PREPARATION OF NBS COPPER-BASE SPECTROCHEMICAL STANDARDS Robert E. Michaelis, LeRoy L. Wyman, and Richard Flitsch A procedure has been developed for the preparation Spectrochemical analysis of alloys usually is complicated by the fact that the component elements react to form intermediate phases, and these constituents represent different energy relationships from each other. As a consequence, it requires different amounts of energy input to excite a given component atom, depending on how tightly it is bonded to itself or to other components in the various constituents of an alloy system. Because of this, the uniformity of the microstructures of samples for spectrochemical standards is vitally important. Particularly is this true for the copper base alloys such as the brasses and the bronzes. The need for copper-base spectrochemical standards has existed for a considerable length of time. For example, a survey made in 1950 by ASTM Committee E-2 revealed that standards were required to cover more than 50 different types of copper-base alloys. Of greater significance than numbers of standards, however, was the realization that the effects of disperse phases and physical characteristics on the spectral excitation seriously complicated the spectroscopic analysis for many of the copper alloys. To illustrate, it was known that as the copper-rich end of an alloy system was approached, a small change in composition could markedly affect the electrical properties and heat conductivity, and hence the volatility rates in spectral excitation. It was no wonder, then, that chill-cast versus wrought samples frequently led to major differences in the spectrochemical results. Moreover, differences often were observed depending on the size, shape, and material of the molds used in obtaining chill-cast samples, the type of heat treatment for wrought samples, and the surface preparation of both types of samples for spectrochemical analysis. In 1955, a cooperative program was initiated between the NBS and a Task Group of Subcommittee V, ASTM Committee E-2, to plan for the preparation of a limited number of needed copper-base standards. The main purposes of this paper are to describe the development of a satisfactory method for the preparation of the copper-base standards and to evaluate the application of this method to 21 prepared copper-base compositions for NBS spectrochemical standards. 2. PRELIMINARY PLANNING As a result of several initial meetings with the ASTM Task Group, three main recommendations were adopted. (1) Standard samples were to be prepared for seven principal copper-base alloys: cartridge, free-cutting, naval, red, and aluminum brasses, gilding metal, and commercial bronze. Close liaison was maintained with the Copper and Copper Alloy Spectrographic Committee of the Canadian Association for Applied Spectroscopy to prevent duplication of efforts since this group also was planning the preparation of standards. (2) Three standards were to be provided for each alloy consisting of a nominal, a high, and a low composition. Thus, individual analytical curves could be established to eliminate the effects of matrix composition for the seven alloys; but equally significant, standards would be available for study of these effects for application to other copper-base alloys. (3) Finally, it was considered important to prepare both chill-cast samples primarily for the producer and wrought samples primarily for the consumer for each composition. By so doing, the problems attributed to chill-cast versus wrought samples or standards would not exist. At the same time as consideration was being given at NBS to the preparation of the copper-base standards, studies were also being devoted to the preparation of a set of eight white cast iron spectrochemical standards [1]. For both, it was noted that the preparation of spectrochemical standard samples to be provided in the "as cast" condition presented several difficulties not encountered in those which could be provided in the wrought condition. Further complicating the preparation of the copper-base standards was the necessity of providing both chill-cast and wrought samples. Since it is next to impossible to obtain complete homcgeneity from conventional castings or ingots of any size from melts having complex compositions, it was evident that advantage must be taken of the generally accepted method to minimize segregation, namely, to solidify the molten metal as rapidly as possible. For the cast iron standards, it was believed that a scheme of rapid unidirectional solidification from the melt would provide samples of suitable homogeneity which would be useful both in optical emission and in x-ray spectrochemical analysis. Although differing in details from the cast iron, another scheme utilizing rapid unidirectional solidification was considered for the preparation of the copper-base material by which, and on further processing, it was believed that both chill-cast and wrought samples could be obtained from the same casting. The main advantage of rapid unidirectional solidification over other types for the preparation of spectrochemical standards is not that it eliminates segregation, but that it does force the segregation to occur largely at the top of the casting where it may either be removed or where it at least does not interfere with optical emission or x-ray spectrochemical analysis. 4. CHEMICAL COMPOSITIONS As indicated previously [1], modern metallurgy is concerned not only with those elements that may be considered the main components of an alloy, but also with the effects of minute amounts of other elements. For the copper-base standards, it was deemed essential to provide suitable standards containing all of the trace elements of interest; however, it was not believed necessary to provide a graded and certified series for each of the seven alloys. In 1955 the NBS, in cooperation with the copper industry, planned the chemical compositions for three standards each. of seven copper-base alloys. The final compositions adopted by NBS and accepted by industry are shown in table 1. As can be seen, each alloy is covered by a nominal, high, and low composition standard. All of the elements covered by the then present and proposed chemical specifications were included for each alloy type; additionally, all of the trace elements of interest were purposely included only in the cartridge brass series. |