Internal friction behaviour of Ni–Mn–Ga

Abstract

The internal friction (IF) behaviour of shape memory alloys (SMA) is characterised by an IF peak and a minimum of the elastic modulus during the martensitic transformation (MT), and a higher IF value in the martensitic state than in parent phase. The IF peak is considered to be built of three contributions, the most important of them being the so-called “transient” one, existing only at non-zero temperature rate. On the other hand, the ferromagnetic Ni–Mn–Ga system alloys undergoes a MT from the L21 ordered parent phase to martensite, the characteristics of the transformation depending largely on the e/a ratio of the alloys. Indeed, a variety of transformation sequences, including intermediate phases between parent and martensite and intermartensitic transformations, have been observed for a wide set of studied alloys. Furthermore, the IF and modulus behaviour during cooling and heating these alloys show specific characteristics, such as modulus anomalies, strong temperature dependence of the elastic modulus, temperature dependent internal friction in martensite, and, as a general trend, a low transient contribution to the IF. In the present work, the IF and modulus behaviour of several Ni–Mn–Ga alloys will be reviewed and compared to that observed for “classical” systems like Cu- or NiTi-based shape memory alloys.