Here is a quote from a large aerospace manual
"Effects of Temperature – Copper alloys are not recommended
for usage above 700°F. Hydrogen embrittlement
of most copper alloys begins at 700° F and increases
rapidly with increase in temperature. The copper–nickel
and copper–aluminum alloys are best suited for elevated
temperature usage because they have the highest resistance
to oxidation. Copper alloys remain ductile at
temperatures as low as –300°F."