Thermal stability of Alloys 718 and Allvac 718-ER (R)

Thermal stability of Allvac((R)) alloy 718-ER(R) and alloy 718 was evaluated by the change in microstructure and mechanical properties after long time thermal exposure. The major microstructural changes due to thermal exposure included gamma" and gamma' particle growth, formation of a gamma"-depletion zone, and occurrence of large, blocky particles resulting from gamma" to delta phase transformation. All three changes were quite similar in both alloys, indicating that P and B did not have a noticeable effect on microstructural degradation in alloy 718. Tensile strength at room and elevated temperatures dropped 10-17% after thermal exposure in both alloys. Stress rupture life was reduced by nearly 90%, but creep resistance dropped by more than two orders of magnitude. The degradation of creep rupture properties in terms of percentage reduction was significantly greater in alloy 718-ER, and most of the improvement from P and B modification was lost after thermal exposure. The strengthening effect of P and B modification was also a function of test temperature. It was concluded that P and B did not increase the thermal stability of alloy 718 at 704 degreesC x 1000 hrs. exposure and that the loss of rupture and creep properties was caused mainly by gamma" dissolution and the formation of a gamma"-depleted zone at grain boundaries. The effect of P and B on rupture and creep properties of alloy 718 appears to be closely associated with gamma" precipitates.