The martensitic transformation in Co37Ni34Al29 ribbon is characterized in detail by means of in-situ thermostatic x-ray diffraction and magnetic measurements.The results show a structural transition from the body-centred cubic to martensite with a tetragonal structure during cooling.Comparison between the results of the diffraction intensity with the magnetic susceptibility measurements indicates that the martensitic transformation takes place in several different steps during cooling from 273 to 163 K.During heating from 313 to 873 K,the peak width becomes very wide and the intensity turns very low.The γ-phase(face-centred cubic structure) emerges and increases gradually with temperature increasing from 873 to 1073 K.
The phase transition characteristics of the prepared Ni46Mn35Ga19 single crystal have been systematically investigated by measuring resistance,strain,heat flow,and ac magnetic susceptibility.The experimental results indicate that both magnetic transition and martensitic transformation occurred simultaneously and the corresponding transition temperature was about 356 K.Meanwhile,the prepared single crystal also exhibited spontaneous and magnetic-field-enhanced two-way thermal shape memory effect with the strains up to-0.89% and-1.90% with and without a magnetic field of 1.6 T,respectively.The possible mechanism of the large spontaneous and magnetic enhanced deformation were analyzed based on the phase transition characteristics of Ni46Mn35Ga19 single crystal and the preferential orientation of martensitic variants.
