In situ solidification experiment on Al and Mg alloys

D. Tolnai1, G. Requena2, F. Wilde1

1Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max Planck-Str. 1, 21502 Geesthacht, Germany 2 Institute of Materials Science and Technology, Vienna University of Technology, Karlsplatz 13/E308 Vienna, Austria

Multiphase Al and Mg alloys have complex internal architectures that determine their macroscopic mechanical properties. These alloys can be produced in the form of castings, especially Mg alloys, and as such, their microstructure is determined during solidification. Therefore, the understanding of the sequence of formation and evolution of the meta-stable and stable phases during solidification is a prerequisite to achieve control of the formation of the architecture of these alloys. The proposed experiments aimed at following the formation of the microstructural phases in situ as a function of temperature and cooling-rate by synchrotron tomography while cooling the molten metal until achieving the fully solidified structure. Previous results obtained by the proposers using in situ X-ray imaging during solidification of cast Al-Mg-Si alloys have proven the reliability of this technique to capture the microstructure formation during cooling from the liquid state. A furnace was constructed by the proposers (Figure 1) at Helmholtz-Zentrum Geesthacht to meet the requirements of the in situ imaging at the P05(IBL) beamline.

Figure 1. The in situ furnace, constructed at HZG, mounted at the P05 (IBL) beamline

The furnace consists of two heating zones in order to conduct directional solidification. The samples were contained in a ceramic crucible, during the measurement Argon was used in the case of Mg samples. A Mg10Gd1Nd alloy was selected to test the furnace since its high rare earth content provides good contrast with the Mg matrix. The beam energy was set to 21 keV, the voxel size was 1 µm. Both heating zones were heated up to 750°C to melt the complete sample. A gradient of 50°C was introduced during cooling in order to create a solidification front and to observe the dendritic solidification by radiography (Figure 2a). After the cooling finished and the sample was completely solidified, tomography was performed to image the 3D microstructure of the alloy (Figure 2b-c).

Figure 2. a) Radiography of Mg dendrites growing during directional solidification of a

Mg10Gd1Nd alloy, b-c) Rare-Earth-containing intermetallic phases in betwen the Mg dendrites. The different microstuctures originate from the different positions in the directional solidifications,

while the colors mark the mean curvature.

The furnace constructed for this beamline was stable during the measurements, which indicates that even longer experiments are possible with this equipment. The data acquired during this session is under quantitative evaluation.