Determination of hydrogen in titanium alloys by cold neutron prompt gamma activation analysis
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I.
INTRODUCTION
THE presence of hydrogen in even small amounts in metals is known to cause embrittlement. Studies have shown that hydrogen affects the cracking strength of titanium alloys at mass fractions, even below 50 mg/kg,[1] far below the safe hydrogen content of 125 mg/kg permitted for aircraft structural applications.[2] The analysis of hydrogen in metals conventionally is carried out either by degassing the sample below its melting point in a vacuum or inert gas, or by fusing the sample at high temperatures, also in a vacuum or inert gas.[3] Although these techniques give reliable analysis of hydrogen at mg/kg concentrations, a significant disadvantage is that both involve destruction of the sample. Furthermore, since these methods require reliable standards for accurate determination of H concentrations, it is desirable that such standards be calibrated using an independent, and preferably nondestructive, analytical technique. We have found cold neutron prompt gamma-ray activation analysis (CNPGAA) to be suitable for the nondestructive determination of trace amounts of hydrogen.[4] The technique is described in detail elsewhere;[5] the basics are as follows. When a sample is placed in a neutron beam, nuclei of many elements in the sample absorb neutrons and are transformed to an isotope of higher mass number. Prompt gamma rays, emitted by de-excitation of the compound nuclei, are then measured using a high-resolution gamma-ray detector. Qualitative analysis is accomplished by identification of the gamma-ray energies, while comparison of gamma-ray intensities with those emitted by a standard yields quantitative analysis. The use of long-wavelength ‘‘cold’’ neutrons enhances the sensitivity and reduces the background. The analysis is essentially nondestructive, although, because of size constraints, large samples may require cutting (Section II). Furthermore, since both the neutron and gamma radiation are penetrating, the entire volume of sample irradiated by the neutron beam RICK L. PAUL, RICHARD M. LINDSTROM, and ROBERT R. GREENBERG, Research Chemists, are with the Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. HUGH M. PRIVETT III, Senior Materials Engineer, is with Pratt and Whitney, West Palm Beach, FL 33410-9600. WADE J. RICHARDS, Chief, is with the Nuclear Section, McClellan Air Force Base, CA 95652-2504. Manuscript submitted November 10, 1995. 3682—VOLUME 27A, NOVEMBER 1996
is analyzed. Because the analytical signal results from a nuclear and not a chemical reaction, the results are independent of the chemical form of the element being measured. In this article, we briefly describe the instrument, discuss sources of background and their effect on H detection limits, and also relate sources of measurement uncertainty and their impact on the accuracy and precision of the method for measurement of trace hydrogen in titanium. We also cite examples of how the technique has been used to study hydrogen embrittlement in titanium alloys. II.
MATERIAL
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