**6. Attempts towards the development of high temperature Aluminum alloys**

Over the years, alloying pure Aluminum with various elements showed good promises for elevated temperature applications. Such elements included rare earths (e.g. Erbium, Ytterbium, Scandium etc.) as well as Zirconium, Silicon etc. [25–27]. For these alloys, formation of coherent precipitates with cubic L12 crystal structure and reduced interfacial energy is the key for their high temperature stability against precipitate coarsening upon thermal exposure [26]. The other unique exploration is the formation of core-shell structure for the strengthening precipitates which also provides excellent coarsening resistance through the minimization of interfacial energy. For example, addition of 0.06 at% Zr or 0.03 at% Er in Aluminum individually form ordered L12 Al3Zr or Al3Er precipitates and show moderate coarsening resistance at elevated temperature [25]. Simultaneous addition of Zr and Er in similar quantity however, leads to Al3(Er, Zr) precipitates with unique core shell structure, which made them coarsening resistant up to 400°C for 750 hours. The difference in diffusivity between Zr and Er was held responsible for formation of such core-shell structure; while Er having higher diffusivity forms the primary precipitate with Aluminum, slower diffusing Zr segregates later at the interfaces of these primary precipitates resulting in the core-shell structure. Similarly, addition of Sc to Al-Zr-Sc-Er alloy (concentrations of both Sc and Er are 0.06 at%) leads to a dual shell layer of Zr and Sc according to their respective diffusivity in Al matrix over the Al3Er core precipitate [26].

Furthermore, excellent creep resistance was observed for Al-0.1 at% Zr and Al-0.1 at% Zr-0.1 at% Ti alloy systems at 300°C, 350°C and 400°C, which is attributed to the high temperature stability of Al3Zr precipitates [28]. Out of the two alloys, ternary Al-Zr-Ti alloy showed comparatively lower creep resistance than binary Al-Zr alloy due to the lower lattice parameter mismatch between Al3(Zr1-xTix) core-shell precipitates with the parent α-Al matrix. The addition of Yb similarly resulted in excellent thermal stability for Al-0.9 at% Zr- 1.73 at% Yb alloys having Al3(Zr,Yb) precipitates up to 400–425°C [29].

For Al-Si system, Al-Si-Cu-Mg alloys are traditionally used for making high temperature pistons for automobile engines [30, 31]. These alloys show satisfactory microstructural stability as well as fatigue resistance at high temperatures which are essential requirements for automotive applications. A viable route for further improving their high temperature performance is by addition of transition metals that forms thermally stable intermetallic precipitates. For example, controlled Zr addition (up to 0.11 wt%) increases the ultimate tensile strength (UTS) of Al-Si-Zr piston alloys by 3.8% at 350°C due to the alteration in the morphology of strengthening ZrAlSi precipitates from flake to block shape [32, 33]. However, increase in Zr content up to 0.46 wt% resulted in a decrease in UTS by 5%. Similarly, A356 alloy (Al-7Si-0.4 Mg) modified with 0.25 wt% Er and nominal amount of Zr (0 to 0.6 wt%)

showed improved high temperature mechanical properties [34]. With increase in Zr content up to 0.59 wt%, both hardness and tensile strength increases at room and elevated temperatures due to the formation of Al3(Er,Zr) precipitates.

Hypoeutectic Al-7 wt%Si-1wt%Cu-0.5 wt% Mg alloys also shows excellent retention of hardness and tensile strength up to 240–260°C when micro-alloyed with 0.15 wt% Zr, 0.28 wt% V and 0.18 wt% Ti [35]. Further exposure to 475°C upto 128 hours led to additional improvement in hardness which can be attributed to the accelerated precipitation of Al3(Zr,V,Ti) and Q<sup>0</sup> precipitates. Similarly, addition of minor Ti (0.22 wt%), Zr (0.39 and 0.19 wt%) and Ni (0.46–0.21 wt%) to commercial 354 alloy showed improvement in tensile properties up to 300°C compared to the base alloy [36]. In both cases, micro-alloying elements synergistically result in unique and complex precipitate formation which improved the high temperature stability of the corresponding alloys. For hypereutectic Al-Si alloys, Ni addition up to 1–4 wt% to Al-12 wt%Si-0.9 wt% Cu-0.8 wt% Mg alloy resulted in retention of room temperature mechanical properties, including creep resistance up to 250°C due to the formation of thermally stable Al3Ni precipitates [37]. In addition to primary and eutectic Si, incorporation of 1 wt% ZnO nanoparticles (particle size �40 nm) also enhance the high temperature tensile strength and elongation for Al-20 wt% Si alloys [38].

