The mechanical properties of aged Cu-Ni-Al steel were studied. Four kinds of steels with compositions 1-2% Cu,3.2-3.6% Ni, 1.3-1.6% Al, after solution treatment at 850℃ and then air-cooled, were aged at temperature range of 400-600℃ for 0.5-128 hours. When aged at 500℃ for over 8hours, they showed maximum hardening and their hardness, Y.S., U.T.S. reached 39-41.5$H_{RC}$, 105-115Kg/㎟, 120-130KG/㎟, respectively. Elongation, however, decreased with hardness increasing and the transition temperatures of V-notch impact strength of steels aged at 550℃ for 4hours were measured to be near 150℃. And at room temperature the values of their impact strength were only 0.4-0.5Kg.m.
By electron microscopy study, it was revealed that two kinds of precipitates, $\epsilon-Cu$ with fcc structure and CsCl type coherent NiAl bring hardening. But the two showed somewhat different hardening behavior, that is to say, NiAl work-hardened the ferrite matrix more potentially than $\epsilon-Cu$ did, which made steels with more NiAl precipitates have higher U.T.S., although their Y.S. values were lower than those with less NiAl but more $\epsilon-Cu$ precipitates.
The solubility of $\epsilon-Cu$ in NiAl is larger than that of NiAl in $\epsilon-Cu$, so Cu supersaturated in the matrix is expected to increase the quantities of NiAl that precipitates. But the solubilities of the two in each other are limited, so the ratios of Cu and (Ni+Al) supersaturated in the matrix seem to determine the increments.
The $\epsilon-Cu$ particles that precipitate during solution treatment are expected to provide the nucleation sites for the NiAl particles and to supply Cu to these. By the above fact it is reasonably thought those steels with higher Cu content (1.5 and 2.0%) would have larger NiAl particles than that with lower Cu (1.0%) and the latter have larger work-hardening effect as the results of this experiment show.
NiAl particles maintain coherency with the matrix even after overaging by the relation $(100)_{Fe}$// $(100)_{NiAl}$, $100_{Fe}$ // $100_{NiAl}$ and their growth seemed to be only small, but $\epsilon-Cu$ grew rods when overaged.
