Thermal Cycling Effect on Transformation Temperatures of Different Transformation Sequences in TiNi-Based Shape Memory Alloys
oleh: Shyi-Kaan Wu, Yi-Ching Chang
| Format: | Article |
|---|---|
| Diterbitkan: | MDPI AG 2019-08-01 |
Deskripsi
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, <b><i>s</i></b>, 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 <b><i>s</i></b> values. This phenomenon also affected the suppression of B19 ↔ B19′ and R ↔ B19′ transformation peak temperatures in two-stage transformation. Both Ti<sub>50</sub>Ni<sub>48</sub>Fe<sub>2</sub> and Ti<sub>48.7</sub>Ni<sub>51.3</sub> 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 Ti<sub>3</sub>Ni<sub>4</sub> precipitates. This causesd the decrease of the R ↔ B19′ transformation peak temperature in the Ti<sub>50</sub>Ni<sub>48</sub>Fe<sub>2</sub> SMA to be much higher than that in Ti<sub>48.7</sub>Ni<sub>51.3</sub> SMAs aged at 450 °C for 4 h, which directly affected the sequential B2 ↔ R transformation of Ti<sub>50</sub>Ni<sub>48</sub>Fe<sub>2</sub> SMA in the next thermal cycle and decreased this transformation peak temperature. The Ti<sub>48</sub>Ni<sub>52</sub> 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.