Copper alloy

A copper-beryllium alloy consisting essentially of about 0.2% to about 0.5% beryllium, about 0.2% to about 0.5% cobalt and the balance essentially copper which has been solution treated at a temperature not exceeding about 1750.degree. F., then cold worked at least about 70% and aged in the temperature range of about 700.degree. F. to about 800.degree. F. for about 1 to about 5 hours and which has a Percent Remaining Stress of at least about 85% after 1000 hours of stress relaxation testing at 300.degree. F. at an initial stress of 75% of the 0.2% offset yield strength, a formability not exceeding 1.25 in the longitudinal bend test, a tensile strength of about 95 to about 125 ksi (655 to 860 MPa), 0.2% offset yield strength of about 85 to about 120 ksi (580 to 825 MPa), elongations of about 5% to about 15% and conductivity of about 40% to about 65% I.A.C.S. and by a microstructure, in the condition resulting from an anneal at 1650.degree. F. followed by an age at 950.degree. F., exhibiting a particle density of the principal hardening precipitates of about 2.3.times.10.sup.9 precipitates per square millimeter in a transmission electron microscope thin foil, and only about 240 cobalt-beryllium intermetallic particles of at least 2.5 microns diameter per square millimeter in an optical metallograph.