Martensitic transformation and pseudoelasticity of aged Ti50.1Ni49.7Si0.2 shape memory ribbon

The effect of aging on transformation sequence, microstructure evolution, hardness change and pseudoelasticity (PE) of Ti50.1Ni49.7Si0.2 ribbon is investigated. Ti50.1Ni49.7Si0.2 ribbon aged at 500 °C for ≧1 h undergoes a two-stage B2→R→B19′ transformation during cooling. The microstructure and hardness of the aged ribbon are closely related to its composition. The matrix of the aged ribbon becomes Ni-rich because of its high Si content and low Ti/Ni ratio. A very high density Ti3Ni4 precipitate is observed in Ti50.1Ni49.7Si0.2 ribbons aged at 500 °C for ≧1 h, which gives the aged ribbons high hardness. Nanoindentation tests indicate that Ti50.1Ni49.7Si0.2 ribbon aged at 500 °C for 15 min has a maximal hardness of 5.5 GPa and that aged for 336 h decreases to 4.0 GPa. The PE recovery of the ribbon is improved from 55% to 75% under a 100 mN loading after aging at 500 °C for 15 min. The experimental results show that adjusting the composition of TiNi ribbon can effectively modify the microstructure, increase the hardness and the ribbon still retains excellent PE recovery.