GRAIN BOUNDARY CAVITATION DURING HIGH TEMPERATURE FATIGUE OF COPPER-ALUMINUM AND ALUMINUM-MAGNESIUM ALLOYS (SANS, ELECTRON MICROSCOPY)

The nucleation and growth of grain boundary cavities during fully reversed, high temperature fatigue of copper-aluminum alloys, containing 5 and 15 atomic percent aluminum, and an aluminum-magnesium alloy, containing 3.5 at .% magnesium, have been experimentally investigated by means of optical and transmission electron microscopy and small angle neutron scattering (SANS).; Observations were made on the siting, morphology and size of the cavities, mainly by using high voltage electron microscopy, while measurements of SANS from fatigued specimens of Cu-15Al were used to calculate cavity total volume fractions, number density and size distributions as a function of temperature and time of fatigue.; It was found that the cavitation level in Cu-Al alloys decreased with increasing Al content. The Al-3.5Mg alloy showed a marked resistance to cavitation, but lower than pure aluminum, which did not cavitate.; In the Cu-Al alloys, cavities were located at sites likely to develop high stress concentrations during grain boundary sliding, such as grain boundary serrations and triple grain junctions. Such detailed observations were not made on the Al-3.5Mg alloy.; The SANS data from Cu-15Al specimens with moderate cavitation levels (volume fractions between 5 x 10('-6) and 1 x 10('-4)) show increasing cavity volume fractions and number densities as the time of fatigue increases. Fatiguing at 480 C appeared to produce the largest volume fractions for a given fatigue time, but the lowest number densities; in contrast, fatiguing at 570 C produced very large densities of small cavities. Testing at 390 C produced intermediate results.; Approximate individual cavity growth rate predictions from J. R. Weertman's theory of high temperature fatigue cavitation gave a good agreement with estimates made from the experimental data. However, the numerical techniques involved in the handling of SANS data need further development.; A strong obstacle to modelling grain boundary cavitation has been recognized in the, as yet, unpredictable migrating behavior of grain boundaries. Our results seem to indicate a marked dependence of cavitation on the details of the migrating behavior of grain boundaries.