Shape-memory effects superelasticity

Figures 20.6 and 20.7 illustrate the relation between shape memory and superelasticity. Shape memory occurs when the deformation takes place at a temperature below the Mf. Superelasticity occurs when the deformation is at a temperature above the Af. For both the memory effect and superelasticity, the alloy must be ordered, there must be a martensitic transformation, and the variant boundaries must be mobile.

Schematic illustration of the difference between shape memory and superelastic effects. For shape memory, the deformation occurs at a temperature for which the material is martensitic. A superelastic effect occurs when the deformation occurs just above the Af temperature. From J. A. Shaw, Int. J. Plasticity 16 (2000) 542. 

Ferromagnetic materials exhibiting shape memory effects and superelasticity under action of a magnetic field turn out to have a great potential for applications. This problem was discussed in a talk by A. Vasiliev (Shape memory alloys of the Ni-Mn-Ga type). 

Rubber-like behavior also constitutes a mechanical type of shape memory as does the process of SIM formation. But rubber-like behavior is characteristic of a fully martensitic structure whereas superelastic behavior is associated with formation of martensite under stress. These two types of behavior collectively fall in the category of pseudoelasticity , but one should use care in the interest of preciseness. Broadly speaking, Olander s (1932) report of rubber-like behavior in Au-Cd was the first indication of the existence of a shape-memory effect. It is ironic that even today, 60 years later, the origin of rubber-like behavior remains obscure. 

The structural transformation between austenite and martensite occurs when the mechanical stress attains a certain level, or with an appropriate temperature change, A reversible twinning process takes place at the atomic level, which can result in superelastic behaviour and shape memory [8], The properties of the nickel-titanium endodontic instruments and orthodontic wires depend critically upon the nature and proportions of the NiTi phases in their microstructures, as discussed in the following sections. While X-ray diffraction has been used to study the phases in nickel-titanium endodontic instruments [15,16] and orthodontic wires [7,17,18], this analytical technique is limited to a near-surface region less than 50 pm in depth for metallic materials [19], and study of the phase transformations with temperature is not generally convenient. In contrast, DSC can provide information about the phases present in bulk nickel-titanium endodontic instruments and orthodontic wires with facility, and the effect of temperature changes on the NiTi phase transformations is easily studied. 

Three nickel-titanium wires studied by TMDSC had been previously investigated by conventional DSC [25] superelastic Nitinol SE (3M Unitek) Neo Sentalloy, which has in vivo shape memory (GAC International, Bohemia, NY, USA) and nonsuperelastic Nitinol. The Nitinol SE and Neo Sentalloy archwires had 0.016 inch x 0.022 inch cross-sections, and the Nitinol archwires were 0.019 x 0.025 inch, also a clinically popular size. In addition, the effect of permanent deformation on transformations in Nitinol SE and Neo Sentalloy was studied after bending the archwires to 135° with orthodontic pliers. 

Fe, Al, Cr, Co, and V tend to substitute for nickel, but sharply depress Mg (Ref 14 to 16), with V and Co being the weakest suppressants and Cr the strongest. These elements are added to suppress Mg while maintaining stability and ductility. Their practical effect is to stiffen a superelastic alloy, to create a cryogenic shape memory alloy, or to increase the separation of the R-phase from martensite. 

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