Internal friction study of substitutional-Interstitial interaction in niobium-vanadium alloys
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I.
INTRODUCTION
THE interaction
of substitutional and interstitial solutes in bcc metals is a subject of continuing interest. Internal friction measurements are widely used to investigate the nature of this interaction. The group VA metals are ideally suited for this type of study due to their high solid solubility for interstitial solutes, notably oxygen and nitrogen. Evidence for trapping of oxygen by vanadium in niobium has been reported from diffusivity 1,2 and activity coefficient measurements. 2'3'4 De Avillez et al. 4 interpreted the decrease in the diffusivity ~and activity coefficient 3 of oxygen by the addition of vanadium to niobium as due to trapping. The diffusion data of Altstetter and coworkers 5'6 show a weak interaction between oxygen and niobium in vanadiumrich V-Nb alloys; however, the activity coefficient data of de Avillez 7 do not indicate that niobium traps oxygen in this alloy. The two principal factors that contribute to trapping according to current models 1'8'9 are chemical interactions and elastic interactions. The criterion that has been used to determine the degree of chemical interaction is the difference in the heats of formation of the oxides whereas the elastic interaction is controlled primarily by the size factor. The purpose of this investigation was to determine the nature of the s-i interaction for oxygen in dilute alloys at either end of the Nb-V system by use of internal friction measurements and to determine the relaxation parameters of the observed oxygen relaxations as precisely as possible.
II.
EXPERIMENTAL
Internal friction measurements were carried out in an inverted pendulum apparatus operating at a frequency of oscillation between 0.20 and 1.70 Hz. The quantity, Q - I , O. N. CARLSON is a Professor Emeritus with the Department of Materials Science and Engineering, Iowa State University, and Ames Laboratory, iowa State University, Ames, IA 50011. H. INDRAWIRAWAN, formerly a Graduate Student at Ames Laboratory, is living in Indonesia. C. V. OWEN. Associate Metallurgist, and O. BUCK, Senior Metallurgist, are with the Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted November 6, 1986 METALLURGICAL TRANSACTIONS A
obtained from the decay of the amplitude of the freely oscillating pendulum, was determined for oxygen in Nb0.24 at. pct V, Nb-0.50 at. pct V, and V-0.50 at. pct Nb alloys between room temperature and 773 K. The maximum shear strain was 3 x 10 -6. The alloys were prepared by arc melting high purity niobium and vanadium together in an inert atmosphere. The arc-melted bars were then swaged, with intermittent annealing, to 1.0 mm diameter wires. These were cut into 75-mm long sections and oxygen of different concentrations was introduced into the specimens by gas-phase charging. This was followed by a homogenization anneal in vacuum at 1575 K for up to five hours. The chemical compositions of the alloys shown in Table I are based on a combination of vacuum fusion analysis and hardness measurements. III.
RESULTS
A. Niobium-Rich All
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