Effects of subsequent heat treatment on the shape memory behaviors of a Ti/Ni sheet fabricated by bonding and cold rolling of Ti/Ni multilayers

TiNi shape memory alloy (SMA) thin sheets with a thickness of 120 μm were fabricated by the method of bonding and cold rolling and subsequent heat treatment of elemental Ti and Ni foils. The annealing temperature and time have been investigated with regard to their effects on the phase composites and microstructures of the as-roll-bonded Ti/Ni multilayers. It is found that the TiNi phase which shows shape memory effect is easily to be synthesized by the subsequent heat treatment even though it was preformed by annealing at a relatively low temperature of 650 °C and for a short time of 6 h. The shape memory effect has been examined by DSC for the fabricated TiNi thin sheets and it reveals a good martensitic and reverse transformation behavior from B2 to B19′ of M_s at around 60 °C and transition hysteresis of about 30 °C. A tensile test at the temperature above A_f revealed that recovery strains of average 1.5% in superelasticity has been obtained for the TiNi thin sheets.