Phase stability and hardness of some ternary Ti-Zr based shape memory alloys

The phase stability and hardness of Ti-30Zr-M (M=Nb, V, Cr, Mo, Fe, Ni and Al) shape memory alloys were investigated by structural observations, d-electron alloy design and microhardness tests. Optical microscopy and X-ray diffraction results show that Nb, V, Cr, Mo and Fe are all β-stability elements in Ti-30Zr-M alloys with the effect enhanced by the presence of Zr. The composition of the least stable β-phase alloy values of Nb, V, Cr, Mo and Fe in Ti-30Zr alloys are 12%, 9%, 5%, 3% and 2%, respectively, indicating an increasing sequence of the α-stability ability. Ni and Al show β-stability or moderate effects and the addition of Ni induces the appearance of (Ti, Zr)₂Ni phase in Ti-30Zr-5/15Ni alloys. Based on the d-electron alloy design method, the bond order of Ti and alloying atoms (Bo) and the metal d-orbital energy level (Md) are calculated to build the - diagram of Ti-30Zr-M alloys associated with the phase structure identification. The microhardness of Ti-30Zr alloy is increased by addition of a small amount of the alloying element, except for Nb. There is a drop of the microhardness of each Ti-30Zr-M alloys when phase appears by the addition of more alloying elements.