Abstract
It has been widely reported that the microstructure refinement of TiAl alloys can be achieved by massive transformation and subsequent annealing in α2 + γ two phase field. To achieve this goal, several heat treatment parameters must be adjusted, including the heat treatment temperature around single α phase field, the annealing temperature, and the annealing time for the precipitation of α2 phase. Thus, a systematic study is needed for each alloy with different compositions. In this study, the heat treatment parameters for grain refinement via massive transformation of a high Nb-containing TiAl are investigated. Precipitation of α2 phase during annealing is observed by transmission electron microscopy. It is found that 30 min at single α phase field is appropriate for the massive transformation; a full, massively transformed microstructure cannot be obtained by oil or water quenching. A short annealing time can result in a refined microstructure, whereas the sizes of the precipitated α2 phase increases with the increase of annealing time. The α2 phase can form at the interface of twin boundaries of the γ phase, following the Blackburn orientation relationship with both sides. The Vickers hardness is measured for the annealed samples, which remains relatively stable for different annealing times.
Highlights
TiAl alloys have been considered as promising high-temperature structural materials for years, and their application on blades has been achieved in several alloy compositions [1,2]
In addition to the grain refinement effect induced by the high amount of Nb, B can strongly lead to a fine-grained microstructure
The βo phase at the lamellar colony boundary is a result of the order-disorder transformation from the high temperature β phase; the subsequent ordering within the βo phase can cause the formation of ordered ω phases, which was reported in detail in our previous research [25]
Summary
TiAl alloys have been considered as promising high-temperature structural materials for years, and their application on blades has been achieved in several alloy compositions [1,2]. For TiAl-based alloys, massive transformation, which is initiated by cooling from high temperature α phase in a controlled cooling rate to create massive γ phase, followed by subsequent annealing at α + γ two phase region is proven to be an effective method to refine the microstructures [5,6,7,8,9,10,11,12]. It is commonly agreed that a heat treatment window exists for each alloy composition; only in this window can the fully massive transformed microstructure be obtained that is suitable for the following annealing treatments, the massive transformation is thought to be displacive only with atom rearrangement across interfaces [7,8,11,13]. For high Nb-TiAl alloys, the microstructure is relatively finer than some other TiAl alloys during the common preparation process, further grain refinement is still necessary, since balanced mechanical properties
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