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

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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 rr′ phase boundary during the creep test. The results of AP analysis suggest that 30 wt.% Ni3Al (lattice constant 3.573 Å) transforms Ni3Cr (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.

Introduction

Negative creep is caused by contraction of the material during the creep test. The phenomenology of negative creep is the changes of the microstructure of nickel-base superalloy during the creep test 1, 2. Using the field ion microscope and the atom probe (AP–FIM) the atomic arrangement of the metal surface can be observed directly and the specimen analyzed chemically, over small regions (a few atoms) with atomic resolution. AP–FIM is therefore well suited for studies of the microstructure of an alloy that has undergone the creep test. The microstructure of the nickel-base superalloy consists essentially of an f.c.c solid solution (r) and finely-dispersed ordered precipitate (r′). In general, the mechanisms of phase transformations and short-range ordering have been proposed for explanation of the negative creep phenomenon 3, 4in the nickel-base superalloy, but experimental evidence is still lacking. The purpose of the present work is to study the coarseness and the microstructure of precipitates in the nickel base superalloy by AP–FIM during the creep test. The changes in the concentration of Ni, Al and Cr were determined by following the sample AP depth profile. The AP experiment established for the first time that reciprocal diffusion of Al and Cr atoms is created in the rr′ phase boundary when the sample has suffered creep. The replacement of Al atoms by Cr atoms in the crystal lattice of Ni3Al induces the lattice constant to decrease. It is shown that the negative creep effect in this material results mostly from reciprocal diffusion of Al and Cr between the rr′ phase boundary.

Section snippets

The experimental procedure and results

The experiments were carried out in an AP–FIM, the detailed description of the equipment being available elsewhere [5]. The field ion microscope is capable of imaging the surface of a tip and each distinct point on the image is an individual atom, provided that the specimen is cooled by liquid nitrogen. The mass spectrometer is used to analyze the sample chemically, with single atom sensitivity for all elements. The instrument has a straight flight tube (mass spectrometer of time-flight) and is

Discussion

The microstructure of the nickel-base superalloy consists of essentially a f.c.c. solid solution (r) and dispersed ordered precipitated (r′), which r′ phase occupied about 45 wt.%, but the lattice parameter is decreased by about 4×10−3 A [6]. The AP mass spectrum of the nickel-base superalloy by AP selective area analysis is shown in Fig. 3, where (a) displays the result of AP analysis of the r′ phase and (b) of the r phase. The mass spectrum shows the difference of the concentrations atoms of

Acknowledgements

This project was supported by NSFC and NAMCC under grant number 59291000. The author would like to thank Mr Zhang Yu for providing the samples for the tests.

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