Firat University Abstract Shape memory effect is a peculiar property exhibited by certain alloy system. This behavior is facilitated by martensitic transformation, and shape memory properties are intimately related to the microstructures of alloys; in particular, the morphology and orientation relationship between the various martensite variants.
Martensitic transformation occurs in thermal manner, on cooling the materials from high temperature parent phase region. Thermal induced martensite called self-accommodated martensite or multivariant martensite occurs as multivariant martensite in self-accommodating manner and consists of lattice twins.
Shape memory alloys are deformed in low temperature martensitic phase condition, and deformation proceeds through a martensite variant reorientation. Shape memory effect, martensitic transformation, self-accommodation, twinning, layered structures. Shape memory effect is a peculiar property exhibited by certain alloy system.
Thermal induced martensite called self-accommodated martensite or multivariant martensite occurs as multivariant martensite in self- accommodating manner and consists of lattice twins.
Introduction Martensitic transformations are first order lattice-distorting phase transformations and occur as twinned martensite with the cooperative movement of atoms by means of lattice invariant shears in the materials on cooling from high temperature parent austenite phase region.
The twinned martensites turn into reoriented martensite by the deformation in martensitic condition. Shape memory effect is based on martensitic transformation, and microstructural mechanisms are responsible for shape memory behavior. In particular, the detwinning is essential as well as martensitic transformation in reversible shape memory effect [1, 2].
Shape memory alloys can be deformed plastically in low temperature martensitic condition, and recover the original shape on heating over the austenite finish temperature. The material cycles between the deformed and original shapes on cooling and heating.
By applying external stress, the martensitic variants are forced to reorient into a single variant leading inelastic strains, and deformation of shape memory alloys in martensitic state proceeds through a martensite variant reorientation or detwinning of twins. The twinning occurs with internal stresses, while detwinning occurs with the external stresses.
The basic mechanism of shape memory effect is schematically illustrated in Figure 1 [1, 2]. The deformed material recovers the original shape on heating over the austenite finish temperature, and returns to the multivariant martensite structures in irreversible shape memory effect on cooling below the martensite finish temperature; in contrast, the material returns to the detwinned martensite structure in reversible shape memory case.
The detwinning process has great importance for the occurrence of reversible shape memory effect. Schematic illustration of the mechanism of the shape-memory effect: One is Bain distortion , and second one is lattice invariant shear which is effective in the formation of self-accommodated martensite structures.
This process is schematically illustrated in Figure 2. The basic characteristic of shape memory alloys is the occurrence of different variants in the low temperature martensitic phase. When a shape memory alloy undergoes martensitic transformation, it transforms from the parent phase to one or more of the different variants of the martensitic phase [4, 5].