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This report describes important new findings in the understanding of intergranular cavitation and fracture in alloys which derive their creep strength from the presence of hard second phase particles, such as the nickel base superalloys. The work is primarily theoretical but is supported in most instances, by simple experiments on model alloys. Criteria for the nucleation of cavities at high temperature are developed and it is shown that cavities are most likely to nucleate at particles in the grain boundaries.
The fracture toughness of an oxide-dispersion strengthened copper alloy AL-15 has been examined at room temperature and 250°C, in air and in vacuum (
Oxide-dispersion strengthened copper alloys and a precipitation-hardened copper-nickel-beryllium alloy showed a significant reduction in toughness at elevated temperature (250°C). This decrease in toughness was much larger than would be expected from the relatively modest changes in the tensile properties over the same temperature range. However, a copper-chromium-zirconium alloy strengthened by precipitation showed only a small decrease in toughness at the higher temperatures. The embrittled alloys showed a transition in fracture mode, from transgranular microvoid coalescence at room temperature to intergranular with localized ductility at high temperatures. The Cu-Cr-Zr alloy maintained the ductile microvoid coalescence failure mode at all test temperatures.
The technological importance of the high-temperture fracture of copper alloys is established.
Fracture at elevated temperature in dispersion strengthened alloys occurs by preferential nucleation and growth of voids at the inclusions present in the grain boundaries. It is shown how the stress-rupture life depends upon: stress, temperature, grain size, volume fraction of inclusions, size of inclusions and the strength of the inclusion-matrix interface.