Pergamon A<,/u m(lr<‘,~ Vol. 45, No. X. pp. 3177-i:!X2, 1997 !’ 1997 Act3 Metallurgica Inc. Published by Elsemer Science Ltd. All rights reserved Printed in Great Britain PII: S1359-6454(97)00035-9 1359-6454 97 s17 00 + 0.00 STRAIN INDUCED DIRECTIONAL COARSENING IN NICKEL BASED SUPERALLOYS: INVESTIGATION ON KINETICS USING THE SMALL ANGLE NEUTRON SCATTERING (SANS) TECHNIQUE M. Vl?RON’ and P. BASTIE’ ‘L.T.P.C.M.. UMR CNRS no. 5614 BP 75. 38402 Saint Martin d’Htres cedex and ‘Laboratoire de Spectrometrie Physique, UMR CNRS no. 5588. BP 87. 38402 Saint Martin d‘Htres cedex. zyxwvutsrqponmlkji France Abstract-Using the small angle neutron scattering technique, we have observed rafting in nickel based single crystal superalloys. Kinetics of morphological evolution of the precipitates have been studied in zyxwvutsrqponmlk sitar. Therefore we used a special furnace designed for the ageing of prestrained specimens under a neutron beam. The evolution of both the precipitate aspect ratio and the distance between precipitates confirms the importance of strain in the directional coarsening process, Results are presented and discussed regarding kinetics and microstructural aspects. In such conditions, rafts seem to be different from those obtained after a creep test. tJ: 1997 Acta Metallurgica Iw. RBsumGNous avons observk la coalescence orient&e dans les superalliages monocristallins a base de nickel en utilisant la technique de diffusion aux petits angles des neutrons. La cinetique de l’evolution morphologique des precipites a Cte ttudiee “in situ” . Pour cela. nous avons utilist un four developpe specialement. pour effectuer des traitements de vieillissement, sous le faisceau, d‘un echantillon predeforme. L’evolution des facteurs de forme des prtcipites et de la distance moyenne entre ces precipiter confirme l’importance de la predeformation dans le processus de coalescence orientbe. La presentation et la discussion des rtsultats concernent a la fois les aspects cinetiques et microstructuraux. Les radeaux obtenus dans ces conditions semblent differents de ceux resultant d’un essai de fluage. INTRODUCTION Owing to their good mechanical properties at high temperature, nickel based superalloys are commonly used for the manufacture of single crystal turbine blades. These mechanical properties are related to the precipitation of an ordered 7 phase inside they matrix. The morphology of the precipitates depends strongly on the thermomechanical history of the sample. A well-known phenomenon is the directional coarsening of the precipitates which occurs at high temperature (typically 1323 K) under stress and results in a strong anisotropy of the microstructure [ 1,2]. After standard heat treatment, the microstructure of those alloys is composed of a matrix containing about 70% of well-aligned coherent cuboidal precipitates, with an average size of 0.45 pm [3]. Precipitates (ordered ;” phase) and matrix (7 phase) present a misfit defined as 6 = 2(q’ - q)/(q’ + a~) where a;” and q are the lattice parameters of the y’ and ; phases, respectively. Directional coarsening occurs under stress and, for example, during a tensile creep test on the ,commercial AM1 superalloy, the microstructure evolves to form “rafts” of precipitate phase perpendicular to the stress axis [4]. Other orientations of rafts with respect to the stress axis are possible: when they are parallel to the stress axis directional coarsening is called type P. and when they are perpendicular to the stress axis. type N [5]. The coarsening process has been extensively studied both experimentally and theoretically. The direction of rafts has been related to the sign of the stress (the developed microstructure is not I he same after a tensile and a compressive creep test), to the misfit sign, and to the elastic constant of both phases [6, 71. Whereas the microstructures after directional coarsening are well characterised and pertinent parameters for rafting have been put forward, the driving force is still a matter of controversy. Most of the theoretical models deal with elastic energy calculation without taking into account elastic or plastic strain. The effects of elastic strain in a regime where there is no dislocation have been shown to lead to directional coarsening [8]. However. for the temperature and stresses involved, creep does take place and therefore dislocations are induced in the material and cannot be neglected. In a plastic regime. it has been shown that inhomogeneous misfit relaxation by plastically induced dislocations pro- vides the main driving force for the phenomenon [9], and moreover raft direction can be predicted owing to the position of dislocations and their action on 3277