**3. Cu-based SMAs**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

**28**

**Figure 3.**

**Figure 2.**

*Shape memory effect test [21].*

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

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**.

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. Accuracy of 10<sup>−</sup><sup>3</sup> to 10<sup>−</sup><sup>4</sup> at.% is typically essential to obtain reproducibility more desirable 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 transformation strain (ε t ) 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 temperatures from 200 to 150°C with very good thermal stability.

Cu-based SMAs consist of different types of alloys, but the most frequently used 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 alloys and high transformation temperatures.
