**2.1 Shape memory effect property**

Shape memory effect (SME) is a property of SMAs which enable thermoelastic martensitic transformation. Shape memory effect will occur with the deformation of the SMA in the martensitic phase during the loading and unloading at temperatures below Mf. After heating these deformed alloys to a temperature above Af, the austenite phase forms, and thus, the original shape is recovered. **Figure 1** shows a typical loading path 1 → 2 → 3 → 4 → 1, wherein the property of SME is observed [16]. The parent phase transforms into the twined martensite (1 → 2) when it undergoes the cooling process. The stress induced detwinning and inelastic strains can occur when the materials are loaded (2 → 3). The martensite phase is in the same state of the detwinned structure without obtaining any recovered inelastic strains even after the unloaded process (3 → 4). In the final step, the materials are returned to the original shape by recovering the inelastic strains after being heated above Af (4 → 1).

A self-accommodating growth of the martensitic variants (1 → 2) is being produced within the stress-free cooling of austenite phase without observing any macroscopic transformation [17–19]. The essential morphology that characterize the crystallographic of these alloys is the self-accommodating structure. For instance, of the Cu-based shape memory alloys, there are 24 variants of martensite that consist of six self-accommodated groups distributed around <011> poles of

**27**

was achieved.

**Figure 1.**

*of SME [16].*

**2.2 Pseudoelasticity property**

*Cu-Based Shape Memory Alloys: Modified Structures and Their Related Properties*

austenite which exhibit an ordinary diamond morphology. During the growth process of these groups, the macroscopic transformation strain cannot be observed, except that some of the boundaries between the martensite variants and twinning interfaces display very high movements. However, the boundary interfaces together with the detwinning structure is performed at a stress level much lower than the martensite plastic yield limit, where these phenomena is known as a reorientation of variants, which dominates at temperatures lower than Mf. In the second stage (2 → 3), the loading forces are going to reorient the variants of the martensite phase, which result in producing a large value of inelastic strain, and this strain is not recovered upon unloading (3 → 4). During the last step (4 → 1), heating the deformed alloys to a certain temperature above Af induces reverse transformation and the inelastic strain is recovered [9, 16, 20]. The martensitic phase transformation will be unstable after the austenite finish temperature (Af) approached without requirement for any kind of external stress. It resulted in a complete recovery will be achieved, in consequence the martensite variant reorientations do occurred, there will be an additional strain with the same value of the inelastic strain but in opposite direction, and thus, the initial shape will be recovered. Saud et al. [21] was carried out the shape memory effect test using a special designed machine, as presented in **Figure 2**; whereas the test was performed at a temperature below martensite finish temperature (i.e., 100°C), the shape recovery was obtained partially, and then it was followed by a subsequent heating above the austenite finish temperature (Af is 300°C) using an external muffle furnace, where a full recovery

*Schematic diagram of stress-strain-temperature for the involved crystallographic changes during the phenomena* 

The property of pseudoelasticity in the shape memory alloys is mainly related to the induced strain recovery upon unloading at temperatures above Af. Within the general conditions, the thermomechanical loading directions of pseudoelastic are usually started in the austenitic area at zero stress, and then move toward the region

*DOI: http://dx.doi.org/10.5772/intechopen.86193*

*Cu-Based Shape Memory Alloys: Modified Structures and Their Related Properties DOI: http://dx.doi.org/10.5772/intechopen.86193*

