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The cyclic application of the shape memory effect in NiTi based shape memory alloys is associated with a degradation of the functional properties. In the present work, we show how the functional stability of NiTi based shape mem- ory alloys depends on various microstructural defects like grain boundaries, dis- locations and substitutional elements (Fe replacing Ni). As a general trend, it was observed that thermal cycling results in a decrease in phase transformation temperatures. Nano-grained microstructures provide a significantly higher func- tional stability than coarse grained microstructures due to reduced dislocation activity. Cold work impedes phase transformation processes and provides a slight improvement of the functional stability. Fe additions to NiTi are associated with the occurance of two-step transformations (cooling: B2 ⇒ R ⇒ B19', heating: B19' ⇒ R ⇒ B2) which allow partial thermal cycling (B2 ⇔ R and R ⇔ B19'). We show that B2 ⇔ R cycling has a better stability than R ⇔ B19' cycling due to a better crystallographic compatibility between B2 and R.

On the influence of crystal defects on the functional stability of NiTi based shape memory alloys