Characterization techniques

4.2.1 Differential thermal analysis

The Differential Thermal Analysis (DTA) technique was applied to determine the semi-solid range of the studied alloys. Coupons of ca. 0.5 mm³ were carefully prepared from the extruded precursor material and analysed with a Mettler Toledo model TGA/SDTA 851e. The coupons were inserted in stainless steel crucibles of 5 mm diameter and closed with a cap of the same material. Pure aluminum was employed to calibrate the DTA and as reference material. To avoid oxidation and burning of the magnesium samples, pure argon gas at a rate of 50 ml/min. circulated in the system during the experiments. The temperature was varied from 25°C to 700°C with a heating/cooling rate of 10°C/min.

4.2.2 X-ray diffraction

The phases of the alloys in the extruded and the thixocast condition were identified using the technique of X-ray diffraction aided by JCPDS diffraction standards (Joint Committee on Powder Diffraction Standards). For this purpose, a Siemens D5000 diffractometer equipped with a secondary monochromator was employed. A current of 40 mA at a voltage of 40 kV was used during the measurements. The samples were scanned over a range of 30° to 90° at a step size of 0.05° and 10 sec/step. The Powder Diffraction Files (PDF) used in the present work were:

¾ PDF#12-0266, Ca2Mg6Zn3

¾ PDF#13-0450, CaMg2

¾ PDF#17-0401, Mg12Nd

¾ PDF#35-0821, Mg

It should be emphasized that phases present in small quantities (below 5%) cannot be detected by this technique.

4.2.3 Metallographic sample preparation

In order to characterize the microstructural features of the alloys in their different states (extruded, partial remelting, thixocast, heat treated, and after mechanical and creep tests), it was necessary to prepare them using metallographic techniques adapted for magnesium alloys. The samples were firstly cut and embedded in epoxy

1200 and 2500 SiC grit paper using water as cooling fluid and ethylic alcohol as cleaning agent between grinding steps. The samples were polished with OPS™

neutral suspension (0.05 μm SiO2) and distilled soapy water for a few minutes (up to 3 minutes) to avoid oxidation. The samples were then washed with distilled water followed by ethylic alcohol and dried with warm flowing air.

After the metallographic preparation, the samples were etched chemically for the purpose of contrasting grain boundaries and phases and determining grain size and orientation by means of optical microscopy. The etching solution was prepared after Kree [04Kre] with 110 ml ethylic alcohol, 40 ml distilled water, 2 ml acetic acid and 2 gr. picric acid at room temperature. The polished samples were immersed in the etching solution for a few seconds until the surface changed its colour. The samples were then washed immediately with ethylic alcohol and dried with hot air.

The microstructure developed at the different steps of representative casting material is presented as micrographs, in order observe the complete thickness of the step casting sample. Fig. 12 shows schematically where the micrographs come from.

shady area shady area

Fig. 12: Sketch of the casting indicating the area (shady) selected for the micrographs.

4.2.4 Optical microscopy

The microstructure of the samples after chemical etching was determined using a Leica upright optical microscope type DM LM equipped with polarisation filters and a Nikon digital camera type DM1200. The largest magnification available in the equipment was 1000x. The microscope was coupled with the image analysis software (Aquinto AG), which allowed the determination of average grains sizes

according to the linear interception standard method described in ASTM E112-02 [02AST].

4.2.5 Scanning electron microscopy (SEM) coupled with EDX

A Zeiss Scanning Electron Microscope (SEM) model DSM 962 was used to resolve the microstructure of the alloys at higher magnifications and to obtain chemical information of the phases developed in the materials. The specimens were metallographically prepared as described above for optical microscopy. In the case of mounted specimens, it was necessary to use silver painting to assure electrical contact between the polished metallic surfaces and the microscope chamber to avoid charging of the samples. Chemical etching was not necessary, because this technique, in combination with the signal mode Backscattered Electron (BSE), allows phases contrast according to their atomic weight. Fracture analysis was made using the signal mode Secondary Electron (SE), which allow topographic imaging. The chemical characterization of the phases as well as its quantification (semi-quantitative analysis) was made by means of the analytical technique of Energy Dispersive X-ray Spectroscopy (EDX) and with the help of the microanalysis system WINEDS, Version 3.0. An accelerating voltage of 15 keV was used during signal acquisition.

4.2.6 Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) was used at Risø DTU National Laboratory for Sustainable Energy (Roskilde, Denmark) to observe at micro-scale precipitates and substructures developed after casting and after creep tests in the reference alloy (MEZ). The deformed specimens were ground mechanically to about 70 µm and then thinned via electropolishing in a twin jet system using a solution of 5% HClO4 and 95% ethanol at about -30°C and with a voltage of 40 V. After electropolishing, the specimens were cleaned with ethanol several times and then dried. The observations were performed on a JEOL 2000 transmission electron microscope coupled with an energy dispersive X-ray analysis (EDX) system operating at 200 kV. All TEM images were taken under two-beam diffraction conditions using different diffractions vectors and bright field mode.