Optical Microscopy
Casting sections 10 mm (0.39 in) from the bottom sur- face were cut using a vertical band saw and ground using successively finer SiC papers (120, 320, 600 and 1200 grit sizes) with flowing water as a coolant. Polishing was completed using a 3 µm diamond polishing compound. The samples were etched in a solution of 15 % acetic acid, 10 % distilled water and 75 % ethyl alcohol for two min- utes with slight agitation. An ethanol rinse was used to stop the etching process and compressed air was used to dry the samples. All the samples were etched using the same solution batch for consistency and grain size mea- surements were performed immediately after etching. Op- tical micrographs of the castings were taken using a Leitz optical microscope.
SEM Analysis
Scanning electron microscopy analysis was performed on a JEOL SEM using back-scatter electron (BSE) im- aging at a working distance of 11 mm and a potential of 20 kV and a FEI Quanta 200 SEM using a working distance of 8.4 mm and a potential of 30 kV. The SEM samples were examined in both the etched and unetched conditions.
TEM Analysis
Specific castings (1.0 wt.% Al-1Ti-3B and 0.1 wt.% Al- 5Ti-1B) were selected for TEM analysis. The samples
Figure 2. Micrograph of AZ91E.
were prepared by first sectioning a 350 µm slice using a low speed saw at 100 rpm. The samples were then sanded with water as a coolant using 320, 600 and 1200 grit SiC papers on a polishing wheel until the sample thickness was 200 µm. Hand grinding with emery papers was per- formed to reduce the sample thickness to 100 µm. The samples were then cleaned in acetone and the final thin- ning was performed using an ion miller at an energy level of 5 keV, incident angle of 10° and current of 30 µA. The samples were then placed into a Phillips CM-12 120 keV TEM vacuum chamber to minimize oxidation on the sample surface.
(a)
(b)
(c)
(d)
Figure 3. Optical Micrographs of AZ91E + Al-5Ti-1B Castings with Increasing Holding Time (a) 0.1 wt.% Al-5Ti-1B (b) 0.2 wt.% Al-5Ti-1B (c) 0.5 wt.% Al-5Ti-1B (d) 1.0 wt.% Al-5Ti-1B.
32 International Journal of Metalcasting/Spring 11
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