Microstructural Characterization

3.3.1 Metallographic Sample Preparation

Samples were cut from various locations of the final cast components for microstructural characterization. The specimens were prepared by the standard technique of mounting in Bakelite followed by grinding using 120, 500, 800, 1200, 2500 and 4000 grit SiC papers for Al-Si alloys and Mg alloys. The commercial aluminium samples were prepared with two different methods. First method with grinding till 4000 grit SiC paper and then

electro-polished. The electrolyte composition was perchloric acid (HClO₄) 20 ml dissolute in acetic acid (CH3COOH) 80 ml. The sample was dipped in a cool bath and a 30 V (dc) voltage was applied for 2 min. The second method of polishing aluminium was manual using 2500 grit paper and then polishing on cloth sprayed with a paste that contained 3μm diamond suspended particles. Al-Si alloys were ground using - 120, 500, 800, 1200, 2500 and 4000 grit SiC papers and then cloth polished with a silica suspension. The microstructures were examined without etching to reveal primary α-Al grains, intermetallic particles and eutectic phases. In order to reveal grain boundaries, the samples of commercial Al and Al-Si alloys were etched in a solution of 70 ml ethanol, 10 ml water, 20 ml acetic acid and 4.2 grams picric acid for 2 minutes. An optical microscope with polarised light and x plate was used to obtain coloured grain boundaries. For macro-etching the ground samples are immersed in the Tucker’s solution (HF 15 ml, HCl 45 ml, HNO3 15 ml and H2O 25 m) for10-20 seconds was used.

3.3.2 Optical Microscopy (OM)

A Carl Zeiss Axioskop 2 MAT optical microscope equipped with image analysis software, a camera and a computer was used for the OM observation and the quantitative measurements of microstructural features. A software application was used to acquire images from the camera and to perform image analysis. This microscope was fitted with 2.5, 10, 20 and 50 objective lenses and the corresponding magnifications were 25×, 100×, 200× and 500×. The grain sizes and intermetallics were measured from the images taken at various magnifications.

In the bright field (BF) mode, polished surface shows bright in contrast and the surface irregularities such as grain boundaries and intermetallics appear dark in contrast. Plane polarized light (PP) is most commonly used for grain size measurements on colour etched surfaces. The typical micrographs of anodized sample and polished Al-Si alloys are presented in Figure 3.8.

Figure 3.8 Typical micrographs of a colour etched sample and a polished sample (both LM6 with grain refiner addition)

3.3.3 Scanning Electron Microscopy (SEM)

The scanning electron microscope employs the beam electrons directed at the specimen. SEM examinations were performed on the Al-Nb-B novel master alloy sample using a Zeiss Supra 35VP FEG scanning electron microscope. For chemical composition analysis, the SEM equipped with an energy dispersive X-ray spectrometer (EDX), Oxford instruments Inca was used and the data were matrix corrected (ZAF). The advantage of SEM over optical microscopy is the large depth of field and higher resolution, thus producing high resolution images at high magnification (up to 50,000 times). A good explanation of the procedures and the use of SEM and EDX are given by Watt [Watt, 1997]. A sample SEM micrograph and chemical analysis are shown in Figure 3.9.

Figure 3.9 Example of SEM micrographs and EDX analysis of the specimen.

500 μm 100 μm

3.3.4 Transmission Electron Microscope (TEM)

The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. What we can see with a light microscope is limited by the wavelength of light. In the TEM technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through it. An image is formed from the interaction of the electrons transmitted through the specimen.

In this study the TEM was used to investigate the inclusions in the master alloy. A JEOL 2200F field emission gun TEM operating at 200 kV was used in this study. The key step in TEM is to prepare a thin sample suitable to transmit electrons. Discs of 3 mm diameter were punched from the rolled material (∼150 µm thick) using a Gatan hole punch. Thin foils suitable for TEM were prepared by electropolishing using a solution of 33 vol.% HNO3 (nitric acid) and 67 vol.% CH3OH (methanol) at approximately −30°C and ∼25-35 V ( ∼0.1A) on a Struers TenuPol-5 twin-jet electropolishing unit. Upon perforation of the disc, as detected by the photo-sensors, the foil was carefully removed, washed thoroughly in methanol and dried in air. Specimens were observed by TEM within 24 hours of electropolishing. We studied smaller particles down to near atomic levels. The possibility for high magnifications has made the TEM a valuable tool in this research. A typical TEM image is shown in Figure 3.10.

Figure 3.10 TEM image of Al-Nb-B master alloy.

3.3.5 Quantitative Metallographiy

Quantitative metallographic analysis was carried out on OM images using image processing software Axioskop 2 MAT0. The measurements of equiaxed grains and large dendrite grains were carried out on colour etched samples using the mean line intercept method as shown in figure 3.11. The mean intercept length , which is used as the grain size,is calculated from the equation:

(3.1) Where is the total length of the test lines and N_i the total number of grain boundary intersections on each test line. The standard deviation calculations from the average grain size were used as the error in measured grain size.

Figure 3.11 Schematic representation of the mean line intercept method performed on the microstructure of an aluminium alloy