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

These sorts of martensite forms have the ability to be organized independently in a self-accommodation approach by the mechanism of twinning throughout the inadequacy of the practiced stresses, with the consequence that virtually no shape transform can certainly be realized. The results of martensitic phase deformation are able to be detwinned into a single variant corresponded to the applied loads, and consequently a large inelastic strain happened [9, 10]. Heating the deformed alloys to a certain temperature above the austenite temperature will turn the inelastic strain to be recovered through transferring the existed martensite to austenite, this kind of feature is known as shape memory effect (SME) [9]. On the other hands, the pseudo-elasticity (PE) is caused by transferring the twinned martensitic phase into detwinned phase and obtained the shape recovery under the austenite starts temperature; in other words, the deformation of loading and unloading will be occurred in the austenite phase. This kind of structure transfer will be resulted in a large inelastic strain and a consequence of the phase reverse transformation, the initial shape will be restored upon the unloading process. Therefore, these types of materials such as Ti-based, Cu-based, and Fe-based SMAs are capable to demonstrated SME and PE [11–13]. Generally, there are two groups of martensitic transformation, thermoelastic and non-thermoelastic [14]. The thermoelastic martensitic transformations happen during the mobile interfaces between the martensite phase and parent phase. These types of interfaces are able to move during the reverse martensitic transformation as an alternative to the nucleation of the parent phase, which leads to a crystallographically reversible transformation [1]. On the other hand, the non-thermoelastic martensitic transformations are mainly found in ferrous alloys, which are related to the non-mobile interfaces of the martensitic parent phase pinned by permanent defects leading to a successful nucleation and growth. As a result of the austenite re-nucleation during the reversible martensitic transformation, these kinds of transformations are crystallographically nonreversible, in which the martensite phase is not able to return to original phase [15].

Shape memory effect (SME) is a property of SMAs which enable thermoelastic martensitic transformation. Shape memory effect will occur with the deformation of the SMA in the martensitic phase during the loading and unloading at temperatures below Mf. After heating these deformed alloys to a temperature above Af, the austenite phase forms, and thus, the original shape is recovered. **Figure 1** shows a typical loading path 1 → 2 → 3 → 4 → 1, wherein the property of SME is observed [16]. The parent phase transforms into the twined martensite (1 → 2) when it undergoes the cooling process. The stress induced detwinning and inelastic strains can occur when the materials are loaded (2 → 3). The martensite phase is in the same state of the detwinned structure without obtaining any recovered inelastic strains even after the unloaded process (3 → 4). In the final step, the materials are returned to the original shape by recovering the inelastic strains after being heated

A self-accommodating growth of the martensitic variants (1 → 2) is being produced within the stress-free cooling of austenite phase without observing any macroscopic transformation [17–19]. The essential morphology that characterize the crystallographic of these alloys is the self-accommodating structure. For instance, of the Cu-based shape memory alloys, there are 24 variants of martensite that consist of six self-accommodated groups distributed around <011> poles of

**26**

above Af (4 → 1).

**2. Shape memory characteristics**

**2.1 Shape memory effect property**

*Schematic diagram of stress-strain-temperature for the involved crystallographic changes during the phenomena of SME [16].*

austenite which exhibit an ordinary diamond morphology. During the growth process of these groups, the macroscopic transformation strain cannot be observed, except that some of the boundaries between the martensite variants and twinning interfaces display very high movements. However, the boundary interfaces together with the detwinning structure is performed at a stress level much lower than the martensite plastic yield limit, where these phenomena is known as a reorientation of variants, which dominates at temperatures lower than Mf. In the second stage (2 → 3), the loading forces are going to reorient the variants of the martensite phase, which result in producing a large value of inelastic strain, and this strain is not recovered upon unloading (3 → 4). During the last step (4 → 1), heating the deformed alloys to a certain temperature above Af induces reverse transformation and the inelastic strain is recovered [9, 16, 20]. The martensitic phase transformation will be unstable after the austenite finish temperature (Af) approached without requirement for any kind of external stress. It resulted in a complete recovery will be achieved, in consequence the martensite variant reorientations do occurred, there will be an additional strain with the same value of the inelastic strain but in opposite direction, and thus, the initial shape will be recovered. Saud et al. [21] was carried out the shape memory effect test using a special designed machine, as presented in **Figure 2**; whereas the test was performed at a temperature below martensite finish temperature (i.e., 100°C), the shape recovery was obtained partially, and then it was followed by a subsequent heating above the austenite finish temperature (Af is 300°C) using an external muffle furnace, where a full recovery was achieved.

#### **2.2 Pseudoelasticity property**

The property of pseudoelasticity in the shape memory alloys is mainly related to the induced strain recovery upon unloading at temperatures above Af. Within the general conditions, the thermomechanical loading directions of pseudoelastic are usually started in the austenitic area at zero stress, and then move toward the region

**Figure 2.** *Shape memory effect test [21].*

of detwinned martensite, followed by the unloading toward the starting point. **Figure 3** shows the loading and unloading direction that started from point *a*, and moved to *b → c → d → e*, then returned back to point *a*. Other examples are the isothermal and isobaric loading paths shown schematically in **Figure 3**.

