A high-mechanical strength copper alloy, which comprises Ni 3.5 to 4.5% by mass,
Si 0.7 to 1.0% by mass, Mg 0.01 to 0.20% by mass, Sn 0.05 to 1.5% by mass, Zn 0.2
to 1.5% by mass, and S less than 0.005% by mass (including 0% by mass), with the
balance being made of Cu and inevitable impurities, wherein a crystal grain diameter
is from more than 0.001 mm to 0.025 mm; and the ratio (a/b), between a longer diameter
a of a crystal grain on a cross section parallel to a direction of final plastic-working,
and a longer diameter b of a crystal grain on a cross section perpendicular to
the direction of final-plastic working, is 1.5 or less, and wherein the alloy has
a tensile strength of 800 N/mm2 or more.
