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

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

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

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

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

**32**

boundaries [33].

**Figure 5.**

**3.2 Phase transformation morphology**

#### **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 transformation temperatures.

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 that of the β′1 (18R) [49, 54, 55].

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

**Figure 6.**

*Evolution of (a) DSC heating-cooling curves, (b) the Ms transformation temperature, and (c) the transformation enthalpy of the as a function of Al content [53].*

decreased, the transformation temperatures increased. Thus, at less than 12 wt.% of Al, the transformation temperatures are increasing, which is in complete agreement with other researchers [23]. From the same point of view, Miyazaki et al. [56] found that with increase in the amount of Al and Ni in the entire composition of Cu-Al-Ni SMA, the transformation temperatures also tend to decrease. Sugimoto et al. [57] found that with the addition of different percentages of titanium to the Cu-Al-Ni SMA, the transformation temperature are increase. These increases are related to the presence of the X-phase as Ti-rich particles into the microstructure that can reduce the mobility of interfaces between the martensite and β phase. The martensite transformation temperature has behaved according to the type of the alloying element, where it has decreased with increasing Ti amount and increased with increasing the Zr amount as reported by Wayman and Lee [58]. This is attributed to the dissolving percentage of Ti and Zr in the β-phase. Dutkiewicz et al. [59], disagreed that Ti additions decreased the Ms. However, they have proved that the Ms temperature increases as grain size reduces, where the rapid drop of the transformation temperatures is in the smallest grain size range. Saud et al. [60] was shown that the transformation temperature of Cu-Al-Ni SMAs after the addition of Sn which was represented by the exothermic and endothermic curve in **Figure 8**, the results revealed that the behavior of the observed peak tend to be sharp and board at 232 and 350°C, respectively, due to the existence of different types of precipitates that led to limit the stability of the

**35**

**Figure 8.**

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

low temperature phase and resulted in an individual transformation corresponding

*Differential scanning calorimetry profiles for Cu-Al-Ni alloys: (a) Cu-Al-Ni; (b) Cu-Al-Ni-0.2Ti; (c) Cu-Al-*

The sort of thermally introduced martensite is totally dependent primarily on the chemical substance composition of Al and Ni in the Cu-Al-Ni SMAs. Once the martensitic transformation is produced by the deformation loading, the particular martensite acquired is determined by aspects including crystal orientation, chemical compositions of Al/Ni, deformation stress as well as applied temperature. There

*Transformation temperature of Cu-Al-Ni SMAs modified with different percentage of Sn [60]; the magnified* 

*peaks of the (a,b) forwards transformation and (c) reverse transformation.*

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

to the high driving force.

*Ni-0.4Mn; and (d) Cu-Al-Ni-0.2Zr [50].*

**Figure 7.**

**4.2 Martensitic structure of Cu-Al-Ni SMAs**

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

**Figure 7.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

decreased, the transformation temperatures increased. Thus, at less than 12 wt.% of Al, the transformation temperatures are increasing, which is in complete agreement with other researchers [23]. From the same point of view, Miyazaki et al. [56] found that with increase in the amount of Al and Ni in the entire composition of Cu-Al-Ni SMA, the transformation temperatures also tend to decrease. Sugimoto et al. [57] found that with the addition of different percentages of titanium to the Cu-Al-Ni SMA, the transformation temperature are increase. These increases are related to the presence of the X-phase as Ti-rich particles into the microstructure that can reduce the mobility of interfaces between the martensite and β phase. The martensite transformation temperature has behaved according to the type of the alloying element, where it has decreased with increasing Ti amount and increased with increasing the Zr amount as reported by Wayman and Lee [58]. This is attributed to the dissolving percentage of Ti and Zr in the β-phase. Dutkiewicz et al. [59], disagreed that Ti additions decreased the Ms. However, they have proved that the Ms temperature increases as grain size reduces, where the rapid drop of the transformation temperatures is in the smallest grain size range. Saud et al. [60] was shown that the transformation temperature of Cu-Al-Ni SMAs after the addition of Sn which was represented by the exothermic and endothermic curve in **Figure 8**, the results revealed that the behavior of the observed peak tend to be sharp and board at 232 and 350°C, respectively, due to the existence of different types of precipitates that led to limit the stability of the

