Atom probe and field ion microscope investigation of the negative creep mechanism in nickel-base superalloy

Abstract

The purpose of the present work is to study the negative creep mechanism in nickel-base superalloy by AP–FIM. The atom probe and the field ion microscope (AP–FIM) are techniques in which the atomic arrangement of the metal surface is observed directly and the specimen is analyzed chemically in small regions (a few atoms) with atomic resolution. Negative creep is caused by contractions of the nickel base alloy during the creep test, its mechanism not having been understood well until now. Samples that have undergone the creep test and samples that have not been subjected to this test are observed and analyzed by AP–FIM. The field ion image of the superalloy shows that it consists of disordered matrix ( r) and small ordered precipitates ( r′). The size of the precipitate grain in the `without creep' sample is about 100–200 Å in diameter whilst it is 300–400 Å for the `creep' sample. The result shows coarsening of r′ precipitates leading to some extent of ordering during the creep test. The results of the AP profile sample show that the concentration of the Al and Cr were changed from the r to the r′ phase after the creep test. The reciprocal diffusion of Al and Cr atoms is created in the r– r′ phase boundary during the creep test. The results of AP analysis suggest that 30 wt.% Ni 3Al (lattice constant 3.573 Å) transforms Ni 3Cr (3.551 Å) according to the change of the Cr content in the r′ phase. The contrast of lattice constant in the sample was estimated from experimental results to be 3×10 −3 A, which is consistent with creep test result.