The tensile deformation behavior of fine-grained α-plutonium

Abstract

Tensile deformation of extruded monoclinic α-plutonium with an average grain size of 4 μm was studied at stress from 2 500 to 100 000 psi (17.3 to 689 MN/m 2) and temperatures from 22 to 108°C. The strain rate varied from 10 −9 to 7 × 10 −3 sec −1. The relation, ϵ = 2.86 × 10 −7 σ 4.2 exp (−25 600/ RT) sec −1 , was obeyed from 12 000 to 60 000 psi (71.7 to 414 MN/m 2) for strain rates greater than about 10 −6 sec −1. Stress and temperature dependences of creep rate over this stress range were in accord with a dislocation climb controlled creep model, although the power law behavior occurred at stresses higher than theory predicts. The value of 25 600 cal/mole proved a reasonable value for the activation energy for self-diffusion in α-plutonium. At lower stresses the apparent activation energy for creep increased with decreasing stress, and the stress exponent n (= d log ϵ/ d log σ) increased from 4.2 to 7.9. The high apparent activation energies for creep and high n values at low stresses were attributed to grain growth during creep. Tensile elongation increased with decreasing strain rate and increasing temperature over the entire stress range. Low elongation at high stresses was attributed to lack of grain boundary sliding. Grain size changed during creep toward a size determined by stress. At the highest test temperatures and lowest stresses grain growth occurred during large strains, while at high stresses the average grain size decreased.