Title: Mechanism of thermal and mechanical memory in shape memory alloys

Biography:

Abstract

Some materials take place in class of smart materials with adaptive properties and   stimulus response to the external changes. Shape memory alloys take place in this group, due to the shape reversibility and capacity of responding to changes in the environment. These alloys exhibit a peculiar property called shape memory effect, which is characterized by the recoverability of two certain shapes of material at different temperatures. Shape memory effect is initiated by cooling and stressing material and performed thermally on heating and cooling. This behavior is called thermal memory or thermoelasticity, and governed by thermal and stress induced martensitic transformations. Thermal induced transformation occurs on cooling along with lattice twinning, with which ordered parent phase structures turn into twinned martensite structure; and twinned structures turn into detwinned martensite structure by means of stress induced martensitic transformation by stressing material in martensitic condition. Shape memory effect is performed thermally following these treatments and cycles between original and deformed shapes, on heating and cooling, while crystal structures cycle between parent phase and detwinned structures.  It is well known that twinning and detwinning reactions play a considerable role in memory behavior of the shape memory materials. Thermal induced martensitic transformation is lattice-distorting phase transformation and occurs as martensite variants with the cooperative movement of atoms in <110>- type directions on {110}-type planes of austenite matrix which is basal plane or stacking plane for martensite, by means of shear-like mechanism.  In these alloys, parent austenite phase structures have greater crystallographic symmetry than that of the low-temperature product phase.

These alloys exhibit another property called superelasticity, which is operated by mechanically stressing and releasing the material at a constant temperature in parent phase region. Material is deformed at a constant temperature in parent phase region and recovers original shape upon releasing, like standard elastic materials. This behavior is called mechanical memory or superelasticity. Superelasticity is the result of stress-induced martensitic transformation, with which ordered parent phase structures turn into the detwinned structure, and performed in non-linear way. Loading and unloading paths are different, and cycling loop reveals energy dissipation.   

Copper based alloys exhibit this property in metastable β-phase region. Lattice invariant shears are not uniform in these alloys, and the ordered parent phase structures martensitically undergo the complex layered structures on cooling. These structures can be described by different unit cells as 9R or 18R depending on the stacking sequences on the close-packed planes of the ordered lattice.

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 and electron diffraction patterns of these alloys exhibit super lattice reflections inherited from parent phase due to the displacive character of transformation.   X-ray diffractograms taken in a long time interval show that diffraction angles and intensities of diffraction peaks change with the aging time at room temperature. This result reveals a new transformation in diffusive manner.