*Evolution of (a) DSC heating-cooling curves, (b) the Ms transformation temperature, and (c) the* 

*transformation enthalpy of the as a function of Al content [53].*

**34**

**Figure 6.**

*Differential scanning calorimetry profiles for Cu-Al-Ni alloys: (a) Cu-Al-Ni; (b) Cu-Al-Ni-0.2Ti; (c) Cu-Al-Ni-0.4Mn; and (d) Cu-Al-Ni-0.2Zr [50].*

low temperature phase and resulted in an individual transformation corresponding to the high driving force.

#### **4.2 Martensitic structure of Cu-Al-Ni SMAs**

The sort of thermally introduced martensite is totally dependent primarily on the chemical substance composition of Al and Ni in the Cu-Al-Ni SMAs. Once the martensitic transformation is produced by the deformation loading, the particular martensite acquired is determined by aspects including crystal orientation, chemical compositions of Al/Ni, deformation stress as well as applied temperature. There

#### **Figure 8.**

*Transformation temperature of Cu-Al-Ni SMAs modified with different percentage of Sn [60]; the magnified peaks of the (a,b) forwards transformation and (c) reverse transformation.*

are several reasons behind adding the alloying elements to Cu-based shape memory alloys [50, 61–64], including to (1) refine the grain size, (2) restrict the martensite stabilization, (3) adjust the phase diagrams, (4) accommodate the transformation temperature, (5) improve the workability of these alloys, since they are difficult to process, due to a large grain size having formed during the solidification process, and to enhance the service life of copper shape memory alloys in applications.

The microstructure of Cu-Al-Ni SMA can be formed in a needle and/or platelike martensites with self-accommodating morphology [50]. Two different phases are excited during adding 13.3% Al and 4.3% Ni to Cu-Al-Ni SMAs: (i) acicular morphology: β′1; and (ii) self-accommodating morphology: γ′1. The martensite in Cu-Al-Ni alloy has experienced a gradual transition from β′1 to γ′1 via a β′1 + γ′<sup>1</sup> composition when the percentage of Al increased [49, 65]. At high cooling rate, β martensite transformed to β′1 martensite with tiny quantities of γ′1 phase. However, in case of low cooling rate, β′1 transformed to γ′1 martensite. The formation of γ′<sup>1</sup> martensite is inevitable irrespective of the processing conditions if the Al content is >14.2 wt.%. Minor additions to the base Cu-Al-Ni alloy tend to produce intermetallic compounds with Al, when the matrix of Al decreases resulting in the formation of β′1 martensite. If the percentage of Al is less than 11.9 wt.%, large plates of α′ martensite will be formed. Fine plates of β′1 martensite form when the Al content is about 11.9 wt.%. β′1 + γ′1 mixtures are observed in Cu-13.03 wt.% Al-4.09 wt.% Ni [66] and martensite formed mainly the M18R type with an orthorhombic structure [67]. However, Chentouf et al. [68] studied the microstructural and thermodynamic analysis of hypoeutectoidal Cu-Al-Ni shape memory alloys and determined that the amount of Al and Ni has a greater effect on the morphology of the precipitated phase as shown in **Figure 9**.

#### **Figure 9.**

*Optical micrographs for alloys: (a) Cu-9.9 wt.% Al-4.43 wt.% Ni, (b) Cu-11.25 wt.% Al-4.07 wt.% Ni and (c) Cu-11.79 wt.% Al-4.37 wt.% Ni [68].*

