Decomposition of Gamma-Phase Manganese Copper Alloys

The decomposition of γ-Mn–Cu alloys containing more than 60 at.% Mn to the mixed (α+γ) phase has been studied using x-ray and neutron diffractometry and measurements of electrical resistance and Young's modulus of elasticity. There is strong evidence that manganese-enriched clusters having the γ-phase structure are produced prior to the precipitation of α-Mn. When γ-phase alloys were aged at 450°C for increasing periods of time, they remained single phase, but developed long-range antiferromagnetic order associated with a tetragonal (c/a<1) lattice distortion, and with a corresponding rise of the Néel temperature to an asymptotic value of ∼470°K for all compositions. The effects of aging at other temperatures, and the behavior of furnace-cooled alloys are discussed in terms of the pre-precipitation of manganese-rich clusters.