Solid shape memory alloys

The behavior of shape memory alloys can be explained on the basis of solid state phase changes that occur within the material. All SMAs exist in one of two phases, known as martensite and austenite, shown in the diagram on page 132. Austenite is the "parent ... 

Melting is the transition of a material from a solid to a liquid. Transitions from one phase to another also can take place within a solid. A solid can have two phases if it has two possible crystal structures. It is the ability to undergo these changes in crystalline structure that gives shape-memory alloys their properties. 

Fig. 14.27 Automated adsorptive stripping voltammetry (AdSV) of Ni2+ ions (a) calibration plot derived from the peak current of DPVs of automated AdSV using the standard addition method (b) Long-term Ni2+ ions release from surfaces of electropolished NiTi shape memory alloys kept in NaCl solution. NaCl concentrations were either 15wt.% solid line) or 0.9wt.% dotted line).78 (Reproduced by permission of Wiley-VCH Verlag GmbH Co KGaA)...

The shape-memory effect in shape-memory alloys requires that the solid contain ... 

In a shape memory alloy the atoms can exist in two different bonding arrangements, representing two different solid-state phases. (Section 11.7) The higher-temperature phase... 

Shape memory alloys (SMA) undergo solid-to-solid martensitic phase transformations, which allow them to exhibit large, recoverable strains [3]. Nickel-titanium, also known as nitinol (Ni for nickel, Ti for titanium, and nol for Naval Ordnance Lab), are high-performance shape memory alloy actuator materials exhibiting strains of up to 8% by heating the SMA above its phase transformation temperature - a temperature which can be altered by changing the composition of the alloy. 

The term shape memory (SM) refers to the ability of certain materials to annihilate a deformation and to recover a predefined or imprinted shape. Even though the shape memory behavior is also attributed to some plastic materials, in this text only shape memory alloys are considered. The SM effect is based on a solid-solid phase transition of the shape memory alloy that takes place within a specific temperature interval. 

A. Amini, W.Y. Yan, Q.P. Sun, Depth dependency of indentation hardness during solid-state phase transition of shape memory alloys. Appl. Phys. Lett. 99(2), 3603933 (2011)... 

The intermetallic Ni-Ti system has the imusual property of after being distorted, returning to its original shape when heated. This was the first of the shape memory alloys (SMAs) and was discovered by accident at the Naval Ordnance Laboratory, hence its name Nitinol. Other SMAs include Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. The shape memory mechanism depends on a martensitic solid-state phase transition that takes place at a modest temperature (50°C—150°C), depending on the alloy. The high temperature phase is referred to as austenite and the low temperature phase is called martensite (following the terminology of the Fe-FeCa system). 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

Transformations from disordered to ordered solid solutions do also occur in some further binary alloy systems, namely, Au-Cd, Au-Cr, Au-Mn, Au-Nb, Au-Pd, Au-V, and Au-Zn [1-3]. The martensitic transformations associated with the ordering in the Au-Cd and Au-Zn systems are relevant for shape memory applications and are also accompanied with considerable strengthening effects. The transformations in the other alloy systems listed above are, in part, relevant for particular functional applications, but little is known about the impact of the transformations on (mechanical) properties. 

Nitinol is a nickel-titanium alloy known as memory metal. The name nitinol is derived from the s)mnbols for nickel (Ni), titanium (Ti), and the acronym for the Naval Ordinance Laboratory (NOL), where it was discovered. If an object made out of nitinol is heated to about 500 °C for about an hour and then allowed to cool, the original shape of the object is "remembered," even if the object is deformed into a different shape. The original shape can be restored by heating the metal. Because of this property, nitinol has found many uses, especially in medicine and orthodontics (for braces). Nitinol exists in a number of different solid phases. In the so-called aus-terite phase, the metal is relatively soft and elastic. The crystal structure for the austerite phase can be described as a simple cubic lattice of Ti atoms with Ni atoms occupying cubic holes in the lattice of Ti atoms. What is the empirical formula of nitinol and what is the percent by mass of titanium in the alloy ... 

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