Preparation and Certification of Hydrogen in Titanium Alloy Standard Reference Materials
- PDF / 475,124 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 116 Downloads / 232 Views
ODUCTION
COMMERCIAL grades of titanium alloys require hydrogen to be present at levels below about 125 mg/kg.[1] Above this level, the solubility limit for hydrogen is exceeded and hydride formation occurs, which may lead to structural failure. The control of quality in manufacturing requires reliable determination of hydrogen in the product. Analysis is typically performed using hot vacuum or inert gas extraction of hydrogen followed by measurement of hydrogen by gas chromatography or mass spectrometry, for example, the ASTM Standard Test Method E1447 for hydrogen in titanium alloys.[2] Independently produced standards for calibration are required as well as reference materials for continued validation of the analytical procedure. Standard Reference Materials (SRMs) at the National Institute of Standards and Technology are usually certified using two or more independent methods of analysis; i.e., methods that have few or no significant sources of bias in common.[3] Another approach to certifying reference materials is to use certification by preparation, where synthesis of the material takes the place of one of the methods of analysis. A known quantity of the doping constituent is homogeneously added to a known quantity of matrix material that contains a negligible or blank amount of the dopant. As an example, solution SRMs are prepared by dissolving a measured mass of a metal or stoichiometric compound of known assay in high-purity acid and then diluting to a measured volume. An analysis of the product by an independent method validates the preparation.
In this investigation, three hydrogen in titanium alloy SRMs were prepared, at levels bracketing the specification limit of 125 mg/kg, by doping a weighed quantity of degassed titanium alloy (Ti6Al4V) with a measured volume of hydrogen. Prompt gamma-ray activation analysis (PGAA) was used to assess the amount of hydrogen in the degassed alloy as well as to measure the hydrogen mass fraction of the final product. The prompt gamma measurements were used in combination with the preparation data to certify the hydrogen mass fraction of each SRM.
II.
MATERIALS AND METHODS
The procedure used in this investigation for preparing titanium alloy reference materials containing low mass fractions of hydrogen is based on the reversible reaction Ti þ H2 ¼ TiH2 ;
DG0f ¼ 80:3 kJ=mole
½1
where the equilibrium pressure of H2 is below 108 Pa (150 atm) at 1173 K (900 °C). The reaction is rapid above 773 K (500 °C). The product is stable at room temperature. This procedure has been used elsewhere to prepare specimens of known hydrogen content for mechanical testing.[4] It has also been used in our laboratory to fabricate H in Ti alloy standards for nondestructive assay by neutron incoherent scattering (NIS)[5,6] and for neutron tomography.[7] A. Prototype Experiments
RICK L. PAUL and RICHARD M. LINDSTROM, Research Chemists, are with the Analytical Chemistry Division, Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899. Contact
Data Loading...
