Development of directionally solidified γ-TiAl structures
Introduction
Gamma titanium aluminide (γ-TiAl) alloys are candidate materials to replace nickel-base superalloys in some gas turbine engine applications, such as low pressure or power turbine blades [1], [2], [3], [4], [5], [6]. These alloys offer good high-temperature strength, stiffness, and environmental resistance with much lower density than superalloys, thus offering a significant potential for weight savings [7], [8], [9], [10], [11], [12]. Unfortunately, the widespread use of these alloys has been delayed by two risk factors: a relatively poor balance of mechanical properties resulting from the inverse relationship between room-temperature strength/ductility and fracture/creep resistance [1], and uncertainties regarding the feasibility of economic component production [10], [13]. However, the directional solidification (DS) casting process, well developed for the production of nickel-base superalloy turbine blades, has attracted interest as a possible solution to these problems [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. A good balance of mechanical properties may be obtained by generating a DS microstructure consisting of fully lamellar columnar grains, with the lamellar boundaries within each grain aligned parallel to the DS growth direction and the individual grains slightly rotated about their longitudinal axes.
While generating microstructure of fully lamellar columnar grains is feasible through DS processing, aligning the lamellar boundaries within each grain parallel to the growth direction is a more challenging technical issue. This is largely a consequence of the lamellar microstructure forming during solid-state transformations on cooling, governed by specific orientation relationships, and not directly from the liquid phase during solidification [24], [25], [26]. There are two methods of controlling the final orientations of the lamellar boundaries making use of these orientation relationships. One is a seeding technique, analogous to the one used to grow single crystal superalloy components, which relies on controlling the orientation of the high temperature α phase [14], [15], [16], [17], [18], [19], [20]. A seed crystal, or crystals, with lamellar boundaries aligned parallel to the DS growth direction is placed at the bottom of the casting mold. During DS processing, the top of the seed melts and subsequent growth occurs from the seed crystal, causing the orientation of the seed crystal to be repeated in the solidifying alloy. For γ-TiAl alloys, several seed compositions and configurations have been identified and successfully used to align the lamellar boundaries with the DS growth direction. The other method of controlling the orientation of the lamellar boundaries relies on the choice of composition and processing conditions [21], [22], [23]. The main premise of this technique is that if an alloy solidifies completely with β as the primary solidification phase, the resulting columnar grains should have a mixture of lamellar boundaries aligned parallel and at 45° angles to the DS growth direction, while avoiding the brittle normal orientation. Some results show that this method is also capable of aligning most, if not all, of the lamellar boundaries with the DS growth direction.
To further develop the technology associated with DS processing of γ-TiAl a research program was initiated between Carleton University and the Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council Canada. The objectives of this research program were to develop the technology associated with DS processing of γ-TiAl alloys and to develop the equipment and processes for pilot-scale industrial fabrication of DS γ-TiAl structures. From results obtained early in the program [27], [28], [29], [30], it was apparent that the relationships between casting process parameters, solidification conditions, and microstructure formation were complex and not easily determined through analytical or experimental means. As was the case with early nickel-base superalloy DS technology [31], it became clear that numerical process modelling was necessary to help elucidate and to develop a better fundamental understanding of these relationships. To this end, a finite element method (FEM) process model was developed to simulate the DS casting of γ-TiAl and to subsequently use the model as a design tool to aid in the development of the casting process. A brief overview of some results of this research program is given in this paper.
Section snippets
Directional solidification casting trials
DS casting1 trials were done using two γ-TiAl
Directionally solidified microstructures
The as-cast longitudinal macrostructures of several DS γ-TiAl bars processed under various withdrawal rates and casting temperatures are shown in Fig. 3. The columnar grains are relatively fine, continuous, and are aligned along the DS growth direction. At the bottom of the bars, columnar grains nucleate just above the equiaxed grains of the original master alloy bar. These columnar grains grow along the longitudinal axes of the bars. The size and the orientation of the columnar grains are
Summary and conclusions
Fully lamellar, columnar-grained microstructures were produced by DS casting of two γ-TiAl alloy compositions. The as-cast DS microstructure was dominated by relatively fine, continuous, and relatively well-aligned columnar grains. The lamellar orientation within the columnar grains was not optimum. Rather, a mixture of parallel, angled, and normal lamellar orientations were found with a relatively high proportion of the parallel lamellar orientation near the bottoms of the cast bars and
Acknowledgements
This research has been made possible through the NSERC/NRC Research Partnership Program. The authors thank NSERC, the NRC Institute for Aerospace Research, and Pratt and Whitney Canada for their support.
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