A premartensitic anomaly in Ni2MnGa alloys studied by dynamic mechanical analysis

Two successive anomalies in the temperature dependence of internal friction (IF) and elastic modulus have been detected for certain off-stoichiometric Ni2MnGa shape memory alloys both on cooling and heating between the parent and martensite phases. According to the transmission electron microscopy (TEM) observations these anomalies are related to two structural transformations, namely P-->I and I-->M, where I is an intermediate cubic phase obtained by [1/3 1/3 0] transverse displacement modulations of the parent P phase. As for the LF results, the 1 to M transformation behaves as a first order phase transition, whereas the P-->I transition shows some distinctive features such as no temperature-rate dependence of the IF peak.