The Effect of the Annealing Parameters on the Structure and Characteristic Temperatures of the Nonstoichiometric Ni₂MnGa Alloys

Transmission electron microscopy studies of the Ni2MnGa alloy showed that in the diffraction patterns there exist two additional effect apart from the spots given by the parent L21 Heusler phase and the tetragonal no modulate martensite. The first effect was the presence of sharp extra spots, lying on a direction significantly deviated from the <110>* for [001]β zone axis. These spots are connected with modulation of the parent phase. The value of the deviation angle is about 11-12°. The detailed analysis showed that the direction of the propagation vector of modulation is <641> type direction. The second effect visible in the [001]β diffraction patterns is the presence of the arcs located between the 220 type parent phase spots.. The analysis of this objects suggests that some chemical modulation exists with the period of <550>β. In this case short range ordering plays the essential role.