Precipitation and deformation behaviour of cast alloy 718 during creep and thermal exposure

Abstract

The precipitation, deformation, and fracture behaviour of cast alloy 718 during creep rupture tests was investigated, in comparison with thermal exposure tests. Inhomogeneous deformation bands appeared during monotonic or cyclic deformation of alloy 718. The bands were identified as mechanical twins, which are known to be responsible for crystallographic failure during creep rupture at and below 649°C. However, crystallographic failure was observed at temperatures up to 760°C in the present study. No crystallographic failure was observed at and above 816°C. Precipitation of δ phase was observed on deformation bands following creep rupture tests at and above 704°C. The difference in failure mode below 760°C and above 816°C is assumed to be caused by the precipitation of δ phase on the bands. A few discrete δ particles on the bands during 704°C creep rupture tests were not sufficient to prevent decohesion along the twin/matrix interface, and therefore crystallographic failure still occurred. In contrast with little or no precipitation below 704°C, needlelike or platelike δ phase precipitated at and above 816°C. It is postulated that the precipitation of δ phase restricted successive deformation. Since δ phase precipitates on {111} planes where major deformation occurs, this phase usually grows according to the following orientation relationship: (010)_δ∥(111)_γ, [100]_δ∥[11¯0]_γ. Restriction of deformation by the precipitation of δ phase caused the change in failure behaviour at and above 816°C.