Mass Spectrometric Determination of Hydrogen Thermally Evolved from Tungsten-Nickel-Iron Alloys G . Louis Powell Oak Ridge Y-12 Plant, P.O. Box Y , Oak Ridge, Tenn. 37830
A hot-extraction, high-vacuum technique using mass spectrographic detection has been used to determine the hydrogen content in tungsten-nickel-iron alloys within the hydrogen content range of l-weight-partper-billion to 10-weight-parts-per-million. An analytical method for time-resolving hydrogen contained in the bulk metal from hydrogen due to specimen surface contamination, as well as a method for determining the diffusion coefficient for hydrogen in these alloys is described.
TUNGSTEN-3.5 % NICKEL-1.5 % IRON ALLOY (1) (percentage by weight) consists of relatively pure (99 %+) tungsten particles (-40-pm diameter) bound together by a -1-pm thick layer of matrix alloy (nickel-24 iron-22 % tungsten). The 95 % tungsten alloy is prepared by sintering a mixture of tungsten, nickel, and iron powders in a hydrogen atmosphere a t a temperature (1500 "C) above the meltingpoint of the matrix alloy. Following this liquid-phase sintering, the hydrogen dissolved in the alloy is removed by evacuating the hydrogen-sintering atmosphere while the alloy is being cooled from 1500 "C. The solubility of hydrogen in tungsten ( 2 ) is much lower than the solubility of hydrogen in nickel (3)or iron (4),thus residual hydrogen in this two-phase alloy would probably be concentrated in the matrix phase which accounts for only 6 % by weight (14% by volume of the 9 5 z tungsten alloy). Since two weight-parts-per-million (wppm) hydrogen content is sufficient t o change the mechanical properties of some steels (9,a hydrogen content o n the order of 100 weight-parts-perbillion (wppb) might alter the mechanical properties of the 95 Z tungsten alloy. This report describes a set of experiments necessary for the development of a procedure to be used for routine determinations of hydrogen in tungsten-nickel-iron alloys over the range of 1 wppb to 10 wppm.
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EXPERIMENTAL The instrument used for the hydrogen analyses reported here was that described by Condon et al. (6). The instrument employed the high-vacuum, hot-extraction method using a mass spectrometer to monitor mass 2. The total base pressure a t the spectrometer was less than 1 x 10-8 Torr as determined by a n ionization gauge. Liquid nitrogen cold traps were used to maintain a low base pressure without baking the entire vacuum system. The mass spectrometer output and the total amount of hydrogen in micrograms evolved from a sample were displayed o n a two-channel recorder in real time. The instru~
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(1) F. R. Winslow, "The Iron-Nickel-Tungsten Phase Diagram," Union Carbide Corporation, Nuclear Division, Y-1785. Oak Ridge, Tennessee, June: 1971 (Available through National
Technical Information Service, Springfield, Va.). (2) R. Frauenfelder. J . Vuc. Sri. Techriol., 6 , 388 (1969). (3) A. Sieverts, Z. Mc.rcil/k, 21, 37 (1929). (4) A. Sieverts, 2.Phys. Clier77. (Leipzig), 77, 591 (1911). ( 5 ) N. J. Grant and J. L. Lundsford, I r O / i ARC, 175, 92 (1955). (6) J. B. Condon, R. A. Strehlow, and G. L.-Powell, ANAL.CHEM., 43, 1448 (1971).
ment was calibrated against a Boyle's law expansion of 5.02 i 0.05 cm3 of hydrogen maintained at 4.00 + 0.05 Torr--i.e., 20 Torr-cm3 a t 21 " C or 2.1 pg hydrogen. Both volume and pressure measurements used for this calibration were standardized against National Bureau of Standards working standards. The pumping rate for hydrogen from the furnace chamber (the ratio of the pumping speed out of the furnace chamber to the furnace chamber volume) was 1.5 sec-I. The mass spectrometer output was proportional to the pressure in the furnace chamber and proportional to the generation rate of hydrogen in the furnace chamber as long as the hydrogen generation rate was slow relative t o 1.5 sec-l. The mass spectrometer could also be operated in a scanning mode such that several mass peaks could be observed for a given sample. Two types of tungsten-nickel-iron alloys were used. The first type was the two-phase, 95% tungsten alloy (tungsten3.5% nickel-1.5Z iron). The second type was the matrix alloy (nickel-24 iron-22 % tungsten). The two-phase alloy was prepared by liquid-phase sintering in hydrogen a t 1500 "C followed by vacuum removal of hydrogen during the cool-down period. The matrix alloy was prepared by arc-melting the constituent metals followed by skull-casting the molten alloy. The resulting matrix alloy was vacuum annealed for 2 hours at 1000 "C. These alloys were machined into samples that were 0.25 inch in diameter by 0.10 inch thick. The thickness of the individual samples was measured and found to be uniform within 5 % . The hydrogen content of these samples were altered by heating to 1000 "C in vacuum (
