A PREMARTENSITIC ANOMALY IN NI2MNGA ALLOYS STUDIED BY DYNAMIC-MECHANICAL ANALYSIS

Two successive anomalies in the temperature dependence of internal fri ction (IF) and elastic modulus have been detected for certain off-stoi chiometric Ni2MnGa shape memory alloys both on cooling and heating bet ween the parent and martensite phases. According to the transmission e lectron microscopy (TEM) observations these anomalies are related to t wo structural transformations, namely P-->I and I-->M, where I is an i ntermediate cubic phase obtained by [1/3 1/3 0] transverse displacemen t 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 temperatu re-rate dependence of the IF peak.