**37**

**Figure 10.**

*SMAs [53].*

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

In Cu-Al-Ni shape memory alloys, large precipitate (XL) particles are formed resulting in the transformation of the 18R basal plane order into 2H martensite at the interface of the precipitate-free and precipitate-matrix. Ratchev et al. [69] stated that there would be a change in the 18R sequence due to the modification of the stresses around the precipitates. Karagoz and Canbay [52] found that when the percentage of Al addition increased, the β phase leads to the total martensitic transformation of β′1 and γ′1 phases during the homogenization process and the grains formed in V-type shape along with different orientations. Chang [53] with 13 wt.% of Al, martensite exhibited self-accommodating zig zag groups at room temperature, whereas the martensite is typical β′1 martensite with an 18R structure as shown in **Figure 10a**. However, by increasing the Al to 13.5 wt.%, a number of coarse variants of γ′1 (2H) structure exist in the matrix of β′1 (18R), as shown in **Figure 10b**. With further increase in the Al amount to 13.7 and 14 wt.%, the microstructure became more distinct exhibiting a β′1 (18R) or γ′1 (2H) martensite along with the abundant precipitate of γ2 phase as demonstrated in **Figure 10c** and **d**. According to the relationship between the variety of transformed martensite and the composition of Cu-*x*Al-4Ni SMAs reported by Recarte [49, 54, 70], the β′1 (18R) and the γ′1 (2H) martensite should coexist in Cu-13.7Al-4Ni SMA, while only γ′1 (2H) martensite

Sugimoto et al. [57] found that with the addition of Ti to the Cu-Al-Ni SMA, a new phase known as X-phase is going to be formed which is rich in Ti-rich. Also, the volume fraction of this phase is increased linearly with increase in the percentage of Ti addition. Other work has been done by Dutkiewicz et al. [59], where they have agreed that the addition of Ti to the Cu-Al-Ni caused a smaller and elongated grain

*SEM micrographs of (a) Cu-13.0Al-4Ni, (b) Cu-13.5Al-4Ni, (c) Cu-13.7Al-4Ni, and (d) Cu-14.0Al-4Ni* 

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

exists in Cu-14.0Al-4Ni SMA.

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

In Cu-Al-Ni shape memory alloys, large precipitate (XL) particles are formed resulting in the transformation of the 18R basal plane order into 2H martensite at the interface of the precipitate-free and precipitate-matrix. Ratchev et al. [69] stated that there would be a change in the 18R sequence due to the modification of the stresses around the precipitates. Karagoz and Canbay [52] found that when the percentage of Al addition increased, the β phase leads to the total martensitic transformation of β′1 and γ′1 phases during the homogenization process and the grains formed in V-type shape along with different orientations. Chang [53] with 13 wt.% of Al, martensite exhibited self-accommodating zig zag groups at room temperature, whereas the martensite is typical β′1 martensite with an 18R structure as shown in **Figure 10a**. However, by increasing the Al to 13.5 wt.%, a number of coarse variants of γ′1 (2H) structure exist in the matrix of β′1 (18R), as shown in **Figure 10b**. With further increase in the Al amount to 13.7 and 14 wt.%, the microstructure became more distinct exhibiting a β′1 (18R) or γ′1 (2H) martensite along with the abundant precipitate of γ2 phase as demonstrated in **Figure 10c** and **d**. According to the relationship between the variety of transformed martensite and the composition of Cu-*x*Al-4Ni SMAs reported by Recarte [49, 54, 70], the β′1 (18R) and the γ′1 (2H) martensite should coexist in Cu-13.7Al-4Ni SMA, while only γ′1 (2H) martensite exists in Cu-14.0Al-4Ni SMA.

Sugimoto et al. [57] found that with the addition of Ti to the Cu-Al-Ni SMA, a new phase known as X-phase is going to be formed which is rich in Ti-rich. Also, the volume fraction of this phase is increased linearly with increase in the percentage of Ti addition. Other work has been done by Dutkiewicz et al. [59], where they have agreed that the addition of Ti to the Cu-Al-Ni caused a smaller and elongated grain

**Figure 10.**

*SEM micrographs of (a) Cu-13.0Al-4Ni, (b) Cu-13.5Al-4Ni, (c) Cu-13.7Al-4Ni, and (d) Cu-14.0Al-4Ni SMAs [53].*

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

tated phase as shown in **Figure 9**.

