An atom probe investigation of the role of rhenium additions in improving creep resistance of Ni-base superalloys

Abstract

Listen

Translate article

Scripta ~ETALLURglCA Vol. 20, pD. 1395-1400, 1986 Pergamon Journals Ltd. Printed in the U.S.A. All rights reserved AN ATOM PROBE INVESTIGATION OF THE ROLE OF RHENIUM ADDITIONS IN IMPROVING CREEP RESISTANCE OF Ni-BASE SUPERALLOYS D. Blavette*, P. Caron** and T. Khan** * Facult~ des Sciences de Rouen BP 67, 76130 Mont Saint Aignan, France ** ONERA, BP 72, 92322 Ch~tillon, France (Received June 23, 1986) [Revised July 29, 1986) Introduction Superalloys for turbine blade applications have constantly been improved through chemistry modifications and better processing techniques. The improvement in mechanical strength is attri- buted partly to the increasing amounts of refractory elements such as Ta, W and/or Mo. For ins- tance, among the modern single crystal superalloys, PWA 1480 (i) contains 12 wt% Ta and CMSX-2 (2) has 8% W and 6% Ta. More recently, the element Re has been shown to be an efficient streng- thener in creep both in the Ni-base eutectlc composites and single crystal superalloys (3, 4). It has also been reported that this element partitions solely to the matrix, retards the coar- sening rate of the ~' strengthening phase and increases the ~'/~' misfit (5). However, the physi- cal mechanisms responsible for the better creep resistance of the Re-contalning superalloys have not been elucidated. The purpose of this paper is to report and discuss the results of an atom probe' analysis on some single crystal superalloys in order to clarify the role of Re, on an ato- mic scale, in improving the creep strength. Materials and Experimental Procedure The chemical compositions of the single crystal alloys used in this investigation are given in Table i. The element Re was partly or entirely substituted for W in the reference alloys CMSX-2 and PWA 1480. Single crystals were grown parallel to the [001] orientation using the seeded technique under a temperature gradient of 250°C/cm and at a withdrawal rate of 15 cm/hour. Laue back reflection technique was used to check the orientations. All specimens were given a solutionlng and homogenizing treatment between 1280 and 1325°C for 15 hours fol- lowed by air quenching and the following precipitation heat treatment : IIO0°C/4h/AC + 850°C/48h/ Specimens within 5 ° of [001] orientation were prepared both for the tensile creep tests and atom probe analyses of the and ~' phases. The ~/~ microstructure of a typical single crystal alloy having about 70 vol.% of the ~ phase is illustrated in Fig. i. A FIM atom probe combines a field ion microscope in which an atomic resolution image of a specimen can be obtained with a mass spectrometer of single atom sensitivity. The principle of this technique is based on field ionization of a rare gas near the surface of a sharply pointed specimen and controlled field evaporation of surface atoms. The chemical nature of those ions evaporated from a small selected area is determined by means of time-of-flight spectrometry, the details of which can be found elsewhere (6). The unique advantage of FIM-atom probe, compared to the other microanalytical techniques, resides in its high spatial resolution coupled with the quantitative analysis capabilities. The lateral resolution at the tip surface can be varied from .5 to 5 nm and composition profiles can be obtained with an ultimate depth resolution of one atomic layer. The atom probe is therefore a unique instrument for the analysis of ~/~ super- alloys on a very fine scale. The details of the specific experimental conditions for the ana- lysis of Ni-base superalloys have been reported elsewhere (7). 1395 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals Ltd.