| Research Abstract |
Extensive progress and improvements in two-phase TiAl alloys, which have great potential as a new class of light-weight high-temperature structural materials, have been made in the last about five years. Based on these achievements, several engineering two-phase TiAl alloys in duplex form with room-temperature ductility up to 3-4% and improved strength have been developed. However, the lamellar structures are still poor in ductility, although they are beneficial for toughness and high-temperature strength. More work is thus needed to achieve an improved balance of strength and ductility in the lamellar form. Some alloying elements such as Cr, V and Mn have been reported to increase the room-temperature ductility of the two-phase alloys in duplex form. However, none of them is effective in ductilizing the two-phase alloys in lamellar form.
Recently, tensile ductility and therefore fracture stress of two-phase TiAl alloys have been found to be sensitive to test environment ; they are high
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er when tested in vacuum or in dry air than in air or in hydrogen gas. The tensile elongation and fracture stress are also sensitive to strain rate ; they increase when tested in air or in hydrogen gas while they decrease when tested in vacuum with increasing strain rate. These results clearly suggest that this class of materials is susceptible to environmental embrittlement. However, we have recently found that the environmental loss in ductility may be reduced by alloying additions.
Thus, it is indispensable to clarify the role of alloying elements in the deformation and fracture behavior and the environmental embrittlement of the two-hhase TiAl alloys, in particular those in lamellar form for the development of two-phase TiAl alloys with wellbalanced mechanical properties. In this project, we attempt to clarify the effects of aluminum content and then alloying additions of transition and non-transition elements and finally test environment on the deformation and fracture behavior of the lamellar structure. On the basis of these results, we try to specify the role of a particular alloying element in the deformation and environmental embrittlement behavior of the lamellar structure. This leads to the establishment of alloying strategy for the development of two-phase TiAl alloys with enough strength, ductility and toughness for their structural applications.
Not only poor ductility and toughness at room temperature but also excessive fabrication cost have been hindering the structural application of TiAl alloys. Castable TiAl alloys is desirable in respect of fabrication cost. However, the lamellar structure inevitable under as-cast conditions is still poor in the ductility and toughness. We expect that results of this project will have a great significance in alloying strategy to improve the mechanical properties of the lamellar structure and thereby put a considerable impact on the development of castable TiAl alloys. Less
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