Some Aspects of Structural Behaviour of Ni-Mn-Ga Alloys

The temperature induced transformation of single crystalline Ni - 31.1% Mn - 17.7% Ga (at%) shape memory alloy with Ms ~ 446 K has been studied by means of dynamic mechanical analysis (DMA), calorimetry (DSC), dilatometry, three point bending tests and TEM. Two maxima in internal friction (IF) spectra on cooling and a single maximum on heating, concurrently with kink-like anomalous changes of elastic modulus (E) have been found. Corresponding changes have been observed using the other techniques. The evolution of electron diffraction patterns and microstructure during in situ cooling and heating experiments performed in the TEM have revealed a two-stage martensitic transformation on cooling (namely, from the cubic L2, ordered parent phase (P) to a martensitic phase with a ten-layered modulated lattice (10M) and from the latter to the monoclinic distorted martensite with seven-layer type of modulation (7M) ). On heating, only one-stage transformation from 7M martensite to parent phase has been observed. The intermartensitic transformation 10M→7M and the martensitic P←→10M show the characteristics of first order transformations, such as transformation heat and transient contribution to the corresponding IF peaks