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V. Prieb, TU Berlin ("ACTUATOR 94": Proc. of the 4th Int. Conf. on New Actuators, Germany, Bremen, 1994, p. 365) Abstract: The stress-induced martensitic transformation with a single interface in a single crystal of the Cu-Zn-Al alloy is investigated. The transformation with entry two once more as for the usual betta-18R transformation starts as a martensitic band nucleation after the critical barrier surmounting. The thermoelastic equilibrium after the band nucleation is not reached. Therefore the subsequent martensitic deformation accompanies with a continuous stress-drop at a constant temperature (pseudoelasticity) or with a temperature increesing at a constant load (shape memory). The transformation has a very narrow hysteresis about 1 K. Introduction: The martensitic transformation with a single interface is a case investigated by that no elastic energy should be saved and no transformation thermoelasticity is expected. The last means that the martensite and therefore martensitic deformation increases with the temperature decreasing or stress increasing. But there is no clear definition what is a transformation with a single interface as an investigation object: microscopical growth of one of the more isolated, not interacted martensitic plate that is to macroscopical behaviour of the specimen integrated or real the nucleation and growth of the single martensit crystal that defines one to one the macroscopical characteristics martensit fraction, marten?sitic deformation etc. The results of microscopical observations show either no dependence of the interface position change on the temperature or a light slope of that, whereas macroscopical properties are not investigated. More investigations of the pseudoelastic behaviour correspon?ding to the transformations with a single interface shown the unloading of samples during the martensitic deformation after the critical stress. It has been also shown theoretical from the analysis of a nucleation kinetics and a kinetical relation for the quasi-static stress induced martensitic transformation with a single interface that the load drops after nucleation at a defined rates ratio of the loading and the interface motion. On the other hand the thermodynamical treatment of martensitic multi-interface transformations and pseudoelasticity give a stress-strain line with a negative slope and a strain-temperature line with a positive slope that corresponds to an instable two-phase equilibrium. Experimentally this line is found in partial cycles, whereas the martensitic deformation depends not on the load. There is not a talk of thermoelasticity in this theory, because the elastic coherent interface energy is relevant to the hysteresis, and it follows that one is narrow for the transformation with a single interface. In this paper are investigated the thermodynamical and kinetical characteristics of the stress-induced martensitic transformation, its hyste?resis and corresponded shape-memory and pseudoelastic behaviour in the alloy, which is also investigated in the case of a multi-interface transformation. Investigation of these transformation corresponded properties could be very interesting for the practical purposes because close temperature and stress intervalls are required for the certain applications of the memory elements. Conclusion: The macroscopical behaviour corresponded to the single-interface transformation and its kinetics have been investigated with the following results: 1. The stress induced single-interface transformation needs for its nucleation a higher shear stress and the formation of 18R phase twins. 2. The stress-temperature dependence of both are distingished in a slope about two times as it is found in other cases of single-interface transformations. 3. The transformation has no thermoelastic character and is determined as a local transformation. More: Article (full script, english) as PDF-file Home |