The influence of coherent ticu plate-like precipitates on the thermoelastic martensitic transformation in melt-spun Ti50Ni25Cu25 shape memory alloys

Abstract Amorphous melt-spun ribbons with the composition Ti 50 Ni 25 Cu 25 were heat-treated for various times above, near or below the crystallization temperature in order to achieve three different states concerning the quantity of precipitates. In detail: homogenized material without precipitates as well as with a moderate and a high density of precipitates, respectively. The thermoelastic transformation behavior and the arising microstructures were characterized by means of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Precipitates appeared in form of thin coherent TiCu plates on {100}-planes of the parent B2-phase during isothermal heat-treatments. Especially, long-term annealing led to formation of rectangular cell structures as a result of densely precipitated plates. As to this microstructure, the peak form of the transformation curves was broadened and the transformation temperatures were lowered. A thermodynamic description is proposed elucidating the observed transformation behavior. The potential reasons, that is, internal stresses, grain size effect, decomposition, ordering/disordering, are discussed.