In situ synchrotron diffraction of the solidification of Mg4Y3Nd D. Tolnai n , C.L. Mendis, A. Stark, G. Szakács, B. Wiese, K.U. Kainer, N. Hort Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck Str.1, D-21502 Geesthacht, Germany article info Article history: Received 28 February 2013 Accepted 22 March 2013 Available online 30 March 2013 Keywords: Mg alloys Solidification In situ Synchrotron diffraction abstract In situ synchrotron diffraction experiments were performed during the solidification of a Mg4Y3Nd alloy. The material was melted and solidified inside a sealed stainless steel crucible in the chamber of a Bähr 805 A/D dilatometer. The sample was heated up to 680 1C and kept at this temperature for 5 min to ensure it is molten. Afterwards it was cooled down to the fully solidified state with a cooling rate of 10 K/ min. During the T(t) program diffraction patterns were acquired continuously in every 25 s (∼5 K). The forming phases were identified as α-Mg at 625 1C, Mg 12 Nd and Mg 14 Y 4 Nd at 545 1C, and Mg 24 Y 5 at 320 1C. The experimental results were correlated with simulations based on thermodynamic databases. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Mg–RE based alloys are attractive for structural [1] as well as for biomaterial applications [2] due to the good combination of mechan- ical and corrosion properties. Commercial Mg–Y–Nd based alloys, such as WE43 and WE54 form an important class of alloys used in elevated temperature applications with superior mechanical properties at both room and elevated temperatures [3]. The WE54 and WE43 alloys are conventionally used in the T6 condition. The age hardening response as well as the precipitation sequence and crystal structures of the resultant phases investigated previously [4–7]. The intermetallic particles found in the as-cast Mg5Y2Nd (wt%) and Mg4Y2Nd (wt%) alloys have been characterized, with transmission electron microcopy to be a ternary phase with a face centered cubic crystal structure with a ¼ 2.223 nm and a composition of Mg 14 Nd 2 Y [8] or Mg 15 Nd 2 Y [7]. The macroscopic mechanical properties of multiphase materials depend strongly on the chemical composition, volume fraction and spatial distribution of the intermetallic phases [9]. The thermodynamic databases and binary phase diagrams [10] provide information on the presumed solidification paths of the simple alloy systems. Based on these databases, the resultant microstructure can be modeled [11]. The majority of the commercial Mg alloys are not binary, but contain- ing more than one alloying addition resulting in complex inter metallics. For these complex phases the information in the simulation databases is limited. Thus the simulation of the experimentally observed microstructure and modeling of the solidification sequence is not feasible in every case due to the lack of available data. The continuous development of the X-ray acquisition systems at synchrotron sources provides a unique tool to characterize the phase formation and evolution during the solidification in situ. Recording the diffraction patterns while cooling the alloy system down, allows identifying the internal phases, to determine their solidification sequence [12,13] and to use these results as experi- mental validation of the existing thermodynamic databases and in certain cases contribute to their further development. The present study reports results on the in situ synchrotron diffraction during solidification of a Mg4Y3Nd alloy, and their comparison with the existing thermodynamic data on this system. 2. Materials and methods The alloy Mg4Y3Nd (wt%) was chosen for the experiments, as it is of particular interest in structural and in biomaterial applications. The microstructure of the direct chill cast alloy was examined with a Carl Zeiss Gemini Ultra 55 scanning electron microscope (SEM) operating at 15 kV. The samples for the synchrotron diffraction measurements were cut into small cubes and encapsulated in steel crucibles. The experiments were carried out at the HARWI II beamline of the Hamburger Synchrotronstrahlungslabor (HASYLAB) at the Deutsches Elektronen-Synchrotron (DESY). The measurements were performed in the chamber of a DIL 805 A/D (Bähr-Thermoanalyse GmbH, Hüllhorst, Germany) dilatometer the experimental setup is shown in Fig. 1. During the tests the samples were held in a sealed stainless steel crucible in order to hold the molten metal and to isolate it from the surrounding atmosphere. The dilatometer has been modified to meet the requirements of in situ synchrotron measurements. The two windows on the sides are covered by Kapton foils, to prevent interference with the X-ray beam. The induction coil is modified so the beam to only passes through the sample [14]. The temperature was regulated by a type S thermocouple welded on the steel crucible Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.110 n Corresponding author. Tel.: +49 4152 871974; fax: +49 4152 871909. E-mail address: domonkos.tolnai@hzg.de (D. Tolnai). Materials Letters 102–103 (2013) 62–64