Effect of Nb addition on the plastic behavior of TiAI crystals containing oriented lamellae

TiAl has been focussed on as a potential high temperature material because of its high strength-to-weight and good oxidation resistance. One of the major problems to be solved before practical application is the improvement of ductility of low temperatures. Ti-rich TiAl containing a well-developed lamellar structure composed of α2 and γ phases shows higher strength and better ductility than the single phase Al-rich TiAl. The mechanical properties of TiAl alloys are well known to be strongly affected by the details of the lamellar structure. In this paper, the authors succeed in obtaining binary TiAl crystal s (48.1 ∼ 51.6at%Al) containing a single set of lamellae with different lamellar spacings by altering the crystal growth rate and the alloy composition. The mechanical properties are closely related to their lamellar structures and the yield stress is represented as a function of the reciprocal square root of the lamellar spacing similar to a Hall-Petch type relation. Furthermore, the authors find that the deformation mode of the α2 phase in the lamellae strongly affects the plastic behavior of lamellar TiAl crystals. When the compressive axis is parallel to the lamellar planes (φ=0 degrees), shear deformation occurs across them and 10 bar 10 <1 bar 210 < slip can be activated in the α2 phase. Addition of niobium to Ti3Al is known to activate the motion of <11Ba26> type dislocations of 22 bar 21 and to improve the ductility. Therefore, in this paper we report the mechanical properties and microstructure of ternary TiAl crystal containing 1.2at%Nb with a single set of lamellae, focussing on the effect of the deformation model t-of the α2 phase