Transformation and ageing behaviour of melt-spun Ni-Mn-Ga shape memory alloys

The effects of rapid quenching on the transformation behaviour of three mel t-spun ribbons of Ni-Mn-Ga alloys has been studied by X-ray and electron di ffraction, calorimetry, thermomechanical and magnetisation tests. One step martensitic transformation (MT) has been detected for each ribbon accompani ed by the formation of modulated martensitic crystal structures with 7- or 10-layer periods, which differ from those observed for single crystals of t he same compositions. The MT temperatures can be raised or lowered by agein g in the parent phase, depending on composition. The ageing also causes a s ignificant increase of both Curie temperature and magnetization in all allo ys. These effects indicate the occurrence of some atomic rearrangement proc esses leading to a more uniform element distribution and, probably, change of atomic order. The activation energies for these processes have been esti mated to be between 0.2 and 0.7 eV. (C) 1999 Elsevier Science S.A. All righ ts reserved.