Effects of Ni and Mn content on continuous/discontinuous precipitation in Cu–Ni–Mn alloys

This study examines discontinuous precipitation (DP) in Cu–Ni–Mn alloys during aging, and evaluates the influence of Ni and Mn content on DP kinetics and the associated strengthening effect. The results have shown that DP occurs in Cu15Ni15Mn, Cu15Ni20Mn, Cu20Ni15Mn, and Cu20Ni20Mn alloys at 350 °C, representing the dominant phase precipitation mode. At a Ni/Mn mass ratio of 1, increasing the overall Ni and Mn content accelerates DP, enabling the alloy to reach a fully DP state at a shorter period of aging. An increase in the Ni/Mn mass ratio to 1.55 promotes continuous precipitation (CP) of the NiMn precipitate, resulting in a combined DP and CP in Cu20Ni15Mn. The NiMn phase precipitated via CP inhibits the development of DP regions, ultimately leading to a lower volume fraction of DP regions in Cu20Ni15Mn when compared with the other alloys for the same aging duration. The nanoscale Cu/NiMn dual phase structure formed by DP significantly enhances alloy strength. When a fully DP state is achieved, the yield strengths of Cu15Ni15Mn, Cu15Ni20Mn, and Cu20Ni20Mn exceed that of the corresponding alloy “solution” states by more than 800 MPa. The strengthening effect of DP is primarily determined by the volume fraction of DP regions and lamellar thickness of the Cu phase. An increase in both Ni and Mn contents effectively refines the lamellar Cu phase in the DP regions, which contributes to a higher strength of Cu20Ni20Mn than the Cu15Ni15Mn and Cu15Ni20Mn alloys in the fully DP state.