**4. Conclusions**

172 Recent Trends in Processing and Degradation of Aluminium Alloys

coefficient value on time of annealing enabled comparison of stability of mechanical

(a)

(b)

Fig. 23. Relative change of ultimate tensile strength (a) and 0.2% offset yield strength (b) of

Repeatability of the mechanical properties of AlCu4Ni2Mg and AlCu6Ni alloys after longterm annealing was determined on the basis of variation of the static tensile test results (table 5). Five specimens were tested for each temperature and time of annealing. Coefficient

> 100 *<sup>z</sup> <sup>s</sup> <sup>W</sup>*

*<sup>x</sup>* = × (1)

the AlCu4Ni2Mg and AlCu6Ni alloys as a function of time of annealing at 573K

of variation was calculated using formula:

where: *s* **–** standard deviation, *x* **–** average value

properties of the investigated alloys (fig. 22 and 23).

In the AlCu4Ni2Mg and AlCu6Ni alloys degradation of the microstructure takes place as a result of long-term thermal loading. It consists largely in coarsening and the change of the shape of hardening phase particles (θ'-Al2Cu). The changes are proportional to the annealing time and temperature and lead to significant decrease of the mechanical properties of the alloys. The alloys studied are characterized by different content of Cu – primary element forming hardening phase. Increased Cu content in AlCu6Ni alloy caused only slight improvement of the stability of its strength properties. The AlCu4Ni2Mg alloy containing less Cu but with addition of Mg is characterized by better strength properties than AlCu6Ni alloy in T6 condition and preserves relatively high tensile strength and good ductility after long-term thermal loading. Taking into account criterion of mechanical properties and their stability both alloys studied can be successfully applied for highly stressed elements of aircraft structures operating in the temperature range of 523-573K.