**29**

*Cu-Based Shape Memory Alloys: Modified Structures and Their Related Properties*

There are two main types of Cu-based SMAs; binary alloys of Cu-Al and Cu-Zn, in which both systems performed their shape memory features in the domain of β-phase, moreover, the third element addition to the binary and/or ternary is aimed to modify and control the transformation temperatures in comprehensive range in meet the application requirements, i.e., T ≈ 100–370°C. From this point of view, it was proven that the transformation temperatures are highly sensitive to the composition of alloys.

able than 5°C. Copper-based alloys commonly display considerably less hysteresis as compared to NiTi. Cu-Zn-Al alloy is not difficult to produce and is quite inexpensive. It decomposes into the equilibrium phases whenever overheated, therefore leading to a stabilization of the martensite. The properties of Cu-Al-Ni and Cu-Zn-Al SMAs are listed in **Table 1**. The availability of additives, including Co, Zr, B or Ti, is vital to provide grains from 50 to 100 nm in size. Add-on of boron is also used to enhance the ductility of the material. Cu-Al-Ni is substantially less vulnerable to stabilize as well as aging phenomena. This alloy performs with less hysteresis than NiTi and turns brittle as Ni increases much beyond 4 at.% [22]. It is also prevalent for Ni to be retained at a constant 4 at.% and this alloy is composed of Cu96-xAlxNi4 [23, 24]. In general, increasing the Al amount can lead to increase the stability of martensite. The purpose of the Al addition is to reduce the transformation temperatures. This variety is nearly entirely linear, ranging from Mf = 203 K and Af = 250 K for a 14.4 at.% Al to Mf = 308 K and Af = 348 K for a 13.6 at.% Al [22]. However, as the temperatures tend to be operated over a wide range; the sensible higher limit for transformation is 473 K. Above this temperature there is certainly an immediate degradation in the transformation as a result of aging effects. The typical Cu-based SMAs are able to exhibit a pseudoelastic strain of about of 4–6%. With the martensite to martensite transformation, very high pseudoelastic strain levels are displayed. A single crystal of the Cu81.8Al14Ni4.2 SMA can exhibit approximately 18% of the pseudoelastic strain associated with 100% of the shape recovery [25]. Cu-Zn alloy with the addition of the third element of Sn with a weight percentage of 34.7% has exhibited very low transformation temperatures, around Mf of 208 K and an Af of 235 K [26]. As well this addition has exhibited a

at.% is typically essential to obtain reproducibility more desir-

) with applied strain of 2.5% along with a pseudoelastic strain

of around 8% by obtaining a full strain recovery [26]. In recent years, a minor amount (about 0.6 wt.%) of beryllium was added as a third element to the binary alloy of Cu-Al, and it was found that this addition led to reduce the transformation tempera-

Cu-based SMAs consist of different types of alloys, but the most frequently used

**Figure 4** displayed the cross section of the ternary alloys of Cu-Al-at 3 wt.% of nickel. The alloy may possibly demonstrate shape memory characteristics as long as the martensitic transformation materialized. With the intention to ascertain undercooling, in which it vital to enforce the martensitic transformation, with a long of fully consideration that the heat treatment can never be prevented. It comes with annealing in the temperature variety of stable β phase to ensuing water quenching

alloys are Cu-Zn-Al and Cu-Al-Ni due to their inexpensive production cost and high resistance to the degradation of functional properties that occurred during the aging processes. There are many features that characterized the Cu-Al-Ni SMA rather than other shape memory alloys, such as considerably cheaper than Ni-Ti

*DOI: http://dx.doi.org/10.5772/intechopen.86193*

to 10<sup>−</sup><sup>4</sup>

**3. Cu-based SMAs**

Accuracy of 10<sup>−</sup><sup>3</sup>

transformation strain (ε

t

alloys and high transformation temperatures.

**3.1 Phase diagram of Cu-Al-Ni SMAs**

and resulted in the formation of β phase.

tures from 200 to 150°C with very good thermal stability.

**Figure 3.** *The two loading paths discussed for pseudoelasticity in single crystal SMA [16].*
