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

**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 and resulted in the formation of β phase.


**31**

equation [32]:

**Figure 4.**

*Ms*

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

The shape memory characteristics of Cu-Al-Ni SMA are mainly dependent on the properties of the body centered cubic β phase for the binary alloys of Cu-Al [29]. During the cooling of β phase from 565°C, this phase undergoes the eutectoid decomposition of β → α + γ2. However, the high cooling rates are able to prevent this phase from eutectoid decomposition and enable the martensitic transformation. When the Cu-Al-Ni SMA possess an Al content of more than 11 wt.%, the structure of body center cubic transforms to a DO3-type superlattice by transferring the β to order β1 phase prior to martensitic transformation. In this case, the martensite "inherits" the ordered structure. At Al content between 11 and 13 wt.%, β′1 martensite, having a monoclinic 18R1 structure prevails. At Al content over 13 wt.%, orthorhombic 2H-type 1 martensite prevails. Which of them will appear depends on the temperature and the stress condition. In addition to these two, other types of

The characteristic temperatures of Cu-Al-Ni alloys can lie between −200 and 200°C dependent on content of Al and Ni; the content of Al has great influence, giving them the permittivity to be used for high temperature applications. The transformation temperatures of Ni–Ti alloys can be adjusted in the range between −200 and 120°C [31]. The Af temperature of Fe-based SMAs can increase to approximately 300°C; but at the same time, the Ms remains at room temperature or even below. The Ms temperature can be estimated using the following empirical

The addition of Al wt.% to the Cu-based shape memory alloys can lead to reduce the transformation temperature, for instance, the addition of 14 w.% Al, the martensitic transformation start will lie around the room temperature. In spite of

C)=2020 − 134 × (*wt*.%Al) − 45 × (*wt*.%*Ni*) (1)

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

martensite can also form (see in **Figure 5**).

*Cross-section diagram of the ternary alloy of Cu-Al-3 wt.% Ni [28].*

(°

**Table 1.**

*Properties of copper-based shape memory alloys [27].*

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

**Figure 4.** *Cross-section diagram of the ternary alloy of Cu-Al-3 wt.% Ni [28].*

The shape memory characteristics of Cu-Al-Ni SMA are mainly dependent on the properties of the body centered cubic β phase for the binary alloys of Cu-Al [29]. During the cooling of β phase from 565°C, this phase undergoes the eutectoid decomposition of β → α + γ2. However, the high cooling rates are able to prevent this phase from eutectoid decomposition and enable the martensitic transformation. When the Cu-Al-Ni SMA possess an Al content of more than 11 wt.%, the structure of body center cubic transforms to a DO3-type superlattice by transferring the β to order β1 phase prior to martensitic transformation. In this case, the martensite "inherits" the ordered structure. At Al content between 11 and 13 wt.%, β′1 martensite, having a monoclinic 18R1 structure prevails. At Al content over 13 wt.%, orthorhombic 2H-type 1 martensite prevails. Which of them will appear depends on the temperature and the stress condition. In addition to these two, other types of martensite can also form (see in **Figure 5**).

The characteristic temperatures of Cu-Al-Ni alloys can lie between −200 and 200°C dependent on content of Al and Ni; the content of Al has great influence, giving them the permittivity to be used for high temperature applications. The transformation temperatures of Ni–Ti alloys can be adjusted in the range between −200 and 120°C [31]. The Af temperature of Fe-based SMAs can increase to approximately 300°C; but at the same time, the Ms remains at room temperature or even below. The Ms temperature can be estimated using the following empirical equation [32]:

$$\mathbf{M\_{i}}^{\prime}\mathbf{C}^{\prime} = \mathbf{2020} - \mathbf{134} \times \text{(wt.96Al)} - 45 \times \text{(wt.96Ni)}\tag{1}$$

The addition of Al wt.% to the Cu-based shape memory alloys can lead to reduce the transformation temperature, for instance, the addition of 14 w.% Al, the martensitic transformation start will lie around the room temperature. In spite of

