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 β-stability ability. Ni and Al show α-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 β phase appears by the addition of more alloying elements.