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

Amorphous melt-spun ribbons with the composition Ti50Ni25Cu25 were heat-tre ated for various times above, near or below the crystallization temperature in order to achieve three different states concerning the quantify of prec ipitates. In detail: homogenized material without precipitates as well as w ith a moderate and a high density of precipitates, respectively. The thermo elastic transformation behavior and the arising microstructures were charac terized by means of differential scanning calorimetry (DSC) and transmissio n 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 mic rostructure, the peak form of the transformation curves was broadened and t he transformation temperatures were lowered. A thermodynamic description is proposed elucidating the observed transformation behavior. The potential r easons, that is, internal stresses, grain size effect, decomposition, order ing/disordering, are discussed. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.