Structure and mechanical properties of Ta-Mo alloy single crystals

Abstract

High purity single crystals of the Ta-Mo alloy series were grown near 10 at. % increments by electron beam zone refining. The methods of investigation of the prepared crystals included: specific resistivity at 273°K, 77°K and 4.2°K, critical resolved shear stress for plastic flow or cleavage at 273°K, lattice parameter determinations, transmission electron microscopy and diffraction and quantitative X-ray diffracted intensity measurements of {200} and {220} reflections. It was found that alloys of compositions between Mo-19 Ta and Mo-58 Ta cleaved on the {100} planes. Lattice parameter measurements and electron microscopy showed that the alloy series is b.c.c. with no long range order, coherent over dimensions greater than 300 Å, nor intermediate phases. Kikuchi electron diffraction proved that there exist deviations from random solid solution in the brittle crystals. This result was confirmed by X-ray intensity measurements of 200 and 220 lines. The ratio of constituent atoms on {100} and {110} differ from the ratio of atoms corresponding to the total composition of the crystal in such a way as to generate quasi-cubic domains for {100} and quasi-dodecahedral domains for {110} deviations. It is suggested that the increase in critical resolved shear stress for plastic flow with alloy composition results from a corresponding increase in the Peierls-Nabarro friction of dislocations produced by alloying, composition deviations and concomitant changes in the elastic coefficients in a local region of the crystal. Both Mo and Ta are anisotropic and have very different elastic constants. Alloying and the existence of composition deviations change the elastic coefficients, in the range of alloys which cleave, to values which marginally fulfill the strain energy restrictions. The presence of a critical density of dislocations in a local region can change the values of the elastic coefficients such that the crystal is locally energetically unstable. The crystal then cleaves on a {100} plane which relieves the maximum strain energy. By analogy, other solid solutions are expected to become brittle when the components have widely differing and anisotropic elastic constants. Marked changes in the slope of ( ρ273°K minus ρ4.2°K) versus alloy composition at Mo-20 Ta, Mo-50 Ta and Ta-12 Mo indicate that there is a relatively large change occurring in the density of states in the d and s bands and/or marked changes in the elastic coefficients at these compositions.