Nucleation and Dynamic Glide of Dislocations as Co-Determinants of the Low-Temperature Flow-Stress Anomaly and the Stress-Relaxation Rate in Copper and Dilute Brasses between 15 and 300 K

Abstract

The dependences of the logarithmic stress‐relaxation rate s ≡ − dσ/d 1n(1 + t/t0) of coarse‐grained high‐purity copper and brasses (c = 0.03 to 1.0 at% Zn) on the initial level of the compressive stress, σ0, and on temperature are determined, following incremental pre‐strains of ½ to 1% in the near‐linear work‐hardening stage, at total strains of 2 to 5%. The results are correctly encompassed by the relation s ˜ σ0T(T + Td)−1, representing a stochastic process of “slip nucleation and dynamic glide”, the latter occurring by percolation of dislocations through accessible zones of the glide planes. T outside the bracket refers to the temperature dependence of the nucleation rate, and (T + Td)−1 to the post‐nucleation slip distance. Its length is assumed to be limited by the frictional resistance to the movement of the dislocations, due to phonon scattering (T inside the bracket), and that arising from forest dislocations, solute atoms, etc. (Td). The theory facilitates derivation of the hitherto empirical “stress‐concentration factor” used in descriptions of the low‐temperature flow‐stress anomaly, which is manifest also in the present materials, and provides a clear indication of its origin.

Rate controlling process of stress relaxation in high purity irradiated titanium

“…Thus their propagation becomes more difficult leading to hardening. The progressive reduction in the mobility of dislocations engenders a decrease in the relaxation rate and an increase in the stress concentration at the clusters of vacancies and/or interstitials [16].…”