Effect of Aging on the High Temperature Mechanical Properties of a Novel Co-‎‎7Al-7W-4Ti-2Ta Superalloy

The aim of this study was to evaluate the effect of aging on the stress rupture and hot pressure of novel cobalt base superalloy of Co-7Al-7W-4Ti-2Ta composition. After melting the superalloy by VIM method and remelting by VAR method, the ingot was homogenized at 1250 °C for 10 hours. Then, aging treatment was performed on homogenized samples at 800 °C for 8, 16 and 24 hours. After microstructural and phase analysis by SEM and X-ray diffraction pattern testing, the high temperature mechanical properties including hot pressure and stress rupture tests were performed. The results showed that the alloy in the cast condition contains ɣ, ɣ‘, a and β phases in which the ɣ‘, a and β phases are dissolved in ɣ phase by homogenization treatment. By aging of the studied alloy, ɣ‘ precipitates nucleate and grow, where in by increasing aging time the volume fraction and size of these precipitates increases. The results of pressure tests in the temperature range of 500 °C to 1000 °C showed that the strength of the alloy increased at a temperature of 700 °C, which indicates the occurrence of the stress anomaly phenomenon as a consequence of ɣ‘precipitation. Besides, the highest resistance to stress rupture at 770 °C under 300 MPa for 22 hours was achieved for the specimen aged at 800 °C for 16 hours.