EFFECTS OF Cr ON THE LONGTIME EMBRITTLEMENT OF ALLOY 718

Current studies of high temperature materials have focused on the longtime behavior of materials in an effort to extend the life of service components. High temperature materials which appear to be stable in short exposure times can become unstable when exposed in a temperature range of up to 1200°F for times of 10,000 to 100,000 hours. Most alloys used in this temperature range are high Cr alloys and the superalloy most commonly used is alloy 718. Whether alloy 718 is used in aerospace or other industrial applications, the behavior of Cr on the formation of carbides,sigma phase, or other Cr rich intermetallic phase plays an important role in the eventual embrittlement of the material.Alloy 718 is limited to temperatures of 1200°F or below because of the occurrence of large amounts of transformations of the strengthening phase γ′to the delta plate phase with the resultant loss of impact and yield strength.In ongoing longtime behavior studies of alloy 718 at 1200°F,the impact property of alloy 718 drops from 38 ft-lbs to less than 8 ft-lbs in exposures of 50,000 hours. The initial drop in impact occurs without apparent changes in the strengthening phase; however, an αCr phase is starting to form and grows with time of exposure.The results of a study of the role of αCr on the longtime embrittled engine run turbine disks and isothermally exposed alloy 718 will be presented. The factors which affect the αCr formation will be discussed.

Segregation of alloying atoms at a tilt symmetric grain boundary in tungsten and their strengthening and embrittling effects

We investigate the segregation behavior of alloying atoms （Sr, Th, In, Cd, Ag, Sc, Au, Zn, Cu, Mn, Cr, and Ti） near Z3 （ 111 ） [1]-0] tilt symmetric grain boundary （GB） in tungsten and their effects on the intergranular embrittlement by performing first-principles calculations. The calculated segregation energies suggest that Ag, Au, Cd, In, Sc, Sr, Th, and Ti prefer to occupy the site in the mirror plane of the GB, while Cu, Cr, Mn, and Zn intend to locate at the first layer nearby the GB core. The calculated strengthening energies predict Sr, Th, In, Cd, Ag, Sc, Au, Ti, and Zn act as embrittlers while Cu, Cr, and Mn act as cohesion enhancers. The correlation of the alloying atom＇s metal radius with strengthening energy is strong enough to predict the strengthening and embrittling behavior of alloying atoms; that is, the alloying atom with larger metal radius than W acts as an embrittler and the one with smaller metal radius acts as a cohesion enhancer.