J. Frenzel, C. Somsen, M. F. -. X. Wagner, G. Eggeler, Ruhr University Bochum, Bochum, Germany; E. P. George, University of Tennessee/Oak Ridge National Laboratory, Knoxville, TN; A. Dlouhy, Institute of Physics of Materials, Brno, Czech Republic
In the present work, we provide a new set of data on phase transformation temperatures, hysteresis widths and transformation heats in NiTi. It is well known that the phase transformation temperatures in binary near equiatomic NiTi shape memory alloys decrease with increasing Ni-content. However, a large experimental scatter was observed in previous studies and even today, a high precision reference data set on the relation between alloy composition and transformation temperatures is not available. NiTi ingots with different Ni-levels were produced using an optimized melting procedure and analyzed by differential scanning calorimetry. We show why it is important to document details of the melting procedure and how precise Ni-concentrations of the NiTi matrix can be obtained. We provide clear experimental evidence showing that the predictions of Tang et al. (Acta Mat. 1999) on the increasing deviations from a linear relation between the thermodynamic equilibrium temperature and the Ni-content are correct. We also provide easy to handle numerical expressions for the effect of Ni on MS, MF, AS, AF and T0. We show that increasing Ni-contents not only affect phase transition temperatures but moreover result in decreasing widths of thermal hysteresis and in decreasing heats of transformation and rationalize these findings on the basis of crystallographic data by Prokoshkin et al. (Acta Mat. 2004) in the light of the theory of Ball and James (Phil. Trans. Royal Soc. A 1992).
Summary: In the present work, we provide a new set of reference data on phase transformation temperatures, hysteresis widths and transformation heats in NiTi. Ingots with different Ni-levels were produced using an optimized melting procedure and analyzed by differential scanning calorimetry. We show that increasing Ni-contents not only affect phase transition temperatures but moreover result in decreasing widths of thermal hysteresis and in decreasing heats of transformation and rationalize these findings on the basis of crystallographic data by Prokoshkin et al. (Acta Mat. 2004) in the light of the theory of Ball and James (Phil. Trans. Royal Soc. A 1992).
