameters. INTRODUCTION Over the last few years much progress has been made in the field of quantitative high resolution transmission electron microscopy (HRTEM) based on pattern recognition techniques for image comparison and automatic structure determination/refinement [1-7]. These techniques are particularly important in the analysis of nanoscale structures, which may not be stable at larger size and are often inaccessible to X-ray analysis. The present work is an application of HRTEM to the study of Al2 CuMg intermetallic S-phase precipitates, and forms part of an ongoing study of the formation mechanisms, morphology and interface structure of S-phase precipitates in Al-CuMg alloys [8]. Al-Cu-Mg based alloys are of significant interest for many structural applications, due to their low weight, mechanical strength and corrosion resistance. The mechanical properties of these alloys are based on a dispersion of S-phase precipitates which have been shown to alter the deformation mode [9]. S-phase precipitates have the composition AI2 CuMg and form as laths along Ai, with {0 12}A1habit [10]. The following crystallographic orientation relationship is observed[ 11-13]: [p00]S//[100]AI (lath axis)
(001)s//( 02 1)Al (habitplane)
[010]s
[01 2]AI,
indicating the presence of 12 S-phase orientation variants in the Al-matrix. The crystal structure of this intermetallic phase has been studied using different diffraction techniques for more than five decades. Since the mechanical behavior of Al alloys strengthened by the S-phase cannot be fully understood without a clear understanding of its crystal structure, morphology and interface structure, quantitative characterization of these crystallographic and microstructural features is essential for future alloy development. Several models have been proposed for the structure of the S-phase. The first model was given by Perlitz and Westgren (PW) [14] based on X-ray diffraction. The unit cell of the PW model is shown in Fig. 1. The structure is orthorhombic with unit cell dimensions a = 0.4 nm, b = 0.923 nm, and c = 0.714 nm, space group Cmcm, containing 16 atoms in the ratio AI:Cu:Mg = 2:1:1. Mondolfo [15] suggested a modified PW model with slightly different lattice parameters (a = 0.4 nm, b = 0.925 nm, and c = 0.718 nm). Cuisiat et al. [16] offered a model with a unit cell dimensions identical to Mondolfo's, but with space group Im2m containing only 8 instead of 16 273 Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
atoms. Yan et al. [17] proposed an orthorhombic structure with space group Pmm2 (No. 25), lattice parameters a = 0.4 nm, b = 0.461 nm, c = 0.718 nm and four atoms per unit cell in the ratio AI:Cu:Mg = 2:1:1. However,
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