Thermal Cycling Effect on Transformation Temperatures of Different Transformation Sequences in TiNi-Based Shape...

In TiNi-based shape memory alloys (SMAs), the effects of thermal cycling on the transformation peak temperatures of B2 ↔ B19', B2 ↔ R, B2 ↔ B19, B2 ↔ R ↔ B19', and B2 ↔ B19 ↔ B19' one-stage and two-stage transformations have been investigated and compared. Experimental results of the differential scanning calorimeter and hardness tests indicate that the alloy's intrinsic hardness and the shear strain, , associated with martensitic transformation, are two important factors, due to their relation to the ease of introducing dislocations during cycling. The temperature decrease by cycling for one-stage transformation was in the order of B2 ↔ B19' > B2 ↔ B19 > B2 ↔ R according to the orders of magnitude of their values. This phenomenon also affected the suppression of B19 ↔ B19' and R ↔ B19' transformation peak temperatures in two-stage transformation. Both Ti₅₀Ni₄₈Fe₂ and Ti_48.7Ni_51.3 SMAs aged at 450 °C for 4 h exhibited B2 ↔ R ↔ B19' transformation, but the hardness of the latter was much higher than that of the former due to the precipitation hardening of the Ti₃Ni₄ precipitates. This causesd the decrease of the R ↔ B19' transformation peak temperature in the Ti₅₀Ni₄₈Fe₂ SMA to be much higher than that in Ti_48.7Ni_51.3 SMAs aged at 450 °C for 4 h, which directly affected the sequential B2 ↔ R transformation of Ti₅₀Ni₄₈Fe₂ SMA in the next thermal cycle and decreased this transformation peak temperature. The Ti₄₈Ni₅₂ SMA aged at 600 °C for 150 h underwent B2 ↔ B19' transformation and then B2 → R → B19'/B19' → B2 transformation as the cycle number increased, in which the B2 ↔ R transformation peak temperature raised slightly by cycling. This characteristic is uncommon and may have resulted from the strain field around the thermal-cycled dislocations favoring the formation of the R-phase.