Effects of oxygen on the mechanical behavior of hydrogenated V, Nb, and Ta
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I.
INTRODUCTION
PREVIOUSstudies have shown that increasing the concentration of carbon, oxygen, or nitrogen enhances the hydrogen solubility in V, Nb, and Ta.l-4 Increasing the hydrogen solubility may affect the mechanical properties of these hydride forming metals. A previous paper evaluated the effect of nitrogen on the strength and ductility of hydrogenated V, Nb, and Ta. 5 This paper extends this previous work by analyzing the effects of oxygen on the mechanical behavior of hydrogenated V, Nb, and Ta.
of ductility and was determined from micrographs of the fracture surfaces using a polar planimeter. Metallographic examination for hydride formation at subambient temperatures was made on an inverted microscope.6 Several samples were also monitored for hydride formation using a torsion pendulum procedure described previously) The temperatures reported for the hydride precipitation temperature were obtained during cooling because this best represented actual testing conditions. III.
II.
MATERIALS AND PROCEDURES
The refining and processing of the V, Nb, and Ta used in this study have been described previously. 5 Oxygen alloys were prepared by adding different amounts of oxygen master alloys to high purity V, Nb, or Ta during the arc-melting consolidation step. Oxygen analyses performed on test specimens after testing indicated good homogeneity. The chemical analysis of the V, Nb, and Ta after melting is shown in Table I. Also included in this table are the chemical analyses of the oxygen containing alloys with and without subsequent hydrogenation. The hydrogen charging procedure was the same as that described previously. 6 The grain sizes of the V, Nb, and Ta and the oxygen alloys are given in Table II. The samples containing oxygen had a smaller grain size, but this difference in grain size did not appear to have any significant effect on mechanical properties. Tensile tests were run at an initial strain rate of 8.3 x 10-5 s -1. Constant temperature liquid baths were used for test temperatures below 295 K. All test specimens were quenched to the test temperature to avoid changes in hydride morphology that could arise because of differences in the rate of cooling .7 Reduction of area was used as the measure W. A. SPITZIG, Senior Metallurgist, and C. V. OWEN, Associate Metallurgist, are with Ames Laboratory, Iowa State University, Ames, IA 50011. T.E. SCOTT, formerly with Ames Laboratory, is with Michigan Technological University, Houghton, MI 49931. Manuscript submitted June 3, 1985. METALLURGICALTRANSACTIONS A
EXPERIMENTAL RESULTS
A. Yield Stress
The effects of oxygen and combined oxygen and hydrogen on the temperature dependence of the yield stress of V, Nb, and Ta are shown in Figures 1, 2, and 3, respectively. The effects of hydrogen alone on the mechanical behavior of the V, Nb, and Ta have been discussed previously, 5 and curves representing the temperature dependence of corresponding hydrogenated alloys without oxygen are included in Figures 1, 2, and 3 for comparison purposes. Comparison of these fi
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