**2. Material and methodology**

The investigation was performed on the two casting alumium alloys AlCu4Ni2Mg and AlCu6Ni. AlCu4Ni2Mg is the standard alloy used currently for highly stressed structural elements of engines and AlCu6Ni1 is an experimental alloy that was chosen to investigate the influence of the increased content of Cu on the phase composition, microstructure morphology and mechanical, technological and operational properties. The alloys were cast into metal moulds and subjected to X-ray inspection in order to exclude the presence of porosity or oxide films.

The alloys were subjected to heat treatment T6 followed by annealing at 523 K and 573 K for 150 and 500 hours. After analysis of the results of preliminary tests it was found, that it is advisable to apply additional annealing times at particular temperature, i.e. 100, 300 and 750 hours.

Heat treatment conditions were established on the basis of the phase equilibrium diagrams Al-Si and Al-Cu and available heat treatment data for the alloys with similar chemical composition (both from literature and used in industry practice). The consideration was also given to requirements concerning mechanical properties of the alloys resulting from operation condition of the structural elements made of these alloys. The chemical composition of the investigated alloys and heat treatment parameters are presented in table 1.


Table 1. Composition of AlCu4Ni2Mg and AlCu6Ni alloys and heat treatment parameters

Examination of the alloys microstructure was carried out using light microscope (LM), as well as scanning (SEM) and transmission (TEM) electron microscopes.
