LOW-CYCLE FATIGUE BEHAVIOR OF OXYGEN-FREE HIGH-CONDUCTIVITY COPPER AT 300-DEGREES-C IN HIGH-VACUUM

In-vacuum fatigue tests were performed on commercially pure OFHC copper and 35% Au-65% Cu brazing filler metal at 300 degree C. Excessive recrystallization due to exposure in the 1025 degree C brazing temperature cycle was detrimental to the fatigue life of the base metal; cold work was beneficial to the fatigue resistance. Triple-point cracking and grain boundary sliding were the prevailing modes of fatigue failure observed in the full-size specimens. However, a mixed morphology of ductile and cleavagelike fracture was observed on the fracture surface of the subsize specimen in which the grain structure appeared to have undergone a change because of the presence of surface cold work. The braze has superior fatigue resistance, but to exploit the maximum strength, the brazed joint must be devoid of defects such as cavities and cracks.