Stress relaxation behavior in single crystal superalloys

The objective of this study was to examine the stress relaxation response and corresponding changes in microstructure of single crystal superalloys. The effects of temperature and time on stress relaxation response were examined for single crystal superalloys CMSX-4 and EPM-102. Stress-strain rate data from relaxation tests was in good agreement with constant load tests; it was further refined by testing samples from the same casting, and by eliminating the effects of primary creep in the stress relaxation test. From these tests, it was determined that EPM-102 had higher resistance to stress relaxation than CMSX-4 at and above 982 °C. Additionally, time-dependent strain recovery (“viscoelasticity”) was observed after unloading during these tests. The single crystal alloys displayed directional coarsening after stress relaxation testing at high temperatures.