are several reasons behind adding the alloying elements to Cu-based shape memory alloys [50, 61–64], including to (1) refine the grain size, (2) restrict the martensite stabilization, (3) adjust the phase diagrams, (4) accommodate the transformation temperature, (5) improve the workability of these alloys, since they are difficult to process, due to a large grain size having formed during the solidification process, and to enhance the service life of copper shape memory alloys in applications. The microstructure of Cu-Al-Ni SMA can be formed in a needle and/or platelike martensites with self-accommodating morphology [50]. Two different phases are excited during adding 13.3% Al and 4.3% Ni to Cu-Al-Ni SMAs: (i) acicular morphology: β′1; and (ii) self-accommodating morphology: γ′1. The martensite in Cu-Al-Ni alloy has experienced a gradual transition from β′1 to γ′1 via a β′1 + γ′<sup>1</sup> composition when the percentage of Al increased [49, 65]. At high cooling rate, β martensite transformed to β′1 martensite with tiny quantities of γ′1 phase. However, in case of low cooling rate, β′1 transformed to γ′1 martensite. The formation of γ′<sup>1</sup> martensite is inevitable irrespective of the processing conditions if the Al content is >14.2 wt.%. Minor additions to the base Cu-Al-Ni alloy tend to produce intermetallic compounds with Al, when the matrix of Al decreases resulting in the formation of β′1 martensite. If the percentage of Al is less than 11.9 wt.%, large plates of α′ martensite will be formed. Fine plates of β′1 martensite form when the Al content is about 11.9 wt.%. β′1 + γ′1 mixtures are observed in Cu-13.03 wt.% Al-4.09 wt.% Ni [66] and martensite formed mainly the M18R type with an orthorhombic structure [67]. However, Chentouf et al. [68] studied the microstructural and thermodynamic analysis of hypoeutectoidal Cu-Al-Ni shape memory alloys and determined that the amount of Al and Ni has a greater effect on the morphology of the precipi-

*Optical micrographs for alloys: (a) Cu-9.9 wt.% Al-4.43 wt.% Ni, (b) Cu-11.25 wt.% Al-4.07 wt.% Ni and* 

**36**

**Figure 9.**

*(c) Cu-11.79 wt.% Al-4.37 wt.% Ni [68].*

size because the Ti addition restricted the grain growth as shown in **Figure 11(a–d)**. Font et al. [71] found that the addition of Mn and B along with different thermal cycling have an effect on the parameters on the martensite morphologies and orientations. They found that the martensite formed in two morphologies: plates and thin needles. The plates martensite form as self-accommodation variant groups. However, some particles have been observed to form between the plates and needles and their size is almost same with different amounts of Mn and B added. The distribution of these particles are mainly dependent on the thermal treatment conditions and by using energy dispersive spectroscopy, it was found that these particles are Mn and/ or aluminum boride, a result which is in complete agreement with Morris [72]. The existence of these particles is due to difficulties dissolving Mn/B into the matrix. Sampath [50] has shown that two different morphologies are formed into the microstructure of Cu-13.3 wt.% Al-4.3 wt.% Ni SMA and these morphologies are (γ′1 with a self-accommodating structure and β′1 with a acicular structure). Also, it was found that with adding a minor addition of Ti, Mn, or Zr to the base alloy, new precipitations/compounds have formed with Al element as shown in **Figure 12(a–d)**. These precipitations are able to enhance the formation of martensite β′1 phase. Saud et al. [21] presented the changes in the microstructure changes of Cu-Al-Ni SMAs after the addition of different percentages of Ti and the microstructure changes were exhibited in **Figure 13(a–d)**. it was revealed that the presence of γ′1 and β′1 phases, on the other hands, there is an irregular phase was observed in the modified microstructure in the shape of flower and it has been formed randomly between β′1 plates and needles, which this phase was called as X-phase.

