2Outline Part 2. Metals and Alloys 2.1 Preparation of slices 2.2 Preparation of TEM discs2.3 Twin-jet electro-polishing2.4 Multiphase Materials: Remarks
32.1 Preparation of slicesThe given alloy is cut into thin slices with thickness of the order of 250 μm.A low speed diamond can be used for slicing.Samples with regular/irregular shapes with size about 5 cm in diameter and 10 cm in length can be accommodated in the specimen holder.The holder is fixed to a specimen arm, which can be raised or lowered with respect to the diamond-wafering blade.The slices can be cut with a blade with a rotation speed of about r.p.m.
42.2 Preparation of TEM discs From the slices prepared using the procedure in §2.1, discs with the desired diameter are punched by using a disc punch or ultrasonic disc cutter.The diameter of the discs depends on the specifications of the TEM specimen holder (many holders accommodate discs with a diameter of 3 mm).The discs are then mechanically lapped/polished to about μm thickness using grinding media.Emery paper (silicon carbide grinding paper) is cheaper and available with different grit numbers (such as grit No. 320, 600 and 1200).The grinding medium should be chosen according to the hardness of the specimen.
5To improve the surface finish, a polishing cloth is taken and diamond paste (particle size of ~ 3 μm) along with few drops of lubrication oil is spread on it.Lapping along a constant direction results in scratches, which may be difficult to remove if lapping is continued along the same direction. Hence, the direction of polishing should be changed continuously so as to minimize the scratches.Fig. 1 Optical images of (a) sample, (b) slice sectioned from the sample using low-speed diamond saw (isomet), (c) slice after punching TEM disc, (d) TEM disc (e) TEM disc after grinding and electropolishing (note the shiny contrast in the central portion due to electropolishing). The major divisions in the scale are in cm.
62.3 Twin-jet electro-polishing Electropolishing refers to the process in which the sample (anode) can be polished by electrochemical action, by keeping it in an electrolyte and applying a suitable potential. The electrolyte, its temperature and bias voltage are some important parameters in controlling the rate of dissolution of the sample.To perform electropolishing, the following procedure is followed:Preparation of electrolyte: About 500 ml of the electrolyte is made in a pyrex glass container having a diameter of about 125 mm and a height of about 65 mm. The electrolyte is material-specific; an exhaustive survey of electrolytes suitable for different metals/alloys is available in the literature .Temperature of electrolyte: The rate of dissolution of the sample depends on the temperature of the electrolyte. Some samples are polished at room temperature. However, many samples are polished at low (sub-zero) temperatures, as this offers better control over the electropolishing.
7(c) Jet speed: The nozzles are submerged in the electrolyte and the speed of the jets is adjusted. (d) Specimen holder: The disc prepared as described in §2.2 is loaded in the sample holder. It acts as the anode. It is kept between the nozzles in the electrolyte.(e) Voltage-current behavior: The variation of current with voltage is studied by applying a bias between the anode and cathode, after enabling the jets. The voltage is gradually increased and the corresponding values of the current are noted. A schematic plot is shown in Fig. 2.(f) Inspection: During electropolishing, the sample holder is removed from the electrolyte after switching off the bias voltage. It is dipped in a beaker containing methanol. The sample is then examined under a table lamp, without unclamping it from the holder, to verify the quality of polishing. the electrolyte and/or electropolishing conditions (bias voltage, temperature of electrolyte and speed of jets) need to be fine tuned for optimal polishing.
8Figure 3 shows representative regions of an electropolished TEM disc. Fig. 2 Schematic showing the current-voltagecharacteristics of an alloy in electrolyte.Figure 3 shows representative regions of an electropolished TEM disc.Fig. 3 Optical images of an electropolished TEM disc observed in (a) reflection and (b) transmission modes. The edges of the hole at an enlarged magnification, in reflection mode, are shown in (c).
92.4 Multiphase Materials: Remarks Multiphase materials consist of chemically distinct phases. The phases exhibit different electrochemical properties for a chosen electrolyte.The electropolishing conditions such as jet speed, electrolyte temperature and voltage can be varied on a trial and error basis, as the nature of polishing is a function of these parameters.