Evidence for dislocation glide controlled creep in niobium-base alloys

Abstract

It was shown several years ago that a coarse-grained Nb-5.4Hf-2.0Ti alloy (known commercially as C103) could exhibit large tensile elongations of over 200%; the phenomenon was observed at intermediate homologous temperatures (about 0.7 T{sub m}, where T{sub m} is the absolute melting temperature) and at intermediate strain rates (of about 10{sup {minus}3} s{sup {minus}1}). The extended ductility in C103 was attributed to the fact that it is representative of a special group of alloys known as Class I solid solutions. This paper reports that based on these descriptions, Class I solid solutions are found in alloys that have relatively large atomic size mismatches for a given value of elastic modulus, although it has been pointed out that the solute concentration, stress level, and the ratio of climb to glide diffusivities are also important variables. Often there is little or no subgrain formation, and the deformation behavior of the alloys does not exhibit a direct dependence on the stacking fault energy. During creep, the alloys exhibit either little primary creep or an inversion of primary creep that often enters directly into tertiary, or accelerating, creep. The activation energy for creep in Class I solid solutions is for chemical interdiffusion and ismore » often below the activation energy for self-diffusion of the matrix metal.« less