#### **Figure 11.**

*Optical micrographs of (a) Cu-11.85 wt.% Al-3.2 wt.% Ni-3 wt.% Mn, (b) Cu-11.9 wt.% Al-5 wt.% Ni-2 wt.% Mn-1 wt.% Ti, (c) Cu-11.4 wt.% Al-2.5 wt.% Ni-5 wt.% Mn-0.4 wt.% Ti, and (d) Cu-11.8 wt.% Al-5 wt.% Ni-2 wt.% Mn-1 wt.% Ti [59].*

**39**

**Figure 12.**

*(d) Cu-Al-Ni-0.2 Zr [50].*

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

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

**4.3 Mechanical properties of Cu-Al-Ni SMA**

Cu-Al-Ni shape memory alloys (SMA) have been selected as high potential materials for high temperature applications. This is attributed to their high thermal stability at temperatures above 100°C [73–76]. On the other hand, these alloys have their limitations such as high brittleness because of the appearance of brittle phase γ2 at grain boundaries, the enormous increase in grain size duplicated with a high elastic variation [77–81]. Thus, their disadvantages have restricted the usage of these alloys for commercial applications [82–92]. One way to solve this problem is the grain refinement. By adding some of the alloying elements such as Ti, Mn, V, Nb, B and others or varying the compositions of Ni or Al, some improvement in mechanical properties of the conventional Cu-Al-Ni SMAs [86, 93–96] was observed. This improvement is attributed to the addition of alloying elements, where these elements are restricting the grain growth and refining the grains. However, these alloying elements have a significant effect on the mechanical properties of Cu-Al-Ni SMAs due to the formation as a second phase structure in the microstructure [97]. Miyzakai et al. [23, 56] found that varying the percentage of Al and Ni lead to changes in crack formation and propagation. It was also found that increases in the Al and Ni amount from 14 and 3.9 wt.% to 14.2 and 4 wt.% lead to the appearance of clear crack formation. This may be attributed to the amount of thermal stress induced and in accordance to the Clausius-Claperyron equation, the increase in the alloying composition of Al and Ni has an effective influence on the

*Optical micrographs of Cu-Al-Ni alloys: (a) Cu-Al-Ni; (b) Cu-Al-Ni-0.2 Ti; (c) Cu-Al-Ni-0.4 Mn; and* 

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

#### **Figure 12.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

needles, which this phase was called as X-phase.

*Optical micrographs of (a) Cu-11.85 wt.% Al-3.2 wt.% Ni-3 wt.% Mn, (b) Cu-11.9 wt.% Al-5 wt.% Ni-2 wt.% Mn-1 wt.% Ti, (c) Cu-11.4 wt.% Al-2.5 wt.% Ni-5 wt.% Mn-0.4 wt.% Ti, and (d) Cu-11.8 wt.%* 

size because the Ti addition restricted the grain growth as shown in **Figure 11(a–d)**. Font et al. [71] found that the addition of Mn and B along with different thermal cycling have an effect on the parameters on the martensite morphologies and orientations. They found that the martensite formed in two morphologies: plates and thin needles. The plates martensite form as self-accommodation variant groups. However, some particles have been observed to form between the plates and needles and their size is almost same with different amounts of Mn and B added. The distribution of these particles are mainly dependent on the thermal treatment conditions and by using energy dispersive spectroscopy, it was found that these particles are Mn and/ or aluminum boride, a result which is in complete agreement with Morris [72]. The existence of these particles is due to difficulties dissolving Mn/B into the matrix. Sampath [50] has shown that two different morphologies are formed into the microstructure of Cu-13.3 wt.% Al-4.3 wt.% Ni SMA and these morphologies are (γ′1 with a self-accommodating structure and β′1 with a acicular structure). Also, it was found that with adding a minor addition of Ti, Mn, or Zr to the base alloy, new precipitations/compounds have formed with Al element as shown in **Figure 12(a–d)**. These precipitations are able to enhance the formation of martensite β′1 phase. Saud et al. [21] presented the changes in the microstructure changes of Cu-Al-Ni SMAs after the addition of different percentages of Ti and the microstructure changes were exhibited in **Figure 13(a–d)**. it was revealed that the presence of γ′1 and β′1 phases, on the other hands, there is an irregular phase was observed in the modified microstructure in the shape of flower and it has been formed randomly between β′1 plates and