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

**Transformation temperature (°C)**

**Hysteresis (°C)**

1. Cu-Al-Ni 100–400 21.5 3–5 60–90 • Low cost

2. Cu-Zn-Al 120 15–25 4 70–85 • High thermal

3. Cu-Al-Be 150–200 20–25 3–5 80–90 • Reasonable recoverable

4. Cu-Al-Ni-Mn 230–280 15–20 3–4 90–100 • High shape memory

5. Cu-Al-Ni-Ti 120–260 12–20 2.5–4 90–100 • High shape memory

6. Cu-Al-Ni-Fe 210–250 12–15 9 40 • Low shape memory

**Tensile strain (%)**

**Strain recovery (%)**

**Remarks/features**

• Reasonable shape memory

• Good pseudoelastic behavior • Brittle in tension • Stable phase precipitation near 200°C • Reordering causes shift in transformation temperature in quenched

specimen

conductivity

• Inexpensive • Brittle alloys

behavior

behavior

behavior

• Reasonable recoverable shape memory strain

shape memory strain • High transformation temperatures

• High corrosion resistance

• Reasonable materials cost • High transformation temperatures • Good corrosion resistance

• Reasonable materials cost • High transformation temperatures

• High corrosion resistance

• High ductile material • Reasonable materials cost • High transformation temperatures

• High corrosion resistance

**Group No. Alloy** 

Cu-based shape memory alloys

**composition**

**30**

**Table 1.**

*Properties of copper-based shape memory alloys [27].*

**Figure 5.** *Schematic phase diagram of Cu-Al-Ni alloy in temperature-stress coordinates [1, 30].*

this, the Al addition may lead to from new phase known as phase γ2 (i.e., it refers to the cubic intermetallic compound of Cu9Al4), in which it results in increasing the brittleness of the alloy. However, the nickel addition will play an important role of controlling the diffusion rate of Cu into Al, thereby, the may lead to retain single phase of β or β1 till the martensitic phase transformation starts been reached during the cooling process. From another point of view, increasing the percentages of Ni in the ternary alloy of Cu-Al-Ni SMAs will be a result of the high brittleness associated with shifting the eutectoid point to higher values. Therefore, optimizing the chemical composition of the Al and Ni in the range of 14 and 3.5–4 wt.%, respectively [1]. On the other hand, these alloys still have drawbacks such as low reversible transformation that included the 4% of one-way shape memory effect and 1.5% of two-way shape memory effect. These disadvantages are mainly attributed to the intergranular cracks that occurred at a low stress level. The reasons behind the low stress failure are the large grain size, high elastic anisotropy, intense reliance of transformation strain on crystal orientations as well as segregation on grain boundaries. The first three reasons apply when there is high concentration of shear stress at the grain boundaries. The fourth reason is mainly due to weakening of grain boundaries [33].

#### **3.2 Phase transformation morphology**

The martensitic transformation can be induced both thermally and/or through applying an external stress. In other words, applying stress and decreasing the temperature both drive the austenite → martensite transformation. In fact, there is a linear relationship between the two forces that is derived from the thermodynamics relationships of the phase transformation, called the Clausius-Clapeyron relationship. Thermal treatments significantly influence the characteristics of the martensitic transformation [34], such as martensite, transformation temperatures and hysteresis, which are very sensitive to the order degree of the β phase and the precipitation process [35, 36]. The copper-based shape memory alloys exhibit a martensitic transformation from the β-phase to a close-packed structure on cooling. Additionally, the high temperatures of the β-phase for the Cu-Al-Ni alloys have a disordered bcc structure similar to the Cu-Zn-Al alloys [37]. In the Cu-Al-Ni alloys, two types of thermally induced martensites (β′1 and γ′1) form, depending

**33**

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

mation requires higher energy than the reverse transformation [48].

