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|Author(s):||Eric S. Windsor; R Carlton; Scott A. Wight; C Lyman;|
|Title:||SRM 482 Revisited After 30 Years|
|Published:||August 01, 2001|
|Abstract:||The National Institute of Standards and Technology's (NIST) Standard Reference Material (SRM) 482 was issued by The National Bureau of Standards (NBS) in 1969 and has been continuously available to the public for over 30 years . The standard consists of a set of 6 wires. Each wire is approximately 0.5 mm in diameter and 5 cm long. The 6 wires represent different compositions within the binary copper-gold alloy system. Included in the set are the two end member compositions, pure copper and pure gold, along with 4 alloys of nominal composition: Au80-Cu20, Au60-Cu40, Au40-Cu60 and Au20-Cu80. SRM 482 was produced specifically for the purpose of microanalysis using the electron probe microanalyzer (EPMA).Recent metallographic preparation of these wires revealed the presence of surface blemishes on some of the prepared wires (Figure 1a). Two of the authors from different affiliations recently prepared cross-sections of SRM wires from separately purchased sets. Each was unaware of the procedures used by the other. The results were similar. Blemishes were prevalent in the Au20-Cu80 and Au60-Cu40 wires, and also occurred sporadically in the Au40-Cu60 and pure copper wires. The blemishes are circular in cross section and range in size from sub-micrometer to approximately 3 ?m in diameter. They can be observed using reflected light microscopy, and appear red under cross-polars and dark field illumination. At present, we are uncertain whether these blemishes are artifacts of the sample preparation procedure or whether they represent inhomogeneities within the wires themselves.SRM 482 was produced in direct response to a need for microscopically homogeneous materials of known chemical composition. Such materials were necessary to evaluate the merit of the EPMA as a quantitative analytical tool [2,3]. Copper-gold was selected as the binary alloy system because very accurate methods were available for the chemical analysis of these elements. Also, copper and gold are completely miscible throughout their entire composition range. Therefore, these two materials should produce highly homogeneous standards of well-known composition. SRM 482 was manufactured with the highest purity starting materials (99.9999% pure copper and gold). Great care was taken to minimize contamination and maximize homogeneity during the manufacturing process . Each of the 6 wires was drawn as a single continuous wire approximately 150 meters long. The EPMA was used to test homogeneity at three locations along each wire; the two ends and one intermediate position. From these 3 spots, each wire was determined to be homogeneous at a level approaching the precision of the EPMA technique [1,3].The certificate of analysis for SRM 482 does not discuss the metallographic preparation of the wires . NBS Special Publication 260-28, was written to expand upon the information included in the certificate of analysis . It states only that the ...cleaned wires were mounted by press fitting them into holes drilled into blocks of aluminum or brass. and also that the cross-sections at the surface of the mounting blocks were carefully polished, finishing with 1/4-micron diamond paste.. This lack of preparation information is unfortunate because SRM 482 users must either rely on personal knowledge and experience in metallographic sample preparation, or they must consult alternate references for guidance. The current authors prepared the wires in cross-section by first cutting short pieces from each wire. The pieces were then press fit into holes drilled in blocks of aluminum. Grinding was achieved using progressively finer silicon carbide grit papers ending with 600 grit. Rough polishing was performed with diamond impregnated (6 ?m followed by 1 ?m) nylon cloths. Final polishing was achieved using fine alumina (0.05 ?m) on napped cloths. If these blemishes are indeed artifacts of the sample preparation procedure, then they result fro|
|Citation:||Microscopy and Microanalysis|
|Keywords:||Standard Reference Material (SRM)|
|Research Areas:||Chemistry, Nanotechnology|