This paper describes the results of an investigation of the influence of neutron irradiation on mechanical properties of a reactor pressure vessel steel at high strain rates ˙ɛ ≈ (10³ to 10⁴)s⁻¹ produced by means of the Hopkinson split pressure bar technique (HSPBT). Our steel with chemical composition 0.18% C, 1.22% Mn, 0.012% P, 0.016% S, 0.17% Cr, 0.33% Ni, 0.055% Al, 0.028% Ti was irradiated by fast neutrons to an exposure of 1.35 . 10²³ (n)m⁻². The extensive dynamic measurements performed in the temperature range of (90 + 573) °K are analysed in terms of state equation for viscoplastic flow. It is shown that neutron irradiation shifts the transition between thermally activated and viscous dislocation damping mechanism of plastic deformation to the higher strain rates. Particular attention is paid to the comparison of the influence of neutron irradiation and strain aging.

The experimental data obtained indicate that plastic strain rate ˙ɛ is a linear function of dynamic over stress δ_fl/δ₂ − 1, ˙ɛ = = γ (δ_fl/δ₂ − 1) + ˙ɛ₂ where γ is viscosity coefficient, δ_fl denotes flow stress and δ₂, ˙ɛ₂ are stress and strain rate respectively which control the transition between termally activated process and viscous damping mechanism. It appears that δ₂ and ˙ɛ₂ are strongly influenced by neutron irradiation and strain aging although γ is nearly independent of both.