**38**

**Figure 11.**

*Al-5 wt.% Ni-2 wt.% Mn-1 wt.% Ti [59].*

*Optical micrographs of Cu-Al-Ni alloys: (a) Cu-Al-Ni; (b) Cu-Al-Ni-0.2 Ti; (c) Cu-Al-Ni-0.4 Mn; and (d) Cu-Al-Ni-0.2 Zr [50].*

#### **4.3 Mechanical properties of Cu-Al-Ni SMA**

Cu-Al-Ni shape memory alloys (SMA) have been selected as high potential materials for high temperature applications. This is attributed to their high thermal stability at temperatures above 100°C [73–76]. On the other hand, these alloys have their limitations such as high brittleness because of the appearance of brittle phase γ2 at grain boundaries, the enormous increase in grain size duplicated with a high elastic variation [77–81]. Thus, their disadvantages have restricted the usage of these alloys for commercial applications [82–92]. One way to solve this problem is the grain refinement. By adding some of the alloying elements such as Ti, Mn, V, Nb, B and others or varying the compositions of Ni or Al, some improvement in mechanical properties of the conventional Cu-Al-Ni SMAs [86, 93–96] was observed. This improvement is attributed to the addition of alloying elements, where these elements are restricting the grain growth and refining the grains. However, these alloying elements have a significant effect on the mechanical properties of Cu-Al-Ni SMAs due to the formation as a second phase structure in the microstructure [97]. Miyzakai et al. [23, 56] found that varying the percentage of Al and Ni lead to changes in crack formation and propagation. It was also found that increases in the Al and Ni amount from 14 and 3.9 wt.% to 14.2 and 4 wt.% lead to the appearance of clear crack formation. This may be attributed to the amount of thermal stress induced and in accordance to the Clausius-Claperyron equation, the increase in the alloying composition of Al and Ni has an effective influence on the

#### **Figure 13.**

*FESEM micrographs showing the microstructures of the Cu-Al-Ni SMA with different concentration of Ti additions: (a) Cu-Al-Ni (alloy A), (b) Cu-Al-Ni-0.4 mass% Ti (alloy B), (c) Cu-Al-Ni-0.7 mass% Ti (alloy C), (d) Cu-Al-Ni-1 mass% Ti (alloy D) [21].*

martensite thermal stress induced, which lead to crack initiation and propagation. The addition of manganese and boron efficiently refine the grain size, however, increasing of the boron concentration produced the highest strain hardening. Wayman and Lee [58] have found that the addition of boride particles helped to relieve the stress concentrations at the grain boundaries. Morris [72] found that by adding the boron to the Cu-Al-Ni SMAs, the ductility increased. This can also be attributed to the presence of boride particle. Another relevant point is that the boron addition can have an effect on the fracture mode, as it has been transferred from brittle failure to intergranular and transgranular failure. Another work by the same author [98], found that the values of yield stress, hardness and tensile

**41**

**Figure 15.**

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

to the type and amount of the addition element as shown in **Figure 15**.

*Tensile fracture surfaces at room temperature for (a) Cu-13.4Al-3.8Ni SMA, (b) Cu-13.2Al-3.04Ni-0.36Ti SMA, (c) Cu-13.0Al-2.9Ni-0.36 Ti-0.22 Mn SMA and (d) Cu-13.4Al-3.05Ni-0.24Ti-0.63Zr SMA [99].*

strength have been increased with increasing the percentage of boron addition. It seems that the boride particles have restricted the interface movement, therefore the required stress to re-orient the martensite phase is high. These particles have played a significant role by accommodating a new strain concentration generated by the coexistence of the new stress-induced martensite. Roh et al. [99] reported that the fine grained alloys resulting from the addition of Ti, Mn, and Zr to the coarse grained Cu-Al-Ni SMA lead to enhance the fracture stress-strain. It was found that the fracture stress and strain obtained the highest value of 930 MPa and 8.6%, respectively, with the combined addition of 0.3Ti-0.6Zr to Cu-13.4AI-3.05Ni SMA. This improvement is due to grain refinement and the presence of precipitates that formed within grains in the alloy. They have also confirmed other researchers' findings [86, 100, 101] that the tensile properties of (σt, σf, and εt) increased as a function of decreasing grain size, as shown in **Figure 14**. In contrast, the fractured surfaces of Cu-Al-Ni SMA changed from brittle mode to different modes according