**4.1 Martensitic transformation temperature of Cu-Al-Ni SMA**

In copper-based shape memory alloys, the most significant factor that controls the martensite transformation is the alloy chemical composition. In commercial applications, the effect of alloying elements on the martensite transformation temperature is highly beneficial during the design of an alloy with the required characteristics [49]. Grain refiners are added to Cu-Al-Ni shape memory alloys for many reasons. These effects are both direct and indirect, such as [50] (i) the transformation temperatures are modified due to the formation of intermetallics; (ii) the remaining solid solution may increase the strength of β phase, thus leading to reduce the Ms and other temperatures; (iii) producing a chemical contribution; and (v) grain growth which occurs during annealing has an influence on the

For decreasing brittleness, one of the most important defects of Cu-Al-Ni SMAs, Itsumi et al. [51] replaced 2% of the aluminum content with Mn, which suppressed the eutectoid reaction β<sup>1</sup> → α + γ2; Mn does not decrease the transformation temperature. At the same time, they used 1% of the Ti, which resulted in grain refinement and thus intergranular cracking can be eliminated. Karagoz and Canbay [52] studied the variations of Al and Ni percentages on the phase transformation temperatures, and have found that the forward and reverse transformation temperatures are strongly influenced by the variation of Al wt.%, therefore, higher percentage of Al exhibited lowest transformation temperatures. The variation of Ni wt.% was found to be mainly responsible for suppressing the diffusivity of Cu and Al. Chang [53] found that the Ms temperature of Cu-*x*Al-4Ni SMAs decreased significantly from 180.9 to −54.7°C when the content of Al was increased from *x* = 13.0 to 14.5 as shown in **Figure 6(a–c)**. This is consistent with the study by Recarte et al. [49], in which the Ms temperature of Cu-Al-Ni SMA depended strongly on its chemical composition, particularly with the content of Al. Cu-*x*Al-4Ni SMAs with a higher content of Al exhibiting a lower Ms temperature could be ascribed to the fact that the driving force necessary for nucleation of the γ′1 (2H) martensite is higher than

Sampath [50] found that addition of alloying elements and grain refiners are the main factors that can increase solid solution strengthening, as some of these elements are capable of dissolving into the solution leading to the formation of a second phase. Therefore, with the addition of a minor amount of Ti, Zr, and B to the Cu-Al-Ni SMA, the transformation temperatures are led to increase, as shown in **Figure 7(a–d)**. On the other hand, when the weight percentage of Al and Ni are

on the alloy's composition and heat treatment [38–41]. The stability of the β-phase decreases with decreasing temperature. For example, at a lower temperature, the β-phase can remain metastable under proper cooling (air cooling) [42–44]. The stability limit of the overcooled β-phase must then be established to avoid the expansion of the ordination state of the β-phase and/or the precipitation of the stable phases. However, the improved mechanical properties of Cu-Al-Ni SMA are highly related to the production of alloys with a fine grain size [45]. During the heatingcooling processes, the structure of these alloys' changes within the martensitic region. Moreover, usable forces arise during the martensite ⇔ austenite transformation upon thermal cycling due to the shape recovery properties, which allows these alloys to be used as a component in some devices [46, 47]. The martensitic transfor-

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

**4. Effects of alloying elements on the:**

transformation temperatures.

that of the β′1 (18R) [49, 54, 55].

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

on the alloy's composition and heat treatment [38–41]. The stability of the β-phase decreases with decreasing temperature. For example, at a lower temperature, the β-phase can remain metastable under proper cooling (air cooling) [42–44]. The stability limit of the overcooled β-phase must then be established to avoid the expansion of the ordination state of the β-phase and/or the precipitation of the stable phases. However, the improved mechanical properties of Cu-Al-Ni SMA are highly related to the production of alloys with a fine grain size [45]. During the heatingcooling processes, the structure of these alloys' changes within the martensitic region. Moreover, usable forces arise during the martensite ⇔ austenite transformation upon thermal cycling due to the shape recovery properties, which allows these alloys to be used as a component in some devices [46, 47]. The martensitic transformation requires higher energy than the reverse transformation [48].
