Irradiation hardening behavior of polycrystalline metals after low temperature irradiation

Irradiation hardening behaviors of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) alloys and pure metals were characterized after neutron, or proton and neutron irradiations at low temperatures (≤200 °C). In the regression analysis, the radiation-induced increase in yield stress, Δσ_YS, was expressed in the form of a power law: Δσ_YS=h(dpa) ⁿ, where h and n are the regression coefficients and dpa is displacements per atom. The log-log plots of Δσ_YS vs. dpa data showed two distinctive regimes: a low-dose regime where a rapid hardening occurs and a high-dose regime where the log-log plot shows a considerably reduced slope. Mean values for n obtained from the 19 metals were about 0.5 for the low-dose regime and about 0.12 for the high-dose regime. Some ductile metals like Fe, Cu and Zr displayed lower h and n values. Doses to reach the regime of reduced irradiation hardening, D_S, were in the range 0.003-0.07 dpa. Comparisons between radiation effect parameters led to a conclusion that the transition from the low-dose to the high-dose regime in irradiation hardening occurs either when the tensile specimen undergoes prompt plastic instability at yield or when saturation of defect cluster density occurs.