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

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