| Summary: | The hot deformation behavior of Cu-20.0Ni-5.0Sn-0.25Zn-0.22Mn was investigated using a Gleeble-3500 thermal simulator with a temperature range from 720 °C to 880 °C and a strain rate range from 0.001 s<sup>−1</sup> to 1 s<sup>−1</sup>. The results show that the flow stress increased with the increase of the strain rate and the decrease of the temperature. The constitutive equation of the alloy was established based on the peak flow stress. Figures of the power dissipation efficiency and flow instability with the variable of the true strain from 0.2 to 0.8 displayed the dynamic change of power dissipation efficiency and the instability area. The domain of 730–770 °C and 0.001–0.01 s<sup>−1</sup> possessed a power dissipation efficiency over 40% throughout the whole deformation. The flow instability always appeared at a high strain rate from 0.1 s<sup>−1</sup> to 1 s<sup>−1</sup> during the whole deformation process. The nucleation site of the dynamic recrystallization generally appeared along the grain boundaries, indicating the discontinuous dynamic recrystallization mechanism. The appropriate conditions for deformation with a true strain of 0.9 is in a safe domain (820–860 °C with a strain rate of 0.001–0.01 s<sup>−1</sup>). There were four kinds of variation tendencies of the power dissipation efficiency with the increase of the true strain under various conditions, suggesting a changing situation for the main softening mechanisms.
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