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

**Figure 14.** *Variation in the (a) transition stress, (b) fracture strain, and (c) fracture stress versus grain size [99].*

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

strength have been increased with increasing the percentage of boron addition. It seems that the boride particles have restricted the interface movement, therefore the required stress to re-orient the martensite phase is high. These particles have played a significant role by accommodating a new strain concentration generated by the coexistence of the new stress-induced martensite. Roh et al. [99] reported that the fine grained alloys resulting from the addition of Ti, Mn, and Zr to the coarse grained Cu-Al-Ni SMA lead to enhance the fracture stress-strain. It was found that the fracture stress and strain obtained the highest value of 930 MPa and 8.6%, respectively, with the combined addition of 0.3Ti-0.6Zr to Cu-13.4AI-3.05Ni SMA. This improvement is due to grain refinement and the presence of precipitates that formed within grains in the alloy. They have also confirmed other researchers' findings [86, 100, 101] that the tensile properties of (σt, σf, and εt) increased as a function of decreasing grain size, as shown in **Figure 14**. In contrast, the fractured surfaces of Cu-Al-Ni SMA changed from brittle mode to different modes according to the type and amount of the addition element as shown in **Figure 15**.

#### **Figure 15.**

*Tensile fracture surfaces at room temperature for (a) Cu-13.4Al-3.8Ni SMA, (b) Cu-13.2Al-3.04Ni-0.36Ti SMA, (c) Cu-13.0Al-2.9Ni-0.36 Ti-0.22 Mn SMA and (d) Cu-13.4Al-3.05Ni-0.24Ti-0.63Zr SMA [99].*

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

martensite thermal stress induced, which lead to crack initiation and propagation. The addition of manganese and boron efficiently refine the grain size, however, increasing of the boron concentration produced the highest strain hardening. Wayman and Lee [58] have found that the addition of boride particles helped to relieve the stress concentrations at the grain boundaries. Morris [72] found that by adding the boron to the Cu-Al-Ni SMAs, the ductility increased. This can also be attributed to the presence of boride particle. Another relevant point is that the boron addition can have an effect on the fracture mode, as it has been transferred from brittle failure to intergranular and transgranular failure. Another work by the same author [98], found that the values of yield stress, hardness and tensile

*Variation in the (a) transition stress, (b) fracture strain, and (c) fracture stress versus grain size [99].*

*FESEM micrographs showing the microstructures of the Cu-Al-Ni SMA with different concentration of Ti additions: (a) Cu-Al-Ni (alloy A), (b) Cu-Al-Ni-0.4 mass% Ti (alloy B), (c) Cu-Al-Ni-0.7 mass% Ti* 

**40**

**Figure 14.**

**Figure 13.**

*(alloy C), (d) Cu-Al-Ni-1 mass% Ti (alloy D) [21].*

**Figure 16.** *Recoverable strain versus bend-relaxing time of Cu-Al-Ni and Cu-Al-Ni-Be [105].*

Xu et al. [102, 103] found by adding the Be to the Cu-Al-Ni SMAs, the fatigue life has been increased, as the strain recovery has reached 30% higher than base alloy. Increase in the recovery strain is almost equal to the recovery strain of the NiTi. Zhu et al. [97] found the bending performance, tensile strength, and elongation percentage of Cu-Al-Ni-Be are higher than Cu-Al-Ni alloy, where the maximum stress of this alloy could reach to 780 MPa with 18% of strain as shown in **Figures 16** and **17**. This may imply that the mechanical property of Cu-based SMAs can be significantly improved by adding the alloying elements. The additions of Ti, Mn, and Zr to Cu-Al-Ni shape memory alloys have decreased the grain size reported by Sampath [50], therefore the values of hardness increased. This is attributed to the formation of fine precipitates that