In case of age hardening Al-Cu system, several attempts were made in the past to increase their high temperature stability by adopting various strategies. Lin et al. [39] studied the effect of Ni addition (0.5–1.5 wt%) on the elevated temperature mechanical properties of squeeze cast Al-Cu-Mn-Fe alloys. At 300°C, the amount of thermally stable precipitates (e.g. Al9FeNi, Al3CuNi and Al20Cu2Mn3) increases with increasing Ni content which enhances the elevated temperature mechanical properties of the base alloy. Addition of La in Al-Cu alloy similarly results in the formation of Al11La3 precipitates leading to a better high temperature mechanical properties with 0.3 wt% La being the optimized concentration [40]. The addition of 1.6–2.0 wt % Li also shows excellent mechanical properties for AA2099 (Al-Cu-Li) alloys at high temperature, primarily due to the enhanced thermal stability of T1 (Al2CuLi) precipitates compared to other possible strengthening precipitates like θ<sup>0</sup> and S (Al2CuMg) [41]. At higher temperature, T1 precipitates coarsen instead of dissolving unlike θ<sup>0</sup> or S. In addition, AA2219 alloy possesses improved high temperature performance when micro-alloyed with 0.8 wt% Sc, 0.45 wt% Mg and 0.2 wt% Zr from grain refinement and simultaneous precipitation of Al3Sc, Al3Zr and Ω precipitates along with other common strengthening precipitates like <sup>θ</sup><sup>0</sup> and <sup>θ</sup>″ [42].

Another viable strategy of increasing the thermal stability of Al-Cu alloys is by micro-alloying with various secondary elements for stabilization of strengthening metastable θ<sup>0</sup> precipitates. For example, 0.18 at% Sc addition forms thermally stable Al3Sc precipitate wherein Sc atoms tend to segregate at the α-Al matrix/θ<sup>0</sup> precipitate interfaces [43]. Such co-stabilization of two different precipitates renders excellent high temperature property for Al-Cu alloys with Sc addition [44]. Micro alloying with Zr provides similar effects as Sc; Zr forms stable Al3Zr precipitates having L12 structure which can act as a heterogeneous nucleation site for θ<sup>00</sup> precipitates, thus generating a finer scale microstructure for Al-Cu-Zr alloys [45]. The resultant alloy shows excellent thermal stability up to 250–300°C. In Al-Cu-Mg alloys, 0.09–0.13 wt% Mg is reported to accelerate the formation of θ<sup>0</sup> precipitates and the resultant alloy exhibits excellent thermal stability at 300°C for 1000 hours [46]. Similarly, addition of 0.45 wt% Ce provides heterogeneous nucleation sites for precipitation of Ω precipitates and enhances the thermal stability of Al-5.3% Cu-0.8% Mg-0.6% Ag (wt.%) alloys by retarding the diffusion of Cu [47].

Overall, there have been numerous efforts in the past to design high temperature Aluminum alloys from different binary systems (Al-Cu, Al-Si etc.), primarily by micro-alloying with various elements. However, most of these attempts showed

*New-Age Al-Cu-Mn-Zr (ACMZ) Alloy for High Temperature-High Strength Applications… DOI: http://dx.doi.org/10.5772/intechopen.104533*

certain shortcomings. The working temperature of the resultant ternary or quternary alloys could not be increased above 300°C under prolonged exposure. Also, use of exotic elements like rare earth additions hindered their industrial acceptance and commercial viability. Hence, the demand of cost-effective Aluminum alloys for high temperature applications has only increased over the years without much of a success.
