Microstructure evolution and properties of Cu-20Ni-20Mn alloy during aging process

The microstructure evolution and phase transformation of Cu-20Ni-20Mn (mass fraction, %) alloy at 450 °C were investigated by X-ray diffraction and transmission electron microscopy (TEM). The variations of tensile strength, yield strength and hardness of this alloy during aging process were also analyzed. The results show that no significant variations of hardness and strength in the initial stage of aging, with a long incubation period, are observed at 450 °C. Subsequently, the ordered face-centered tetragonal (FCT) NiMn phase nucleates and grows up with prolonging the aging time. The hardness and tensile strength of the alloy increase up to their maximum values with increasing the ordered particle size, i.e., the strength of the alloy reaches 942 MPa after being aged at 450 °C for 40 h. The main cause of the age-hardening is considered to be precipitation strengthening due to the ordered FCT-NiMn particles.