**1. Introduction**

154 Recent Trends in Processing and Degradation of Aluminium Alloys

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The precipitation−strengthened 2xxx series Al−Cu alloys are one of the most important high-strength aluminium alloys. They have been employed extensively in the aircraft and military industries, in which materials are frequently subjected to elevated temperature. The aluminium casting alloys, based on the Al−Cu system are widely used in light−weight constructions and transport applications requiring a combination of high strength and ductility.

Al-Cu alloys are less frequently used than Al-Si-Cu grades due to technological problems in production process (e.g. high propensity to microcracking during casting). However they are the basis for development of multicomponent alloys. Typical alloys for elevated temperature application are Al-Cu-Ni-Mg alloys (containing about 4,5% Cu, 2% Mg and 2%Ni). Their good properties at elevated temperature result from formation of intermetallic phases Al6Cu3Ni and Al2CuMg, both during crystallization and precipitation hardening (El−Magd & Dünnwald, 1996; Martin, 1968; Mrówka-Nowotnik et al., 2007).

Mechanism of precipitation hardening in cast and wrought binary Al-Cu alloys is well known and widely covered in literature. There are some suggestions that decomposition of supersaturated α(Al) solid solution in other precipitation hardened alloys like Al-Cu-Mg, Al-Si-Cu, Al-Mg-Si follows the same route as in the Al-Cu alloys with some specific features of the particular stages of the process (Martin, 1968;). The interest in course and kinetics of the aging process has the practical meaning as the early stages of aging leads to significant improvement of mechanical properties of the alloys. Maximum hardening effect in Al-Cu alloy is a result of in situ transformation of GP zones into transient phase θ". Increase in aging temperature leads to decrease of the hardness of solid solution α(Al) due to precipitation of equilibrium θ phase on the grain boundaries or on the θ'/matrix phase boundaries. Prolonged aging may lead to microstructure degradation related to coagulation and/or coalescence of the highly dispersed hardening phase precipitates resulting in decrease of hardening effect (Mrówka-Nowotnik et al., 2007; Wierzbińska & Sieniawski, 2010). Therefore development of the chemical composition of the alloy, especially intended for long term operation at elevated temperature, requires taking into account factors resulting in deceleration of the coagulation process and obtaining stable microstructure consisting of solid solution α grains and highly dispersed precipitates of the second phase (Wierzbińska & Sieniawski, 2010).

Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature 157

Figs. 1 to 4 show the results of microscopic observations of AlCu4Ni2Mg and AlCu6Ni alloys (in T6 condition). In both of investigated alloys large, irregular shaped precipitates of

Fig. 1. Microstructure of AlCu4Ni2Mg alloy in T6 condition (LM)

phases in interdendritic areas (SEM)

Fig. 2. Microstructure of AlCu4Ni2Mg alloy in T6 condition: precipitations of intermetallic

**3. Results and discussion** 
