High-temperature strength and kink-band formation in mille-feuille-structured Ti/FeTi eutectic alloys

To develop ultrahigh-strength and lightweight structural materials, the temperature dependence of the deformation behavior of Ti/FeTi eutectic alloys was examined using compression tests. An extremely high yield stress of ∼2.3 GPa was obtained at room temperature accompanied by ∼5% of plastic strain by the alignment of a “flower-like” lamellar microstructure. High stress was maintained up to 400 °C; however, the yield stress considerably decreased to ∼0.8 GPa at 600 °C. Shear deformation was observed at room temperature. However, at high temperatures, at 600 °C and above, the formation of a deformation-kink band was observed for the first time as the deformation mechanism, and this resulted in a rapid decrease in yield stress. The size of the kink bands was governed by the diameter of the aligned lamellar grains, which was termed the colony size. Large kink bands were formed in the alloy containing coarse colonies, resulting in the buckling of the specimen. The introduction of primary β-Ti grains in the eutectic microstructure improved the ductility at low temperatures, which was only accompanied by the marginal decrease in yield stress to ∼1.9 GPa. Furthermore, the high-temperature strength of the hypoeutectic alloy at 600 °C showed a higher value of ∼1.0 GPa compared to that in eutectic alloys. The refinement of the colony microstructure of the hypoeutectic alloy suppressed the formation of kink bands. The experimental results suggest that microstructural control is extremely important for improving the high-temperature strength of Ti/FeTi eutectic alloys by increasing the formation stress of the kink bands.