Dislocation Density and Dislocation Arrangement during Creep of 20% Cr–35% Ni Stainless Steels

Measurements of dislocation density and observations of dislocation structure have been carried out on two 20% Cr–35% Ni steels, one (A) solid solution and the other (B) precipitation-hardened by Ni₃ (Ti, Al), both creep-tested at 700° C. The dislocations were arranged in a three-dimensional network. Subgrains were formed only in the solid solution. A linear relationship between creep stress (τ) and (dislocation density)^½ (ρ^½) was established for both materials. This may be considered as supporting the view that the recovery-creep model applies to materials hardened by a second phase. The τ vs. ρ^½ relationship had a positive stress intercept for steel B and may be regarded as a measure of the decrease in the driving force for recovery due to the precipitates. The lower creep rate of alloy B could be entirely accounted for by this decrease in driving force. The solid-solution alloy exhibited a negative stress intercept in the τ vs ρ^½ relation and this is a consequence of the increase in dislocation density during primary creep.