The martensitic phase transformations taking place during Superelastic and Shape Memory behaviour cause characteristic changes in the microstructure. These are particularly striking if viewed dynamically, when the nature of the shear displacements taking place can often be seen very clearly. This is assisted by using viewing conditions such that the martensitic and austenitic phases are readily distinguishable.
The two videos available here show:
1) a martensitic specimen being mechanically compressed, inducing two sequential changes of the orientation of the crystal by twinning.
2) a specimen being cooled, and then heated, inducing transformation to martensite and then reversion to the austenitic phase.
In both cases, the specimens are being viewed by optical microscopy, using Nomarski differential interference contrast. The width of the viewed area is in both cases about 200 µm.
A CuAlNi single crystal (2H orthorhombic phase ) is compressed (vertical axis) at room temperature, causing activation of two sequential twinning deformations. As austenite, the crystal is cube-shaped, whereas in the martensite form it is sheared. Six different sheared martensite crystals, having well - defined prism shapes (three of which appear in this video), can be created by pressing on 3 different faces of the cube . . It is essential that the loading arrangement allows lateral displacements to occur.
A bi-crystal of austenitic CuAlNi is cooled, causing transformation to the martensitic (2H orthorhombic) phase. The process is reversed in the second half of the video, as the specimen is heated again. The rate at which transformation occurs is controlled by heat flow effects. (The shear process itself tends to take place very rapidly.) The martensitic phase is internally twinned. This is very clear within the dark-coloured phase moving in from the left-hand side in the first part of this video.
These videos are made available by the courtesy of Prof. Vaclav Novak and Prof. Petr Sittner, from the Department of Functional Materials, in the Institute of Physics of the ASCR, Prague, Czech Republic. Further technical details are available in the following publication:
V. Novak, P. Sittner, S. Ignacova, T. Cernoch, Transformation behavior of prism-shaped shape memory alloy single crystals , Mat Sci and Eng A , 438-440 (2006) p.755-762.
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