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V. Prieb, M. Feller-Kniepmeier, T. Link TU-Berlin, Str. des 17. Juni 135, D-10623 Berlin, Germany N. A. Polyakova and V. A. Udovenko Institute of Metal Science and Metal Physic, Moscow, Russia (Proceeding of the 1st US-Russian workshop "New Physical and Mathematical Approaches to Computer Aided Design of Shape Memory Alloys". - St. Petersburg, 1995, 145-150) Abstract: Stress-induced martensitic transformations with a single-interface and with multiple-interfaces are investigated on single-crystals of the non-stoichiometric Cu-16.0 at% Zn-15.2 at% Al shape memory alloy. Two different kinetics are caused by the austenite start-structure that was investigated by transmission electron microscopy after different heat treatments. Thermodynamic parameters of the stress-free thermal-induced transformation of the same heat treated single-crystal alloy were also determined by calorimetry. The heterogeneous stress-induced martensitic transformation is energetically more favourable from the homogeneous austenite and the homogeneous stress-induced transformation, from a heterogeneous austenite structure which is interpreted as a (B2+DO3)-mixture. This situation is likely the homogeneous (Luder's line) and heterogeneous (multiple slipping) plastic deformation. The single-interface velocity, the martensitic strain inside of the martensitic single-domain, and the increase/decrease of the sample temperature due to adiabatic single interface propagation are measured. Introduction: It is well known that the stress-induced thermoelastic martensitic transformation can develop on two kinds of kinetic: - the nucleation and growth of separate martensitic lamellas by the continuously increasing external applied stress; - the nucleation of a single martensitic band after the reaching of a certain critical external stress and its following growth by the interface propagation through the whole specimen, known as the transformation with a single interface. The stress-strain diagrams are different correspondingly. In the first case the stress-strain diagram is smooth with, in general, the stress-controlled increasing/decreasing martensitic phase-fraction and the martensitic strain. In the second case the martensitic strain is no controlled more by the external stress. This leads to the stress-drop on the stress-strain diagrams. This peculiarities were observed on samples as well of the same alloys as also of different shape memory alloys. Nevertheless, the general origin of these kinetical differences still is not settled. The aim of the presented work is to investigate and to discuss the structural mechanism of the stress-induced martensitic transformation with the single- and multiple-interfaces on an example of their studying on the same Cu-Zn-Al single-crystals after different heat treatments. Conclusion: Two kinds of the stress-induced transformation kinetics on the same single-crystals of a Cu-Zn-Al shape memory alloy are determined by the start austenite structure depending on the heat treatment. The equilibrium structure of this alloy with an usual intermediate composition between two stoichiometries is a (B2+DO3) -mixture of two austenitic phases with the stoichiometric composition. Crystallographic identical martensitic transformation of the homogeneous and heterogeneous austenite have different kinetics: - the nucleation of multiple martensite crystals and the following increasing of their density during the deformation - the heterogeneous stress-induced martensitic transformation - is more favourable for the homogeneous austenite structure, and, quite the contrary, - the nucleation of the single martensitic band and its growth by the single-interface propagation - the homogeneous stress-induced martensitic transformation - is more favourable for the heterogeneous austenite. This kinetics different is close similar to the heterogeneous plastic deformation by the multiple slipping and the homogeneous plastic deformation by the nucleation and propagation of the single or more Luder's lines and has the same energetic reason. More: Article (full script, english) as PDF-file Home |