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

Abstract

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) 2 Ni 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 Bo-Md 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.

Effects of 3d Transition Metal Substitutions on the Phase Stability and Mechanical Properties of Ti–5.5Al–11.8[Mo]eq Alloys

“…Meanwhile, the Vickers hardness for the other alloys is close to each other ( Figure 4 f). Apart from the effect of the precipitates, the hardness of metastable β-Ti alloys is discussed by the solid-solution strengthening effect and also by the stability of the β phase [ 30 , 31 ], respectively, as described below. First, the alloys exhibiting a single β phase, namely, the (b) Ti–Al–Cr, (c) Ti–Al–Co, (d) Ti–Al–Ni, and (e) Ti–Al–Mo alloys, have almost identical phase stabilities because of equal Mo equivalents.…”