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of NiTi-X-Shape Memory Alloys V.Prieb, V.Wolff "MFS-Büro Dr. Prieb", Berlin, DE "Materials Sciences Transactions", 6(2000)12, p.p. 12-15 Abstract: Polycrystalls of NiTi-based shape memory alloys with a third component X (X = Au, Pt, Pd or Al) were examined with calorimetric measurements to investigate the thermodynamic parameters and characteristics of the hysteresis of the thermoinduced martensitic transformation. All known sequences of transformations of NiTi-based shape memory alloys were observed. The enthalpy and entropy of the martensitic transformations B2-R, R-B19' and B2-B19 were measured and calculated. The smallest dissipative characteristics are found for the transformations B2-B19 with invariant plane. The SMART effect in partial thermocycles was investigated and discussed also. Introduction: The parameters of the transformation hysteresis in shape memory alloys are connected to the dissipation of the elastic energy. The elastic transformation energy arises from incompatibility of lattices at the phase and domain boundaries. This causes stress and rotation moments in the habitus plane, which is relieved by the building of an internal domain structure. This makes the elastic energy of the martensit increase and displace the transformation temperatures out of equilibrium temperature, but minimises the total energy of the two phase system. The energetic state in the habitus plane of martensit changes in TiNi-alloys, when some Ni-atoms are substituted by precious metals Au, Pd and Pt. If the concentration of this elements is about 10at% the B2-B19 transformation takes places with an crystallographic invariant plane. It is the aim of this work to show the changes in the sequence of transformations and to investigate the development of thermodynamic transformation parameters and characteristics of hysteresis during the transition to B2-B19-transformation with invariant plane while the composition in shape memory alloys on NiTi-basis is changed. Conclusion: The experimental data show, that all possible transformations in TiNi-based alloys have very different transformation characteristics and hysteresis parameters. The B2(R)-B19'-transformation of the stoichiometric NiTi-alloy has the highest values of enthalpy and entropy and a wide hysteresis, what causes the highest energy dissipation. The R-B19'-transformation shows the widest hysteresis, although the transformation enthalpy and entropy are much lower than the values for B2(R)-B19'- and more like these values of the B2-B19- and the B2-R-transformations. The transformation B2-R as well as the transformation B2-B19 with invariant plane are accompanied by very small dissipation. Such dissipative characteristics and similar values of entropy have been measured in our investigations on Cu-based single crystals and the stoechiometric NiTi single crystal. One can conclude from this results, that the SMART effect is correlated with the dissipation of the elastic energy and has the same mechanism. The calorimetry is not a method to phase analyse directly, but it is more sensitive than X-ray measurements, and if the values of entropy are known, it can be used for the analysis of the transformation completeness. More: Article (full script, english) as PDF-file Home |