Diffusion of Metallic Solutes in Vanadium Using Spark Source Mass Spectrometry as the Method of Analysis F. A. Schmidt, R. J. Conzemius, 0. N. Carlson, and H. J. Svec Ames Laboratory-USAEC and Departments of Metallurgy and Chernstry lowa State Unwersity Ames lowa 50070
The diffusion coefficients of iron, niobium, nickel, silicon, and titanium in vanadium metal were determined at 1600, 1700, and 1800 "C using a spark source mass spectrometric technique to trace the concentration profiles. Samples containing both a single solute and several solutes were studied and the results compared. The diffusion coefficients obtained for an element when it was the single additive were about the same as when it was present as one of several solutes. In all samples, nickel diffused at the fastest rate, followed by iron. The diffusion rates of niobium, silicon, and titanium were slower at each temperature but not in any particular order. Because of the high sensitivity of the mass spectrometric method of analysis, the diffusing solute could be present in low concentrations of the couple. This represents a distinct advantage of the present method because the activity of the additive in dilute solution is decreased and any solute-solute interactions are minimized.
Since the beginning of this century, vanadium has been used extensively as an alloying element in steels for purposes of grain refinement and hardenability. In recent years vanadium-base alloys, because of the favorable nuclear properties of vanadium, have been considered for use as a fuel element cladding in fast breeder reactors. Vanadium-base alloys are also of interest as a first wall or envelope material for controlled thermonuclear reactors (CTR). In both of these potential nuclear applications, the material will operate a t temperatures in the range of 600 to 900 "C. Thus, a knowledge of the chemical diffusion properties of the metallic impurities present in commercial vanadium or as alloying elements is quite important. The primary purpose of this investigation was to determine the diffusion coefficients of several possible alloy or impurity elements a t temperatures where a measurable amount of diffusion takes place in a reasonable length of time. The data obtained can then be extrapolated to the operating temperatures of the nuclear reactor. One of the principal commercial processes for producing vanadium metal is an aluminothermic process developed a t the Ames Laboratory ( I ) and industrialized by the Wah Chang Corporation ( 2 ) . The major metallic impurities in commercial vanadium are iron, nickel, and silicon. These elements have relatively low vapor pressures ( < 2 Torr) a t the melting point of vanadium and, hence, are not readily removed during electron beam melting, which is the final consolidation and purification step. For this reason, these solutes were included in the present investigation. Titanium and niobium were also included because they have been used as alloying additions to vanadium to improve its high temperature strength properties (3).
The concentration profiles of the diffusion couples were determined using spark source mass spectroscopy as the method of analysis. Because of the high sensitivity of this method, the concentration of the solutes could be kept low, thereby decreasing their activity and assuring a single phase solid solution at the experimental temperature. This use of very dilute alloys also permitted doping of the diffusion couples with several solutes simultaneously. The concentration profiles of these couples could be determined during a single mass spectrometric sample analysis.
EXPERIMENTAL Apparatus. The diffusion couples were heated under a n argon pressure of 650 Torr in a National Research Corporation Resistance Furnace, Type 2940. A matched w/W-2690 Re thermocouple was used to register the experimental temperature which was controlled to within *5 "C. The tungsten wire was shielded from the W-26% Re wire by high purity alumina insulators. In each experiment, four of the diffusion couples were mounted in a support block made of high purity vanadium. The high purity end of the diffusion couple was placed in the block and the thermocouple junction was placed in the center of the group of samples in the vicinity of the welds. The support block was placed inside of a vanadium crucible which was contained in a tantalum crucible. This arrangement is shown in Figure 1. A molybdenum screen was used to separate the tantalum from the vanadium to avoid problems due to the minimum in the solidus and liquidus curves of the tantalum-vanadium system a t about 1800 "C ( 4 ) . The spark source mass spectrometer used in this work was a Model Graf 11-2 manufactured by the Nuclide Corporation, State College, Pa. Improvements in the basic unit include the installation of a versatile electrical ion detection system ( 5 ) , simultaneous control of the spark gap and of the ion illumination angle (6),and development of a special wide-mass-range dual-beam collector to permit elemental ratios to be measured at the image of the optical system ( 7 ) . Materials. The high purity vanadium metal used in this study was obtained from the U.S. Bureau of Mines, Boulder City, Nev. It had been doubly refined by a fused salt electrolysis process (8) and was received under vacuum as coarse dendritic crystals. A chemical analysis of these crystals is given in Table I. Vanadium of this quality was chosen as the starting material since it contains low concentrations of those elements whose diffusion behavior was to be determined. The counter electrode used in the spark ion source consisted of a high purity gold wire obtained from Cominco American Inc. This material was in wire form 0.02 cm in diameter and designated as Grade 69. Procedure. The experimental procedure section is divided into three parts-namely, the preparation of the diffusion couples, the analysis of these couples, and the calculation of the diffusion coefficients of the solute. These efforts are described separately in the following paragraphs. Preparation of Diffusion Couples. The diffusion couples were rods 3.8 cm long and 0.25 cm in diameter. One half of the rod consisted of the above vanadium after it had been arc melted in an atmosphere of purified argon and swaged. The other half of Carlson. D. T . Eash, and A . L. Eustice, A l M E Metallurgical Society Conference, VoI. 2. "Reactive Metals.' lnterscience Publishers, Inc., New York, N Y . 1959, p 277 (5) R J. Conzemius and H. J Svec, Talanta. 16, 365 (1969) (6) R J. Conzemius and H J Svec, Talanta. 20, 477 (1973) ( 7 ) R . J. Conzemius and H . J Svec. Talanta. in press. (8) T. A . Sullivan, J . Metals. 17, 45 (1965) (4) 0 . N .
(1) 0 N
Carlson
F A
Schmidt and W E K r u p p
J
Metals
18, 320
(1966) ( 2 ) C T Wang E F Baroch S A Worcester and Y S Shen Met Trans I , 1683 (1970) (3) G A Whitlow R A Nadler and R C Svedberg Tech Rep WARD- 3791-47, U S At Energy Comm Nov 1970
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fis 4
Figure 1. Section view of support block and crucible assembly used to contain vanadium diffusion couples at temperature ( 1 ) tantalum cap and thermocouple well, (2) molybdenum disk, (3)vanadium cap ( 4 ) tantalum outer crucible (5) vanadium inner crucible, (6) vanadium diffusion couples (solute e n d u p ) , (7) vanadium support block (8) molybdenum spacer screen
Table I. Chemical Analysisa of Vanadium Metal Used in Diffusion Studies Element
Concentration, ppm (w)
A1 B C Ca co Cr
Fe H Mg
cu
Element
Concentration, ppm (w)
3
