2 0 A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 2 Fig. 1 — Recrystallized grains: exponentially decaying spacings at (a) 200 gf; (b) 300 gf; (c) 500 gf with 150 μm spacing; and (d) 1 kgf with 150 μm spacing... Single phase cartridge brass, C26000, an alloy composition of 70% copper and 30% zinc, was selected as the material of choice to create an entry for the 2021 International Metallographic Contest[1]. C26000 was chosen due to the striking twins and large grain size that are easily generated by annealing a cold-worked sample, the large range of contrast and colors that can be created by etching, and the wide availability of the material[2]. To purposefully engineer these microstructures, a Vickers microindenter was used to selectively cold work the C26000 cartridge brass in specific areas to develop the desired patterns. These specific cold-worked areas were annealed to create the intentional images from recrystallized grains. Color etchants were used to obtain contrast in the micrographs and were also chosen so that the image was generated only through light optical microscopy (LOM) techniques with no post processing or false coloring. SAMPLE PREPARATION Samples of fully hard rolled cartridge brass (70% cold work) were annealed at 850°C for 45 minutes to obtain large grains with an average size of 800 µm. Samples were then mounted and polished to 1 µm using conventional metallographic techniques. The samples were indented using a manual Vickers microhardness tester. An array of indentations was created on each sample using forces of 1000, 500, 300, and 200 gram-force (gf). Indentation spacings were varied from 3 mm to the point where they overlapped. Two lines of 10 indentations with equal spacings (150 and 75 um) were created in order to attempt to produce a fine, continuous line of grains. Simple geometric shapes were also created using the same two spacings to attempt curved lines. This pattern of indentations was designed to test for the optimal spacings and load between indentations required to successfully recrystallize the cold-worked areas. The optimal indenter load and indentation spacing were determined to produce the most visible recrystallized grains with the cleanest overlap to create a continuous line. Samples were annealed at various temperatures from 450-600°C for 15 minutes to recrystallize, so the desired final grain size for visual appeal could be established. An anneal procedure of 525°C for 15 minutes was chosen. Using the optimized parameters, Vickers microhardness indentations were performed using 1 kgf, 10 second dwell time, and spacing between 80-100 μm. Samples were demounted and annealed at 525°C for 15 minutes. The samples were then mounted again and repolished taking care to stop the coarse grinding steps once the Vickers microhardness indentations were no longer visible. Next, samples were fine-polished through 1 μm diamond polish followed by a 21-hour vibratory polish with 0.05 µm colloidal silica 138~140V – 63.8 Hz frequency. Samples were etched to reveal the microstructure and produce color etch effects. Klemm’s reagents are recommended by Vander Voort to produce the most vibrant colors[3]. Three reagents were tested, Klemm’s I, II, and III[4]. These reagents contain the same constituents, sodium thiosulfate (Na2S2O3), potassium metabisulfite (K2SsO5), and deionized water, with varying compositions. Table 1 shows the composition of each chemical prescribed by the ASM Handbook along with recommended etch times[4]. To obtain the final colored images, samples were etched using Klemm’s II reagent (50 mL saturated aqueous sodium thiosulfate mixed with 5 g po- TABLE 1 — KLEMM’S REAGENTS, COMPOSITIONS AND ETCH TIMES Etchant/Reagent Composition Time Klemm’s I 50 mL saturated Na2S2O3 (stock solution), 1 g K2SsO5 Up to 3 min Klemm’s II 50 mL saturated Na2S2O3 (stock solution), 5 g K2SsO5 Up to 8 min Klemm’s III 5 mL saturated Na2S2O3 (stock solution), 20 g K2SsO5, 45 mL DI H2O Up to 8 min (a) (d) (c) (b)
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