Serrated yielding in Nb–V dual phase steel

Abstract

The characteristics of serrated yielding (the Portevin–Le Chatelier effect) in a Nb–V dual phase steel have been studied in the temperature range 85–210°C at strain rates between 1·2 × 10⁻⁵ and 1·2 × 10⁻² s⁻¹. Serrated yielding was found to initiate only after a critical strain ɛ_c was reached. The strain between two successive serrations ∆ɛ_s increases almost linearly with strain, while the stress drop ∆σ_c increases with strain up to ∆σ_max, then decreases. The exponent β in the mobile dislocation density–plastic strain relationship (ρ_m= ɛ^β) is 1·09 in the temperature range 85–140°C and 1·34 in the temperature range 140–210°C. The results also indicate that in the same temperature ranges there are two values of activation energy for type A serrations, i.e. 79 and 119 kJ mol⁻¹ respectively. The results are discussed in terms of substitutional–interstitial solute atom interaction and changes of concentration of interstitial atoms.

MST/934