Cu-Al-Ni shape memory alloys

The Cu-Al-Ni shape memory alloys are based on the intermetallic phase CujAl with a disordered A2 structure, which is usually known as the P phase, and which is stable only at high temperatures between 567 and 1049 °C with compositions between 71 and 82 at.% Cu (Murray, 1985). At 567 °C this phase decomposes by a eu-tectoid reaction at such a sluggish rate that it can be retained with the then metastable A2 structure by cooling below this temperature. At about 500 °C the P phase undergoes an ordering reaction to form the metastable Pi phase with a DO3 structure. Both phases can be transformed martensit-ically by quenching to form various types 

The Cu-Al-Ni alloys are advantageous because of their higher stability at higher temperatures compared with the Cu-Zn-Al alloys. However, second-phase precipitation cannot be suppressed and embrittles the Cu-Al-Ni alloys and precludes cold working, i.e. such alloys can only be hot finished (Van Humbeek and Delaey, 1989 Hodgson, 1990). Thermomechanical treatments and microalloying additions - in particular Mn, Ti, and Zr - are used for 

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Some of these binary and ternary phases have been studied to a larger extent because of special physical and/or mechanical properties. Of particular interest are the Cu phases CuZn, Cu j +1, 3,Zn i 2.3,Al, and (Cu,Ni)3Al, on which the Cu-Zn-Al and Cu-Al-Ni shape memory alloys are based and which are the subject of the following sections. In addition, the Cu-Au phases CU3AU and CuAu and the Cu-Sn phases Cu3Sn and Cu Snj will be addressed, which are important constituents of Cu-Au alloys and amalgams for dental restorative applications. 

Cohc, M., R. Rudolf, D. Stamenkovic, I. Anzel, D. Vucevic, M. Jenko, V. Lazic and G. Lojen (2010), Relationship between microstracture, cytotoxicity and corrosion properties of a Cu-Al-Ni shape memory alloy. Acta Biomaterialia, 6(1) ... 

Shape-Memory Alloys. Stoeckel defines a shape-memory alloy as the ability of some plastically deformed metals (and plastics) to resume their original shape upon heating. This effect has been observed in numerous metal alloys, notably the Ni—Ti and copper-based alloys, where commercial utilization of this effect lias been exploited. (An example is valve springs that respond automatically to change in transmission-fluid temperature.) Copper-based alloy systems also exhibit this effect. These have been Cu-Zn-Al and Cu-Al-Ni systems. In fact, the first thermal actuator to utilize this effect /a greenhouse window opener) uses a Cu—Zn-Al spring. 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

As their name implies, shape-memory alloys are able to revert back to their original shape, even if significantly deformed (Figure 3.24). This effect was discovered in 1932 for Au-Cd alloys. However, there were no applications for these materials until the discovery of Ni-Ti alloys (e.g., NiTi, nitinol) in the late 1960s. As significant research has been devoted to the study of these materials, there are now over 15 different binary, ternary, and quaternary alloys that also exhibit this property. Other than the most common Ni-Ti system, other classes include Au-Cu-Zn, Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. 

Cu-Zn-Al Cu-Al-Ni CuZn-Al (Cu.NOjAl shape memory alloys Hodgson (1990)... 

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). 

A relatively new gronp of metals that exhibit an interesting (and practical) phenomenon are the shape-memory alloys (or SMAs). One of these materials, after being deformed, has the ability to return to its predeformed size and shape upon being subjected to an appropriate heat treatment—that is, the material remembers its previous size/shape. Deformation normally is carried out at a relatively low temperature, whereas shape memory occurs upon heating. Materials that have been found to be capable of recovering significant amounts of deformation (i.e., strain) are nickel-titanium alloys (Nitinol, is their trade name) and some copper-base alloys (Cu-Zn-Al and Cu-Al-Ni alloys). 

This chapter begins with a general consideration of the crystallographic features of martensitic transformations. The principles are general, and thus detailed descriptions of the crystal struaures and substructures for individual alloy systems such as Ni-Al versus Cu-Sn are avoided. A brief survey of shape-memory phenomena within the framework of martensite crystallography is presented this subject and the various martensite crystal structures are presented in detail in Chapter 26 by Schetky in Volume 2. Martensitic transformations and shape-memory phenomena are common to many... 

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