Jafar Khalil-Allafia Antonin Dlouhyb GuntherEggeleraa Institut für Werkstoffe, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44 780 Bochum, Germanyb Institute of Physics of Materials, AS CR, Zizkova 22, CZ-616 62 Brno, Czech Republic
Received 22 March 2002, Revised 7 June 2002, Accepted 10 June 2002, Available online 19 September 2002.
The present work studies the microstructure of a Ni-rich NiTi shape memory alloy and its influence on the thermal characteristics of martensitic transformations. The solution annealed material state is subjected to various isothermal aging treatments at 773 K; this results in the nucleation and growth of lenticular coherent Ni4Ti3-precipitates, which were quantitatively characterized using transmission electron microscopy (TEM). Stress free aging for 36 ks results in a heterogeneous microstructure with precipitates near grain boundaries and precipitate free regions in grain interiors; this microstructure shows a three step (’multiple step’) transformation behavior in a differential scanning calorimetry (DSC) experiment on cooling from the B2 regime, which can neither be rationalized on the basis of a coherency stress argument (Bataillard et al., 1997) nor on the basis of varying Ni-concentrations between growing precipitates (Khalil-Allafi et al., 2002). A new interpretation of evolving DSC chart features is proposed which takes the evolution of microstructures during stress free and stress-assisted aging into account. Most importantly it is shown that stresses as small as 2 MPa strongly affect the precipitation process.
Ni-rich NiTi shape memory alloysAgingNi4Ti3 precipitation Martensitic transformation Transmission electron microscopy (TEM)Differential scanning calorimetry (DSC)