Effect of Ni addition on precipitation strengthening of Cu-rich precipitates in Fe-Cu-Mn alloy

The Fe-1.63Cu-2.17Mn (wt%) and Fe-1.57Cu-2.15Mn-3.23Ni alloys were aged at 450 °C to investigate the effects of the addition of Ni on the precipitation process of Cu-rich precipitates. Their micro-hardness and tensile strength in relation to the microstructures were examined. It has been found for the two alloys the peak hardness appeared at the same ageing time; the hardness and strength of Fe-Cu-Mn-Ni alloy were higher than those of Fe-Cu-Mn alloy over the whole range of aging time due to the solid solution strengthening of Ni addition; the response of ageing treatment in the Fe-Cu-Mn-Ni alloy was smaller than that in the Fe-Cu-Mn alloy, because Ni addition decreased the nucleation rate of Cu-rich clusters in alloy and weakened the precipitation strengthening effect. After peak aging, Ni atoms were enriched at the precipitate/matrix interface close to the matrix side, and promoted the segregation of Mn atoms at the interface close to the Cu-rich precipitate, forming the core–shell structure with the core of Cu-rich precipitate, which reduced the rate of growth of the Cu-rich precipitates and the decline rate of hardness of Fe-Cu-Mn-Ni alloy.