Effects of Ni and Mn contents on precipitation and strengthening behavior in Cu-Ni-Mn ternary alloys

The microstructural evolution and precipitation behavior of Cu-Ni-Mn alloys in the presence of solution treatment and aged treatment are examined. Based on a microstructural analysis, the strengthening contributions of the precipitates and solute atoms are calculated quantitatively. The results indicate that the Ni and Mn atoms dissolved in the Cu matrix cause a substantial solid-solution strengthening effect in the Cu-Ni-Mn alloys. The Mn atoms can produce a higher solid–solution strengthening effect than that of the Ni atoms in the solution-treated Cu-Ni-Mn alloys due to the larger atomic size misfit of Mn solute atoms. The continuous precipitation and discontinuous precipitation (DP) occur simultaneously in the Cu-Ni-Mn alloys during aging at 450 °C. The Cu-Ni-Mn alloy with a higher Ni and Mn content exhibits a higher area fraction of DP colonies due to the higher Gibbs chemical energy for the DP colony growth. The dispersed NiMn nanoparticles are precipitated by the continuous precipitation, which inhibits the DP colony growth caused by consuming the Ni and Mn solute atoms in the Cu matrix. The precipitation strengthening pertinent to the NiMn nanoprecipitates is the predominant strengthening mechanism in the peak-aged Cu-Ni-Mn alloys. The precipitation strengthening contribution to the total strength increment exceeds 80% in the Cu-20Ni-20Mn alloy aged for 48 h. Additionally, the volume fraction of the NiMn nanoprecipitates is increased in the Cu-Ni-Mn alloy after aging for 48 h as the Ni and Mn content increases. The increasing volume fraction of the NiMn nanoprecipitates and the grain boundary embrittlement induced by the DP reaction can result in remarkable embrittlement in the aged Cu-Ni-Mn alloys.