Embrittlement of several nickel-base alloys after high-temperature exposure

Abstract

High temperature turbine blades in gas turbines are manufactured mainly from 7'-strengthened Ni-base superalloys. As is characteristic of all complex alloy systems, prolonged exposures at elevated temperatures lead to microstructural and property changes. The most common structural changes in Ni-base superalloys consist of 7' agglomeration, carbide reactions, and formation of 7' network at the grain boundaries. These changes in representataive blade alloys have been reported in the literature (i). The possibility of subtle environmental effects, which are known to affect mechanical properties in metals or simple alloys at lower temperatures, appears to have received an increasing recognition in the case of turbine alloys (2-8). The first detailed study was reported for Rene 77,80 and 100, which were embrittled following relatively short exposures at 980°C in air (2). This embrittlement manifested itself in a drastic reduction in tensile properties in a narrow temperature range around 8600C. Comparatively ductile behavior was retained at both higher and lower temperatures. This early work showed the embrittlement to be related to the penetration of gaseous species along grain boundaries, and as all the alloys studied had formed 7' networks along their grain boundaries by the MC+7 4 M2~CK+7' reaction, it was concluded that this was the embrittled phase and a neCeSsary prerequisite for embrittlement. Recently, a number of studies have been carried out to cover a range of Ni-base superalloys, all of which were shown to be embrittled in creep testing (3-5). Oxygen was identified as the penetrating and damaging species (3,4). In those studies, air exposure was used to induce the Observed environmental effect. In the case of the wrought IN X-750 superalloy, high temperature air exposure was not effective in embrittling the material in creep testing, while the exposure in reduced air pressure environments was extremely damaging (6,7). This was explained on the basis of a mechanism which assumed that in reduced air pressure the flux of oxygen along grain boundaries in th~ alloy IN X-750 was faster than in air (6,7). Hence in vacuum of ~ 2,6x10 Pa, oxygen diffused along grain boundaries without forming any oxides of chromium and nickel and instead formed gas bubbles of CO~ which acted as cavity nuclei (6). Recently the effect of environmental damage caused by thermal exposure on the high temperature low cycle fatigue (HTLCF) behavior of Ni-base superalloy IN738LC at 850°C, has been reported (9). The degradation in the HTLCF life of IN738LC, after thermal exposure at 1000°C for 240h in air, was attributed to the change in the modes of crack initiation and propagation due to the environmental effect. The aim of this paper is to report the results of an extensive study of the effect of prior thermal exposure in air on the tensile properties, mainly tensile ductility at different temperatures, of conventionally cast (CC), directionally solidified (DS) and single crystal (SC) Ni-base superalloy IN738LC. In addition, oxide dispersion strengthened (ODS) Ni-base alloys MA6000 and MA754, were also included in this investigation.