THE HECHAMICAL PROPERTIES OF TANTALUM ^ITH SPECIAL REFEiiENCE TO THE DUCTILE- BRITTLE TRANSITION

The tensile behavior of single phase tantalum-hydrogen alloys and tantalum-molybdenum alloys containing dilute additions of molybdenum has been studied as a function of grain size and alloying addition. Uniaxial tension tests were carried out at 25C and -196C, the strain rate.£ , being fixed throughout the experiments at 10-3 sec-l. The parameters (fi and ky of the Fetch equation were evaluated where possible. It was observed that q"!, the lattice friction stress, was essentially independent of hydrogen content at room temperature whereas ky, which was presumably a measure of the solute-dislocation interaction, was observed to increase by the absorption of hydrogen into tantalum. These results are similar to those found by Wilcox for hydrogen in niobium. The increase in k is believed to be the result of the hydrogen-dislocation pinning interaction. The insensitivity of (f. to hydrogen content is postulated to be due to the adsorption of hydrogen not associated with dislocations at the grain boundaries. No evidence of mechanical twinning was found in any of the Ta-H alloys either at 25C or at -196C. The addition of small amounts of Mo to Ta results in an increase in ky at 25C which can be attributed to a Mo-dislocation interaction. The rate of increase of ky per at.f« Mo is greater than expected on the basis of a linear variation in ky between the pure elements. It was also observed that (J^ is unaffected, at 25C, by the addition of small amounts of Mo. This behavior is not presently understood and raises some doubt as to the physical significance of (T^. Mechanical twinninfr; was observed in several Ta-Mo alloys at -T?6C, but none war, observed for these same alloys at 2^°^. A large increase in ductility accompanied the twinninr. This documentary report has boon reviewed and is approved. W. J. TRAPP Chief, Strength and Dynamics Branch Metals and Ceramics Division Air Force Materials Laboratory