**43**

**Figure 19.**

**Figure 18.**

*Cu-Al-Ni-1 mass% Ti (alloy D).*

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

*Compressive stress–strain of different loading-unloading cycles tested at a temperature of 473 K (200°C); (a) Cu-Al-Ni; (b) Cu-Al-Ni-0.5 wt.% Sn; (c) Cu-Al-Ni-1.0 wt.% Sn; and (d) Cu-Al-Ni-1.5 wt.% Sn [60].*

*Shape memory effect curves of the alloys performed at T < Mf, then preheated to T > Af to obtain the shape recovery [21], Cu-Al-Ni (alloy A), Cu-Al-Ni-0.4 mass% Ti (alloy B), Cu-Al-Ni-0.7 mass% Ti (alloy C),* 

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

**Figure 17.** *Stress-strain curves of SMA samples at room temperature (25°C) [105].*

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

**Figure 18.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

Xu et al. [102, 103] found by adding the Be to the Cu-Al-Ni SMAs, the fatigue life has been increased, as the strain recovery has reached 30% higher than base alloy. Increase in the recovery strain is almost equal to the recovery strain of the NiTi. Zhu et al. [97] found the bending performance, tensile strength, and elongation percentage of Cu-Al-Ni-Be are higher than Cu-Al-Ni alloy, where the maximum stress of this alloy could reach to 780 MPa with 18% of strain as shown in **Figures 16** and **17**. This may imply that the mechanical property of Cu-based SMAs can be significantly improved by adding the alloying elements. The additions of Ti, Mn, and Zr to Cu-Al-Ni shape memory alloys have decreased the grain size reported by Sampath [50], therefore the values of hardness increased. This is attributed to the formation of fine precipitates that

*Recoverable strain versus bend-relaxing time of Cu-Al-Ni and Cu-Al-Ni-Be [105].*

**42**

**Figure 17.**

**Figure 16.**

*Stress-strain curves of SMA samples at room temperature (25°C) [105].*

*Shape memory effect curves of the alloys performed at T < Mf, then preheated to T > Af to obtain the shape recovery [21], Cu-Al-Ni (alloy A), Cu-Al-Ni-0.4 mass% Ti (alloy B), Cu-Al-Ni-0.7 mass% Ti (alloy C), Cu-Al-Ni-1 mass% Ti (alloy D).*

**Figure 19.**

*Compressive stress–strain of different loading-unloading cycles tested at a temperature of 473 K (200°C); (a) Cu-Al-Ni; (b) Cu-Al-Ni-0.5 wt.% Sn; (c) Cu-Al-Ni-1.0 wt.% Sn; and (d) Cu-Al-Ni-1.5 wt.% Sn [60].*

restricted the grain growth by the pinning effect. Also, other elements have shown a significant effect on the mechanical properties of Cu-Al-Ni SMAs during the addition. For example, the rupture strain of Nb and V has increased up to 14 and 6%, respectively, which is much higher than the base alloy as reported by Gomes et al. [104]. The strain recovery by the shape memory effect (εSME) of the Cu-Al-Ni SMAs with and without the Ti additions was studies by Saud et al. [21], as shown in **Figure 18**. The results were shown that the addition of Ti with different mass percentages exhibited an increase in the values of strain recovery by the SME. These enhancements in references the strain recovery were attributed to the existence of the X-phase that was brought about by the Ti additions in the parent phase. Another study by the same authors [60] shown the effect of different percentage of 0.5, 1.0, and 1.5 wt.% of Sn addition on the stress–strain curves under multi-cycles of loading and unloading. It was found that the largest number of cycles was indicated with the Cu-Al-Ni-1 wt.%Sn SMA before the occurrence of fracture, as shown in **Figure 19(a–d)**. This improvement is due to two reasons: low porosity density and the finest particle size among the alloys.
