Preparing and characterizing standards for hydrogen in a titanium alloy
The presence of excess hydrogen in a structural titanium alloy reduces its mechanical strength. If the hydrogen concentration exceeds {approximately}100 mg/kg, titanium hydride precipitates as a separate phase, preferentially at grain boundaries. As a result, controlling the hydrogen concentration is crucial. Standard methods for analysis are destructive, using high temperature in inert gas or in vacuum to extract the hydrogen. These methods are rapid but assume that all the hydrogen present is extracted, and they also require materials with known composition for calibration. In a related area, quantitative nondestructive assay by neutron radiography or tomography also requires artifact standards with known hydrogen content. Methods have been set in place at the National Institute of Standards and Technology to produce standard reference materials (SRMs) and other standards to satisfy these needs. The reaction Ti + H{sub 2} = TiH{sub 2} is reversible. From the free energy of formation (105 kJ/mol), one may calculate that the equilibrium pressure of H{sub 2} is below 10{sup {minus}8} Pa ({lt}10{sup {minus}13} atm) at room temperature, and 5 MPa (50 atm) at 800 C. The reaction is rapid above 500 C. Specimens with known hydrogen content are prepared by degassing a weighed quantity of titanium alloy in vacuum at high temperature and reacting it with a known volume of hydrogen gas in a closed vessel. The composition is stable at room temperature. The accuracy of the concentration thus achieved is limited by the pressure and volume measurements in the reaction system (accuracy {gt}0.5%) and by any residual hydrogen left after bakeout. For additional certainty, the hydrogen concentration measured volumetrically is confirmed by several methods of lower accuracy or sensitivity. For high concentrations (above {approximately}100 mg/kg), weighing the sample before and after treatment gives easily measurable changes in good agreement with the volume dosing. Prompt-gamma activation analysis with cold neutrons (CPGAA) is specific for hydrogen, but limited counting statistics make measurements below 100 mg/kg imprecise. More sensitivity, but more matrix dependence, is obtained from neutron incoherent scattering (NIS). This method can be used to measure as little as a few tens of mg/kg, which is useful to confirm the efficiency of the initial bakeout.
- Research Organization:
- National Inst. of Standards and Technology, Gaithersburg, MD (US)
- OSTI ID:
- 20005722
- Journal Information:
- Transactions of the American Nuclear Society, Vol. 81; Conference: American Nuclear Society 1999 Winter Meeting, Long Beach, CA (US), 11/14/1999--11/18/1999; Other Information: PBD: 1999; ISSN 0003-018X
- Country of Publication:
- United States
- Language:
- English
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