Mechanical behaviour of Alloy 718 at high temperatures

Nickel base superalloys find extensive application in elevated temperature and stress environments. The precipitation strengthened class of these alloys contains mostly the precipitates of either γ′ or γ″ phase. The γ″ strengthened alloys find extensive applications in chemical industry and aircraft engines in different components like turbine blades, shafts, discs, tubes, piping, ammonia cracker tubes etc. Nickel iron based Alloy 718 is extensively used in different components in aircraft engines. Recent trend of formation of many of the components is using powders or through additive manufacturing. The phases that form in this alloy are the γ′ (L1₂ structure), the γ″ (D0₂₂ structure) and the δ (D0_a structure) and they all form within the matrix at different temperatures. Understanding of the basic mechanism of formation of the phases and their stability range is very important to estimate the life of components. Some of the important results of detailed studies performed by the authors in Alloy 718 are elaborated in this lecture. Serrated behaviour has been noticed during elevated temperature tensile testing for all these microstructures and it has been rationalised on the basis of interstitial or substitutional solute atmosphere formation around dislocations depending upon the temperature of test and the strength of bonding of solutes in the precipitates