Thermal Stability Characterization of Ni-Base ATI 718Plus Superalloy

Abstract

Thermal stability of ATI 718Plus Alloy was characterized after thermal exposure at 760°C for up to 500 hrs. Microstructure and mechanical properties, including tensile, stress rupture, creep, low cycle fatigue and fatigue crack growth rate, were tested before and after thermal exposure. For comparison, the same experiments were conducted on alloys 718 and Waspaloy. Changes in microstructure brought out by exposure were a slight dissolution/ coarsening of γ' phase particles and formation and growth of δ phase for 718Plus alloy. The growth rate of γ' phase particles in 718Plus alloy was significantly slower than that of γ phase in 718 and even slower than that of γ' phase in Waspaloy. The amount and formation rate of δ phase formed in 718Plus alloy during exposure was lower than that observed or predicted for alloy 718. The most significant effect of thermal exposure on properties was the acceleration of creep rate, especially Stage I, for all three alloys. Alloy 718Plus suffered moderate thermal degradation in strength and stress rupture life, but there was little change in low cycle fatigue and non-dwell fatigue crack growth rate. In fact, thermal exposure significantly reduced the dwell fatigue crack growth rate in 718Plus alloy. The observed changes in mechanical properties are explained from the standpoint of the microstructural changes in 718Plus alloy and comparisons made to alloys 718 and Waspaloy.