Very different materials exhibit martensitic phase transitions, for example famous shape memory alloys. But even in materials that are more known for their exotic physics, martensitic phase transitions occur precisely at the transition to this low-temperature phase, which is interesting from an electronic point of view, for example in electronically correlated materials, materials that develop charge density waves or exhibit other metal-insulator transitions. Is this a coincidence or does it rather mean that shape memory alloys also belong to this larger material class of electronically driven metal-insulator transitions?

Indeed, for the prototype system NiTi in particular, there is much experimental evidence in favor of a charge density wave type metal-insulator transition: Transport anomalies, nesting of the Fermi surface, phonon softening and also (giant) Kohn anomalies. In particular, the giant Kohn anomalies, which have been known in NiTi for decades, are interpreted in the charge density wave materials community as experimental evidence for charge density wave physics. What does this mean for the development of applied alloys? First of all, electronic aspects are important. It is clear in NiTi, for example, that the electron number of the substituted compound is important for the transition temperature, but normally only a general value from the periodic table of elements is estimated. For the charge carrier density (Hall effect) and the average heat per charge carrier (Seebeck effect), there is little measured data. From a scientific point of view, much more precise findings could be obtained here with relatively easily accessible experiments. In this talk I will discuss the corresponding transport data of a series of NiTi samples (with different nickel content) and Cu-substituted samples and relate them to thermodynamic data of the same material systems. Here it can be clearly seen that the martensitic phase transition in NiTi (+X) is accompanied by a reduction of the charge carrier density at the phase transition, as is typical for metal-insulator transitions. From these data, values for the electronic entropy and enthalpy at the phase transition can be estimated, from which the relevance of the electronic driving forces becomes apparent. However – and this is where the discussion can only begin – it remains completely unclear from these macroscopically averaged transport data how microstructural aspects and electronic aspects of this phase transition interact.


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