SOME ASPECTS OF STRUCTURAL BEHAVIOR 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 similar to 446 K has be en studied by means of dynamic mechanical analysis (DMA), calorimetry (DSC). dilatometry three point bending tests and TEM. Two maxima in in ternal friction (IF) spectra on cooling and a single maximum on heatin g. 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 m icrostructure during in situ cooling and heating experiments performed in the TEM have revealed a two-stage martensitic transformation on co oling (namely, from the cubic L2(1) ordered parent phase (P) to a mart ensitic phase with a ten-layered modulated lattice (10M) and from the latter to the monoclinic distorted martensite with seven-laver type of modulation (7M)). On heating, only one-stage transformation from 7M m artensite to parent phase has been observed. The intermartensitic tran sformation 10M-->7M and the martensitic P <----> 10M show the characte ristics of first order transformations, such as transformation heat an d transient contribution to the corresponding IF peaks.