Dynamic strain ageing of L12-strengthened Ni-Co base high-entropy alloy and unraveling its deformation mechanisms in strain ageing process

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Dynamic strain ageing (DSA) of L12-strengthened Ni-Co base high-entropy alloy (HEA) was examined at temperatures varying from 20 to 600 °C with strain rates between 10–2 to 10–4 s-1. In normal DSA regimes, elevating temperature or lowering strain rate advances the DSA behavior, resulting in the lowered critical strain and raised amplitude of serrations. Based on strain-rate jump tests, the negative strain-rate sensitivity induced by DSA was observed at the elevated temperature regime, and high apparent activation volumes ranging from 97∼ 737b3 correspond to the strong obstacles effect from the precipitates and the additional pinning strengthening of solute atoms. Transmission electron microscopy evidence suggests that stacking faults prevailed at all testing temperatures, while the serration changes are the outcomes of their dynamic interactions with precipitates and condensed Cr, Co-rich solute cloud. Subsequently, in normal DSA regimes, activation energies required for the onset of type A, a mixture of type A and type A + C, and a mixture of type A + B and type C serrations are 30.6, 65.8, and 101.1 kJ/mol determined by strain ageing model at strain rates of 10–2, 10–3, and 10–4 s-1, respectively. Lastly, a two-time parameter-based Cottrell-Bilby strain aging kinetic model that considers the solute-dislocation interaction in a pipe diffusion manner is applied to evaluate the DSA strengthening concerning strain, strain rate, and temperature.