The effect of grain size on the high temperature plastic deformation of nb3sn

A study of high temperature plastic deformation has been undertaken on 10, 20, and 60 μm grain size Nb₃Sn. The materials were produced by the hot isostatic pressing of powder blends. The 20 and 60 μm grain size material involved a stoichiometric blend of Nb and Sn powder, whereas the 10 μm grain size material involved a blend of 30.2 wt pct Sn powder and 69.8 wt pct Nb-1 Zr powder. The ZrO₂ formed during processing limits grain size and NbO formation. Through compression testing and load relaxation testing, deformation has been studied over a strain rate range from 10^-6 to 10^-2 per second and a temperature range from 1150 to 1650 °C. “Power law creep” was generally observed, although stress exponent reduction at the higher temperatures and lower strain rates suggests substructural coarsening. Analysis of stress-strain rate-temperature data projected an activation energy for creep of 400 to 500 kJ/mol. Grain size refinement clearly strengthened the polycrystals. Assuming a Hall-Petch relationship, “lattice friction stresses” and “unpinning constants” were calculated, both increasing with decreased temperature and increased strain rate. Grain size refinement from 60 to 10 μm lowered the ductile-to-brittle transition temperature for simple compression by the order of 125 °C.