Grain growth and particle pinning in a model Ni-based superalloy
Introduction
In metallic alloys, dispersions of second-phase particles may inhibit grain growth by exerting a pinning force on migrating grain boundaries during thermal treatment. Since the driving force for grain growth varies with the grain size, grain growth will proceed until the grain size reaches a limiting value, whereupon the driving force is balanced by the pinning force. This effect was first described by Zener in the late 1940s [1] and is widely known as Zener pinning. Since then, numerous refinements of this model have been proposed in which the details of the interactions between the grain boundaries and the particles are taken into account. These factors include: the degree of contact between the boundary and the particles [2]; the shape of the pinned grain boundaries [3]; the heterogeneity of the grain size distribution [4]; the shape of the pinning particles [5]; the particle-assisted motion of grain boundaries [6] and many more. All of these factors may affect the grain size to some extent and so the application of pinning theory to practical alloy systems can be very challenging.
Ni-based superalloys exhibit an attractive combination of mechanical properties and corrosion/oxidation resistance at high temperature, leading to their use in a variety of gas-turbine engine components including blades and discs. Grain size plays an important role in controlling the mechanical properties of these superalloys, including their tensile strength [7], low cycle fatigue life [8] and creep resistance [9]. Ni-based superalloys generally contain a variety of different precipitates in a face-centered cubic γ-Ni matrix [10]. The majority of these correspond to the L12 (Ni3Al) γ′ phase, while inert precipitates like carbides and borides are present as minority constituents. During the thermo-mechanical processing of the alloys, these second-phase particles could significantly retard the grain boundary migration and limit grain growth. However, although there is a significant body of literature on Zener pinning, there is little detailed information available about the pinning effect of the precipitates in Ni-based superalloys. This is presumably due to both the complexity of the microstructures and the complications associated with impurities and local inhomogeneities in commercial alloys.
In this study, the grain growth and particle pinning phenomena have been examined in a model powder metallurgy (P/M) Ni-based superalloy whose chemistry resembles that of the matrix phase mixture in the commercial alloy IN100. This alloy was produced as part of a program to develop analytical models for the mechanical behavior and microstructural development in alloys such as IN100 [11], and was chosen for the present study because it contains low levels of impurities, has a highly homogenous microstructure and has a fine initial grain size. The samples were heat-treated over a range of temperatures corresponding to those at which Ni-based superalloys are typically processed for gas-turbine engine applications, and microstructures were characterized using electron microscopy techniques. These data were used to elucidate the effects of the various second-phase particles on the grain growth behavior in this alloy.
Section snippets
Materials and experimental methods
The composition of the alloy is Ni–20.3Co–14.8Cr–4.6Al–3.9Mo–3.7Ti–0.08Zr–0.08C–0.02B–0.011Y (all in wt.%). The alloy was heat-treated at elevated temperatures to study the grain growth behavior. First, small coupons of the alloy, around 2 cm × 2 cm × 0.2 cm, were cut from the bulk material. The small sample size was chosen so that the sample could be heated or cooled as quickly as possible. The samples were inserted into a pre-heated tube furnace, and held under flowing Ar for times ranging from 2 min
Microstructural evolution and grain growth
A typical example of a SEM image showing the microstructure exhibited by the alloy in the as-received (AR) state is shown in Fig. 1a. The darker areas correspond to the γ′ phase that was partly etched out during the preparation process. The volume fraction of the γ′ phase in the AR material is around 40%, and the distribution of this phase is complex with coarse recrystallized γ′, finer unrecrystallized γ′ and very fine γ′ precipitates that form during slow cooling from the processing
Conclusions
The microstructures exhibited by a model Ni-based superalloy in the as-received condition and after a series of heat-treatments at different times and temperatures have been studied using SEM and TEM techniques and the following points have been established:
- (1)
The initial microstructure consists of γ grains, a complex distribution of grains/precipitates of the γ′ phase, and four types of inert particles: M3B2-type borides, MC- and M23C6-type carbides, and Y2O3. The M3B2 and M23C6 particles were
Acknowledgements
This work was supported by DARPA/USAF under contract no. F33615-00-2-5216 with Dr. R. Dutton as technical monitor. The authors would like to thank Dr. Mike Savage of Pratt and Whitney for providing the alloys used in this investigation.
References (21)
- et al.
Acta Mater.
(1996) Acta Metall.
(1988)- et al.
Acta Metall.
(1988) - et al.
Acta Metall. Mater.
(1990) Acta Metall.
(1982)- et al.
Scr. Mater.
(2004) - et al.
Mater. Sci. Eng. A
(1997) - et al.
Mater. Sci. Eng.
(2003) - et al.
Scr. Metall.
(1987) - et al.
Acta Mater.
(1997)