NiTiNb: a Stiffer and Stronger Alloy System with Many Potentials
Dr. S. Cai, Research and Development Engineer
Unique microstructures and properties of a ternary Ni46.7Ti42.8Nb10.5 alloy reported in one of our recent studies  shows great potential of this alloy system in applications that require high stiffness and large mechanical energy dissipation. As shown in Fig.1, the material consists of bundles of nano-sized (<100nm) Nb-riched phase (brighter area) embedded in an approximately NiTi-5 at.%Nb matrix (grey area). This nano-sized Nb phase increases the elastic stiffness of the bulk material by load sharing and raises the stress hysteresis by resisting reverse phase transformation during unloading. The Nb dissolved in the NiTi matrix raises the upper plateau stresses by stabilizing the austenite phase and solution strengthening. Fig.2 shows that, after the same heat treatment (e.g. 450?C), the upper plateau stress of the NiTiNb alloy is twice that of the binary NiTi alloy (e.g. ~1240MPa Vs. 600MPa). The modulus of the NiTiNb alloy is ~75 GPa, much higher than the ~48GPa of a straight annealed NiTi alloy. From Fig.2, it can be seen that the stress hysteresis (i.e. difference between the upper and lower plateau stresses) is controllable and can be easily tuned by heat treating in a large range. The stress hysteresis of the sample heat treated at 500?C is more than 100% increase over the binary NiTi alloy, indicating that this alloy could effectively replace binary NiTi in applications such as seismic energy absorbers  and lightweight armor .
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Figure 1 SEM image of the NiTiNb alloy in a longitudunal plane
Figure 2 Stress-strain curves of samples heat treated at different temperatures
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