5 Werkstofftechnik
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- Martensitic transformation (2) (entfernen)
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The crystal structure of martensite in Cu-27at.%Zn-9.0 at.%Al alloy has been studied by using sphericalaberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and geometrical phase analysis (GPA) to examine possible changes in atomic rearrangements during martensitic transformation of this ternary system. Observation along [100]M zone axis is suitable for examining a chemical order of the martensite, and showed that, despite the non-stoichiometry of the alloy, atomic columns containing
Al atoms are imaged and distinguished from the others. On the other hand, observation along [010]M zone axis directly revealed that the parent and martensitic phases possess L21 and 18R (21) structures, respectively. These observations suggested that the martensite retained the local chemical order of the parent phase without shuffling before and after the transformation. GPA revealed that the interface between the two phases was coherent with tilting of the basal plane approximately 6◦ across the boundary, which makes otherwise large inclination small during the martensitic transformation.
Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19' martensite was always present in the extended range of temperatures and stresses investigated here.