Al-4%Cu-4%Mn alloy specimens were chill-cast, homogenized at 793 K for 0.6 ks, water-quenched, preaged at 473 K for 60 ks and subjected to over-aging at 623 K. The crystal structure and precipitation process of ternary phase (named T') appearing during final aging and being effective for the suppression of over-aging of this alloy, were studied by transmission electron microscopy including selected area diffraction. The T' is an orthorhombic crystal (a = 0.339 nm, b = 1.24 nm and c= 1.20 nm) and has an orientation relationship with the matrix : (100)
T'//(100)
Al, [010]
T'//[010]
A. It was found that T' nucleates on the periphery of θ' which has formed during pre-aging. Cold working before pre-aging was performed based on this result. The cold working was significantly effective for an increase in the number density of T', leading to an increase in hardness after over-aging.
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