Nanoscale Aspects of Phase Transformations in Shape Memory Alloys | Abstract
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European Journal of Applied Engineering and Scientific Research


Nanoscale Aspects of Phase Transformations in Shape Memory Alloys

Author(s): Osman Adiguzel

Shape memory effect is a peculiar property exhibited a series alloy systems in the -phase fields. Shape memory alloys are sensitive to external condition and temperature, and crystal structure of these alloys change with changing temperature and stressing, by means of structural phase transformation, called martensitic transformations. Shape memory effect is treated thermally by means of thermal induced martensitic transforma­tion, which occurs as martensite variants with lattice twinning in crystallographic or atomic scale, sub-nano scale, in materials on cooling below martensite finish temperature. Twinned mar­tensite structures turn into detwinned martensite structure by means of stress induced martensitic transformation by stressing material in a strain limit in martensitic condition. Shape mem­ory alloys are in the fully martensitic state below martensite finish temperature with fully twinned structure can be easily deformed through variant reorientation/detwinning process. Therefore, martensite is called soft phase and austenite is also called hard phase. Thermal induced martensitic transforma­tion is lattice-distorting phase transformation and occurs as martensite variants with the cooperative movement of atoms by means of shear-like mechanism. Martensitic transforma­tions occur by two or more lattice invariant shears on {110}- type planes of austenite matrix which is basal plane or stacking plane for martensite. In the martensitic transformation, the lattice of high temperature austenite phase has greater crystal­lographic symmetry than that of the low-temperature product phase. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures at high tempera­ture parent phase field. Lattice invariant shears are not uniform in copper based shape memory alloys, and the ordered parent phase structures martensitically undergo the non-conventional complex layered structures on cooling. The long-period layered structures can be described by different unit cells as 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 direction z, in case of 18R mar­tensite, and unit cells are not periodic in short range in direc­tion z. In the present contribution, x-ray diffraction and trans­mission electron microscope studies were carried out on two copper based CuZnAl and CuAlMn alloys. These alloy samples have been heat treated for homogenization in the -phase fields. X-ray diffraction profiles and electron diffraction patterns re­veal that both alloys exhibit super lattice reflections inherited from parent phase due to the displacive character of marten­sitic 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. In particular, some of the successive peak pairs providing a special relation between Miller indices come close each other, and this result leads to the rearrangement of atoms in diffusive manner.