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Lecture

Crystallographic Processes Governing Memory Behaviour of Shape Memory Alloys

Wednesday (28.09.2016)
12:15 - 12:30
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Shape memory effect is facilitated by crystallographic phase transformations, and shape memory properties are intimately related to the crystal structures of the material. Shape memory effect proceeds by successive martensitic transformations, thermal and strain induced transformations. Thermal induced martensitic transformation in materials occurs on cooling and proceeds by a shear like mechanism in different orientations. Twinned martensite structures occur with thermal induced transformation by lattice twinning. Strain induced transformation is a martensite-martensite transformation and occurs by the deformation in low temperature martensitic condition and proceeds by twin reorientation in crystallographic level.

Martensitic transformation occurs as twinned martensite in self-accommodation manner with lattice invariant shears, which occur in opposite directions, <110 > -type directions on the {110}-type planes of austenite matrix. Twinned martensitic structures turn into detwinned martensitic structures with deformation in low temperature martensitic condition.

Copper based alloys exhibit this property in ?- phase field, which possess simple bcc- structures at high temperature austenite phase. Lattice invariant shears are not uniform in copper based ternary alloys and give rise to the formation of unusual layered structures, like 3R, 9R or 18R depending on the stacking sequences on the close-packed planes of the ordered lattice. The unit cell and periodicity is completed through 18 layers in z-direction.

In the present contribution, x-ray diffraction and transmission electron microscopy studies were carried out on two copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles take in a long term intervals reveal that diffraction peaks and scattering angles change with ageing duration. In particular, some peak pairs which have Miller indices providing a special relation come close each other. This result leads to the rearrangement of atoms in diffusive manner.

Keywords: Martensitic transformation, shape memory effect, lattice invariant shear, twinning and detwinning.

 

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